Forest Service Northern Research Station General Technical Report NRS-180

Critical Loads of Sulfur and Nitrogen 

and Modeled Effects of Deposition 

Reduction for Forested Ecosystems of 

Great Smoky Mountains National Park

Linda H. Pardo, Natasha Duarte, Helga Van Miegroet, 

L. Suzanne Fisher, and Molly J. Robin-Abbott

United States Department of Agriculture

September 2018
Published by

U.S. FOREST SERVICE

11 CAMPUS BLVD SUITE 200

NEWTOWN SQUARE PA 19073

September 2018

Manuscript received for publication 07 February 2018

Abstract

Great Smoky Mountains National Park (GSMNP) is subject to high levels of sulfur (S) and nitrogen 

(N) deposition, which can adversely affect forest vegetation and aquatic biota. We used multiple 

chemical criteria to calculate critical loads for S and N deposition (CL (S + N)) and nutrient N 

(CLnutN) and used the Very Simple Dynamic (VSD) model to predict the effects of deposition 

reduction scenarios on critical thresholds for four forested sites in GSMNP. Critical loads were 

exceeded for current deposition at three of four sites using critical thresholds of aluminum to base 

cations (Al:Bc) = 0.1 or no decrease in base saturation but were not exceeded using the chemical 

criteria of Al = 0.2 meq L-1, Al:Bc = 1.0, and pH = 4.2. With deposition reductions of 48 percent S 

and 56 percent nitrate (NO3−), neither the critical thresholds of acid neutralizing capacity (ANC) 

= 20 μeq L-1 nor a decrease in base saturation was achieved. With deposition reductions of 90 

percent S and 90 percent NO3−, ANC = 100 μeq L-1 was achieved at a single site. Historical ANC 

values affected a site’s ability to achieve ANC critical thresholds. The critical threshold for soil 

solution NO3− was exceeded for all but the most stringent deposition scenarios (–90 percent S and 

–90 percent NO3−). Deposition reductions of 90 percent for NO3− and 80 percent for ammonium 

(NH4

+) were not sufficient to lower deposition below the CLnutN at all sites. Data indicate 

that upper sites at GSMNP are N saturated; to protect these sites from acidification, the more 

protective chemical criteria of no decrease in base saturation and Al:Bc = 0.1 should be used when 

determining critical loads. When choosing chemical criteria in deposition reduction modeling, 

care should be taken to ensure that the criteria chosen will protect sensitive ecosystem elements.

The Authors

LINDA PARDO, environmental engineer, U.S. Forest Service, Northern Research Station, 81 

Carrigan Dr., UVM Aiken Center, Room 204C, Burlington, VT 05405; lpardo@fs.fed.us

NATASHA DUARTE, soil scientist, U.S. Forest Service, Northern Research Station, 705 Spear St., 

South Burlington, VT 05403; nduarte@gmavt.net

HELGA VAN MIEGROET, professor emerita, Utah State University, Department of Wildland 

Resources, 5230 Old Main Hill, Logan UT 84322-5230; helga.vanmiegroet@usu.edu

L. SUZANNE FISHER, senior program manager, Natural Resources, Tennessee Valley Authority, 

Knoxville, TN 37902; lsfisher@tva.gov

MOLLY ROBIN-ABBOTT, contractor, U.S. Forest Service, Northern Research Station, 705 Spear St., 

South Burlington, VT 05403; mjrobina@gmail.com

Cover Photo

A typical view of the mountains in the Great Smoky Mountains National Park, North Carolina. 

Photo by Helga Van Miegroet, used with permission.

Quality Assurance

This publication conforms to the Northern Research Station’s Quality Assurance Implementation 

Plan, which requires technical and policy review for all scientific publications produced or funded 

by the Station. The process included a blind technical review by a reviewer who was selected 

by the Assistant Director for Research and unknown to the author. This review policy promotes 

the Forest Service guiding principles of using the best scientific knowledge, striving for quality 

and excellence, maintaining high ethical and professional standards, and being responsible and 

accountable for what we do.
Contents

Introduction...................................................................................................................................................................1

Materials and Methods..........................................................................................................................................2

Site Description...................................................................................................................................................2

Methods for Calculating Steady-state Mass-balance Critical Loads............................4

Methods for Dynamic Modeling Using the Very Simple Dynamic Model..............7

Results..............................................................................................................................................................................9

Critical Loads of Acidity and Nutrient Nitrogen and Exceedance...................................9

Dynamic Modeling of Soil Solution Trends Over Time ........................................................11

Discussion..................................................................................................................................................................15

Critical Loads and Exceedances for Acidity (S + N)...............................................................16

Critical Loads and Exceedances for Nitrogen..............................................................................16

Trends in Modeled Soil Solution Over Time.................................................................................17

Conclusions...............................................................................................................................................................19

Acknowledgments.................................................................................................................................................20

Literature Cited.........................................................................................................................................................20

General Tecnical Report NRS-180
Spruce -fir forest near Clingman's Dome, North Carolina. Photo by Helga Van Miegroet, used with permission.
1

INTRODUCTION

The Eastern United States has been severely affected 

by elevated atmospheric deposition of sulfur (S) and 

nitrogen (N) over many decades (Driscoll et al. 2001, 

Eagar and Adams 1992, Reuss and Johnson 1986). 

Atmospheric S and N deposition have contributed to 

acidification of soils and surface waters, with harmful 

effects on forest vegetation and aquatic biota (Driscoll 

et al. 2001). Sulfur dioxide (SO2) emissions peaked in 

the United States in 1973 and declined ~67 percent 

between 1980 and 2009 because of the Clean Air Act 

(Burns et al. 2011). This pattern has not, however, 

coincided with widespread recovery of soil and surface 

waters from acidic deposition (Bailey et al. 2004, 2005; 

Rice et al. 2014; Stoddard et al. 2003; Warby et al. 2005, 

2009). Acidification may deplete soils of base cations 

and can mobilize aluminum (Al) in soils (Elliott 

et al. 2008, Reuss 1983, Reuss and Johnson 1985), 

resulting in toxicity to plants and other biota. Nitrogen 

deposition poses a threat to forest health beyond the 

impacts of acidification; it can lead to N saturation, the 

condition where available N exceeds biotic demand 

(Aber et al. 1989). Detrimental responses to elevated 

N deposition, which occur with N saturation, include 

elevated surface water nitrate (NO3−) concentration 

(Aber et al. 2003, Brookshire et al. 2007); biotic 

responses include increases in tissue N chemistry and 

shifts in understory and overstory species composition 

(Boggs et al. 2005, Pardo et al. 2011a), which have been 

observed across the Eastern United States.

The Great Smoky Mountains National Park (GSMNP), 

designated an International Biosphere Reserve in 

1976 and a World Heritage Site in 1983, has long been 

considered a center of vascular plant biodiversity 

in North America (Jenkins 2007). In addition to a 

diverse assemblage of plant communities, GSMNP 

is home to many plants that are endemic to the 

southern Appalachians and large tracts of primary 

forest. These communities are subject to multiple 

threats to forest health, including exotic pests and 

pathogens and airborne pollutants (Jenkins 2007). 

Great Smoky Mountains National Park has received 

among the highest levels of N and S deposition in 

the Eastern United States (Johnson et al. 1991, Lovett 

and Lindberg 1993, Weathers et al. 2006). For 2000, 

Weathers et al. (2006) modeled deposition hotspots 

of 42 kg ha-1 y-1 (2,600 eq ha-1 y-1) S and 31 kg ha-1 y-1

(2,200 eq ha-1 y-1) N. Soils, especially at high elevations, 

have low base saturation and therefore little capacity 

for buffering acidic inputs (Cai et al. 2012, Johnson 

and Lindberg 1992). High-elevation soils and runoff 

show signs of N saturation (Nodvin et al. 1995, Van 

Miegroet et al. 2001). Some tree species in the park, for 

example, red spruce (Picea rubens Sarg.), are sensitive 

to elevated N inputs and acidification (Sullivan et al. 

2001, Webster et al. 2004). Thus, some areas of GSMNP 

may be susceptible to the adverse impacts of N and S 

deposition.

The concept of critical loads was introduced in 

Europe more than two decades ago as a tool for 

negotiating reductions in air pollution to protect 

sensitive ecosystems (Posch et al. 2001). A critical 

load is the estimate of exposure to pollutants below 

which harmful effects on sensitive elements of the 

environment do not occur over the long term based on 

present knowledge (UBA 2004). Critical loads can be 

determined using empirical approaches, steady-state 

mass-balance calculations, and dynamic modeling for 

aquatic or terrestrial systems. Terrestrial critical loads 

for acidity (CL (S + N)) are used to estimate the level 

of N + S deposition that will lead to soil acidification 

and the subsequent detrimental effects to the forest 

ecosystem; aquatic critical loads for acidity determine 

the deposition that will maintain an acid neutralizing 

capability (ANC) level that will protect aquatic biota 

from acidification (Scheffe et al. 2014). Steady-state 

mass-balance critical loads for nutrient N (CLnutN) for 

terrestrial ecosystems designate the N deposition level 

in excess of potential biological and abiotic retention 

within the ecosystem. The exceedance, the extent to 

which current deposition exceeds the critical load 

(exceedance = current deposition – critical load), is a 

simple way of quantifying the risk from atmospheric 

deposition for an ecosystem.

The purpose of this assessment was to calculate critical 

loads for S and N deposition at forest ecosystem 

sites in GSMNP based on available data and using 

four chemical criteria. Our approach to critical load 
2

calculation differs from recent studies (Fakhraei et 

al. 2016, Zhou et al. 2015), which focused primarily 

on impacts of surface water acidity (ANC) on aquatic 

biota. We focus on the potential impacts on soil and 

vegetation and incorporate the effect of disturbance 

on stand dynamics in the high-elevation spruce-fir 

ecosystems. A simple mass-balance model comparing 

ecosystem inputs (including N or S deposition) to 

ecosystem outputs was used to calculate CL (S + N) 

and CLnutN. We used the Very Simple Dynamic (VSD) 

model (Posch and Reinds 2009, Posch et al. 2003) 

to predict when an ecosystem parameter (e.g., soil 

solution chemistry) may achieve a critical value, or 

when the ecosystem may recover. We used dynamic 

modeling to evaluate various deposition scenarios to 

assess emission control strategies. Resource managers 

and policy makers require this information to assess 

the impact of additional pollution sources and to 

determine the level of deposition reduction necessary 

to protect national parks and other protected lands. We 

used the steady-state model to assess critical thresholds 

in critical loads calculations and the dynamic model to 

assess changes in chemical criteria over time.

MATERIALS AND METHODS

Site Description

Great Smoky Mountains National Park covers 2114 

km2 in eastern Tennessee and western North Carolina. 

We used four sites in the park with available data to 

assess the critical loads (Table 1, Figure 1). These sites 

represent three forest types at different elevations: 

• High-elevation spruce-fir. This forest type 

is represented by two sites included in the 

Integrated Forest Study (IFS) (Johnson and 

Lindberg 1992). Both sites have Fraser fir as a 

component of the understory. The soils at both 

sites are classified as Umbric Dystrochrepts 

derived from Thunderhead sandstone 

(Johnson and Lindberg 1992). Soils have a silt 

loam to sandy loam texture, are acidic, and are 

characterized by high organic matter content, 

low base saturation, and high N mineralization 

and nitrification capacity (Cai et al. 2012, 

Garten and Van Miegroet 1994, Johnson et 

Table 1.—Site description

Site Upper Spruce-Fir Lower Spruce-Fir Beech Gap Mixed Hardwood

Elevation (m) 1800 1740 1600 635

Forest type spruce-fir spruce-fir American beech mixed hardwood

Additional site information Red Spruce; 

Becking site IFS 

(SS) a

Red Spruce; Noland 

Divide Tower site IFS

(ST)a

American 

Beech site IFS 

(SB) a

NADP/NTN b 

Elkmont site 

(TN11)

Total S deposition (eq ha-1 y-1) 1958 1958 983 625

Total S deposition (kg ha-1 y-1) 31 31 16 10

Total N deposition (eq ha-1 y-1) 2313 2313 1162 607

Total N deposition (kg ha-1 y-1) 32 32 16 8.5

Total base cation deposition 

(eq ha-1 y-1)

1713 1713 860 173

N sequestration rate (eq ha-1 y-1) 321 45 0 0

Base cation sequestration rate

(eq ha-1 y-1)

562 79 0 0

Weathering rates (eq ha-1 y-1) 770 2632 682 971

Weathering rate source IFS IFS Substrate type– 

clay %c

Substrate type– 

clay %c

a Sites included in the Integrated Forest Study (Johnson and Lindberg 1992).

b National Atmospheric Deposition Program/National Trends Network.

c Sverdrup et al. 1990.
3

al. 1991). Both sites have a long-term mean 

annual precipitation of 200 cm (Vose and 

Swank 1992).

ο The Upper Spruce-Fir site (called the 

Becking site in the IFS) is located at an 

elevation of 1800 m on a southwesterly 

slope west of Noland Divide near Clingmans 

Dome. This site is characterized by abundant 

large sandstone boulders on top of and 

within the soil profile. The vegetation is 

dominated by old-growth red spruce (Picea 

rubens Sarg.) with occasional Fraser fir 

(Abies fraseri Pursh (Poir.).

ο The Lower Spruce-Fir site (called the Tower 

site in the IFS) is located at an elevation of 

1740 m on a southerly slope near Noland 

Divide. The vegetation here is also dominated 

by old-growth red spruce and occasional 

yellow birch (Betula alleghaniensis Britton) 

(Johnson et al. 1991).

Historical logging has affected the western 

part of GSMNP; however, no further logging 

activity occurred after the park was established 

in 1934, and there is no evidence of fire 

disturbance. The Spruce-Fir sites included in 

this study have remained largely undisturbed 

by logging (Pyle 1988). Starting in the 1970s, 

the balsam woolly adelgid (Adelges piceae Ratz.) 

caused dieback of mature Fraser fir in the 

Upper Spruce Fir site. This major disturbance, 

in conjunction with ice storms and hurricaneinduced windthrows, led to changes in the forest 

structure by creating gaps of standing dead fir 

and fallen spruce (Moore et al. 2008, Pauley 

et al. 1996). Post-disturbance recruitment 

of understory trees contributed to increased 

productivity of these forests from 1993 to 2003 

(Moore et al. 2008, Van Miegroet et al. 2007).

• Mid- to high-elevation American beech (Fagus 

grandifolia Ehrh.). This forest type is represented 

by the IFS Beech Gap site, located at an 

elevation of 1600 m on a southerly slope, 1 km 

west of Newfound Gap. The vegetation consists 

primarily of American beech with occasional 

buckeye (Aesculus flava Aiton) and red spruce 

(Lindberg et al. 1992). The soils are Umbric 

Dystrochrepts derived from the Anakeesta 

Formation (shale parent material), which is 

sulfide-bearing and may thus lead to release 

of sulfate (SO4

2–) in soil solution/stream water 

that is not of atmospheric origin (Johnson and 

Lindberg 1992).

• Low- to mid-elevation hardwoods. This forest 

type is represented by the Mixed Hardwood 

site, which is near the National Atmospheric 

Deposition Program (NADP) station in 

Mixed Hardwood (Elkmont)

Upper Spruce-Fir 

(Becking)

Lower Spruce-Fir 

(Tower)

Beech Gap

Figure 1.—Map of site locations 

within Great Smoky Mountains 

National Park. The four sites 

represent three forest types at 

different elevations: (1) highelevation spruce-fir, (2) mid- to 

high-elevation American beech, and 

(3) lower-elevation hardwood. The 

high-elevation spruce-fir forest type 

is represented by two sites included 

in the Integrated Forest Study (IFS) 

(Johnson and Lindberg 1992): the 

Upper Spruce-Fir site (called the 

Becking site in the IFS), and the 

Lower Spruce-Fir site. The mid- to 

high-elevation American beech 

forest type is represented by the IFS 

Beech Gap site. The Mixed Hardwood 

site is near the National Atmospheric 

Deposition Program station in 

Elkmont. The inset shows the eastern 

region of the United States.
4

Elkmont (elevation 635 m) and is mapped as 

the Ditney-Unicoi soil complex, consisting 

typically of 40–50 percent moderately deep 

Ditney soils, 25–36 percent shallow Unicoi 

soils, and about 5 percent rock outcrop (Khiel 

and Thomas 2007). These soils are well drained 

and are weathered from metasedimentary 

rock such as arkose, metagraywacke, 

metasandstone, and quartzite (USDA, n.d.). 

The dominant vegetation comprises oak 

(Quercus spp.), hickory (Carya spp.), and 

yellow pine (Pinus). The Mixed Hardwood site 

has a long-term mean annual precipitation of 

162 cm.

Methods for Calculating Steady-state 

Mass-balance Critical Loads

The critical load is based on a mass balance equation—

the total amount of acidifying deposition that the 

ecosystem can tolerate must be balanced by the net 

input of neutralizing base cations in the ecosystem. 

Therefore, we calculate the sum of the base cation 

inputs (from atmospheric deposition and mineral 

weathering) and outputs (from removal of biomass and 

leaching losses of ANC) for an ecosystem to determine 

the net input of base cations per year. Sulfate and NO3−

deposition is accompanied by hydrogen ions (H+), 

which are acidifying. Per the European protocol (UBA 

2004), we assume that all ammonium (NH4

+) entering 

the ecosystem via deposition is nitrified to NO3− within 

the ecosystem. This assumption is supported by field 

data (Cai et al. 2010). As this process liberates two H+

ions, NH4

+ d eposition can be more acidifying than 

NO3− deposition (Nihlgård 1985). These mobile H+

ions can displace base cations from the soil exchange 

complex and lead to depletion of the base cation pool 

in the soil, which ultimately leads to acidification of 

soils (Reuss and Johnson 1986). This phenomenon has 

been widely observed across the Eastern United States 

(Bailey et al. 2004, 2005; Warby et al. 2005, 2009).

Critical Loads of S + N

Calculations of critical loads of S + N are based on 

a simple mass balance model described in detail 

elsewhere (Pardo 2010, UBA 2004):

CL (S + N) = BCdep –Cldep + BCw – Bcu + 

Ni + Nu + Nde – ANCle,crit [1]

where:

BCdep = sum of base cations (Ca + Mg + Na + K) 

deposition rate (eq ha-1 y-1)

Cldep = chloride deposition

BCw = soil weathering rate of base cations (Ca + Mg 

+ K + Na) (eq ha-1 y-1)

Bcu = net Ca + Mg + K uptake rate (eq ha-1 y-1) 

ultimately removed by harvest or fire

(Bc denotes that Na is not included)

Ni = acceptable net N immobilization (or 

accumulation) rate in the soil (eq ha-1 y-1)

Nu = net N removed via biomass removal (eq ha-1 y-1)

Nde = soil denitrification rate (eq ha-1 y-1)

ANCle,crit = acceptable ANC leaching rate (eq ha-1 y-1).

Equation 1 assumes that the stand is mature and at 

steady state; for the ecosystem as a whole there is no 

net change in standing biomass and therefore no net 

annual nutrient requirement. Nutrient uptake and 

storage by vegetation are accounted for only when 

vegetation biomass is removed from the ecosystem; e.g., 

by harvesting (UBA 2004). We altered equation 1 to 

reflect the net N sequestration (Nse) and net base cation 

sequestration (Bcse) for the Upper and Lower SpruceFir sites, where site disturbance (see Site Description, 

page 2) reduced the number of mature spruce trees 

in the overstory (Pauley et al. 1996, Van Miegroet et 

al. 2001) and led to significant regrowth in the gaps 

(Moore et al. 2008). These aggrading forest conditions 

cause the assumption of steady-state to be violated. 

Equation 2 takes into account the transient assimilation 

and retention of N by the ecosystem over the period 

for which the critical load was calculated (~100 years). 

Not accounting for this N sink would cause us to 

underestimate the critical load.

CL (S + N) = BCdep - Cldep + BCw – Bcu – 

Bcse + Ni + Nu + Nse + Nde – ANCle,crit [2]

Input parameters for calculating steady-state massbalance critical loads at GSMNP are described below 

and summarized in Table 2. Pardo and Duarte (2007) 

provide additional details about the methods, data 

sources, and assumptions.
5

Base Cation Deposition. Base cation deposition (BCdep) 

was calculated for Ca + Mg + Na + K deposition. Total 

BCdep of 1713 eq ha-1 y-1 for the Upper and Lower 

Spruce-Fir sites was estimated using the total 

deposition: throughfall base cation ratio from the IFS 

(0.79:1) (Johnson and Lindberg 1992) and throughfall 

BCdep of 2159 eq ha-1 y-1 collected under canopy at the 

Lower Spruce-Fir site from 1999 to 2004 (B. Robinson, 

personal communication1). Total BCdep of 860 eq ha-1

y-1 for the Beech Gap site (mid- to high-elevation) was 

estimated using year 2000 Beech Gap: Upper SpruceFir total deposition ratio (0.50) (Weathers et al. 2006 

and K. Weathers, personal communication2) and 

Spruce-Fir BCdep estimates. Total BCdep for the Mixed 

Hardwood site of 173 eq ha-1 y1 was estimated using 

annual wet deposition data from the NADP site at 

Elkmont (NADP site # TN-11) from 1999 to 2004, 

using a scaling factor of 1.4:1 for total BCdep: wet BCdep. 

The factor of 1.4 was derived from the ratio of wet + 

dry S deposition: wet S deposition reported at the 

Table 2.—Input for VSD calculations

High elevation 

Noland Divide Mid- to highelevation 

Beech Gap

Lower 

elevation Mixed 

Hardwoodb Parameters Comments a Units

Upper SpruceFirLower SpruceFir

Elevation m 1800 1740 1600 635

Forest type Fraser fir 

red spruce

Fraser fir 

red spruce

beech mixed hardwood

Time frame 1860–2150 1860–2150 1860–2150 1860–2150

Soil depth Measured m 0.46 0.57 0.74 0.83

Bulk density Measured g cm-3 0.92 0.92 1.13 1.09

Soil moisture Estimatedc M m-1 0.12 0.12 0.12 0.15

CEC Measured meq kg-1 180 293 239 132

Base saturation 

(obs)

Measured % 7.6 9 21 11

C pool (obs) Measured g m-2 5500 9500 9250 2409

C:N (obs) Measured 10 12 11 16

Q Modeled m 1.16 1.16 1.16 0.79

Weathering Modeled eq m-2 y-1

eq ha-1 y-1

0.0770

770

0.2632

2632

0.0682d

682

0.0971d

971

cRCOO Estimatede 0 0.0317 0.0317 0.0317 0.0317

lgKAlBCf VSD -0.86345 8.0427 3.3423 7.0982

lgKHBCg VSD 3.5317 6.4225 4.781 6.1211

lgKAloxh Constanti 8.77 8.77 8.77 8.77

Soil solution PCO2 Constanti 17 17 17 17

Nim_acc j Constanti eq m-2 y-1

kg ha-1 y-1

0.0036

0.5

0.0036

0.5

0.0036

0.5

0.0036

0.5

a All measured values are from IFS (Johnson and Lindberg 1992) unless otherwise noted.

b Measured values are from the Natural Resources Conservation Service.

c Based on texture (Brady 1990).

d Substrate type-clay% method.

e Estimated based on total organic carbon and average carboxyl content from Oliver et al. (1983).

f Selectivity constant for Al:Bc exchange.

g Selectivity constant for H+:Bc exchange. h Gibbsite equilibrium constant: Kgibb.

i Based on values from the literature.

j Acceptable N accumulation in soil, Ni

.

1 Bruce Robinson, Emeritus Professor, Department of Civil 

and Environmental Engineering, University of Tennessee.

2 Kathleen Weathers, Senior Scientist, Cary Institute of 

Ecosystem Studies.
6

CASTNET site at Look Rock. Cloud and fog 

contributions were not included in deposition 

estimates for this site, because their inputs at this low 

elevation are negligible.

Base Cation Weathering. Base cation weathering (BCw) 

was calculated for Ca + Mg + Na + K weathering. 

We used mineral weathering rates for the Upper and 

Lower Spruce-Fir sites as reported in the IFS (April and 

Newton 1992). We used the substrate type/clay content 

method to estimate soil mineral weathering (Ouimet 

2006, Sverdrup et al. 1990) for the Mixed Hardwood 

site, which is not an IFS site and therefore did not have 

reported weathering rates, as well as for the Beech 

Gap site, which had an anomalously low weathering 

rate reported in IFS (289 eq ha-1 y-1). Weathering rates 

shown in Table 2 were calculated using depth-weighted 

data weighted by area for all the soil series that make up 

the components of the Natural Resources Conservation 

Service GSMNP order 2 map unit ID (Khiel and 

Thomas 2007) for the sites. Mineral soil depth was 

defined as the depth from the top of the mineral soil 

(A or E horizon) through the bottom of the B horizon 

(excluding BC and Bx horizons, if present); this depth 

was selected to represent the rooting zone. Average clay 

content was calculated as the depth-weighted average 

of clay content in the mineral soil in the rooting zone 

(based on horizon-level data).

Base Cation Uptake, Nitrogen Immobilization, and 

Nitrogen Uptake. No harvesting is permitted in 

GSMNP, so base cation uptake (Bcu) and N uptake (Nu) 

through biomass removal were 0 kg ha-1 y-1. We used an 

acceptable soil N immobilization of 0.5 kg ha-1 y-1; this is 

the upper end of the range, 0.2–0.5 kg ha-1 y-1, suggested 

in the European critical loads protocol (UBA 2004).

Nitrogen and Base Cation Sequestration. We estimated 

net overstory Nu at the sites based on stand inventory 

data for the period 1993–2003 (Barker et al. 2002, 

Van Miegroet et al. 2007). First, we estimated the N 

increment in wood for 1993–2003 by taking the mean 

of the N increment reported for 1993–98 (Barker et al. 

2002) and 1998–2003. The 1998–2003 N increment was 

obtained by dividing carbon (C) increments reported 

in Van Miegroet et al. (2007) by a C:N ratio of 200. 

To calculate the net N increment, we then subtracted 

N release calculated from coarse woody debris 

decomposition reported by Van Miegroet et al. (2007), 

again assuming a C:N ratio of 200. Finally, assuming 

the aggrading period might continue for approximately 

50 years of the 100-year critical load modeling period, 

we divided the current net N increment by 2 to obtain 

an average annual net increment estimate over the 

entire 100-year period. We refer to this value as N 

sequestration (Nse). We used average Nse rates of 321 

eq ha-1 y-1 (4.5 kg ha-1 y-1) for the Upper Spruce-Fir 

site and 45 eq ha-1 y-1 (0.63 kg ha-1 y-1) for the Lower 

Spruce-Fir site. This is consistent with greater growth 

responses at the highest elevation, where disturbance 

was most pronounced (Moore et al. 2008). We used the 

Bc:N ratio of 1.75 for nutrient content in overstory bole 

wood, based on IFS data (Johnson and Lindberg 1992), 

to estimate Bcse (Ca + Mg + K sequestration) rates of 

562 eq ha-1 y-1 for the Upper Spruce-Fir site and 79 eq 

ha-1 y-1 for the Lower Spruce-Fir site. There was no 

long-term nutrient sequestration for the mid- or lowelevation hardwood sites, which were not affected by 

the balsam woolly adelgid infestation.

Acid Neutralizing Capability Leaching. The acceptable 

ANC leaching rate quantifies the loss of ANC from the 

ecosystem. The acceptable ANC leaching rate is not a 

measured value; it is set based on a critical threshold 

that is intended to prevent adverse impacts on the 

forest ecosystem. The ANCle,crit can be calculated in 

multiple ways using different chemical criteria. For 

steady-state calculations of CL (S + N), we used four 

chemical criteria: Al:Bc (Al:(Ca + Mg + K); 0.1 and 

1.0 mol mol-1), base saturation of soil (no decrease), 

Al concentration (0.2 meq L-1), and pH (4.2) in 

soil solution. Pardo and Duarte (2007) include the 

equations that use these criteria to calculate ANCle,crit. 

These chemical criteria are the most widely used in 

critical loads assessment in Europe and North America 

(Cronan and Grigal 1995, Forsius et al. 2010, NEG/

ECP 2003, Ouimet et al. 2001, Sverdrup and Warfvinge 

1993). The Al:Bc ratio in soil solution is the most 

broadly used chemical criterion; the ratio of 1.0 mol-1

is most commonly used in Europe (Reinds et al. 2008). 

We set the lower threshold of 0.1 mol mol-1 to protect 

against decreases in base saturation. The criterion of 

base saturation, with a “no decrease” threshold, should 

have a similar effect. Aluminum and pH were included 

for comparison purposes. Aluminum may be used as 

a chemical criterion, especially if there is a drinking 

water standard that can be used as a critical threshold 

for Al concentration. The soil solution pH is typically 

selected as a criterion in systems with high organic 

matter, which interacts with Al to make it less available 

and thus less reliable as a chemical criterion.
7

Critical Load of Nutrient Nitrogen

Both S and N deposition can acidify the ecosystem; 

atmospheric N deposition also represents a nutrient 

addition that is either stored in or lost from the 

ecosystem (via immobilization in the soil N pool, 

removal in biomass by harvesting, fire, or gaseous 

losses). The CLnutN is defined as the sum of the net N 

accumulation in the soil, net N removed via biomass 

removal, soil denitrification, and acceptable N leaching. 

The acceptable N leaching rate, Nle(acc), is the maximum 

acceptable leaching rate for non-N-saturated ecosystems 

(Table 3) and is given in equation 3:

Nle(acc) = Q × [N]crit [3]

where:

[N]crit = the N concentration in the soil solution 

above which would be considered detrimental to 

ecosystem or soil (see Table 3)

Q = hydrologic flux (m3 ha-1 y-1)

Under steady-state ecosystem conditions, CLnutN is 

expressed as:

CLnutN = Ni + Nu + Nde + Nle(acc) [4]

We altered equation 4 to reflect the net N sequestration 

(Nse) until the stand reaches steady state for the SpruceFir sites, leading to the following modified expression of 

CLnutN:

CLnutN = Ni + Nu + Nse + Nde + Nle [5]

We used an acceptable soil N immobilization of 0.5 

kg ha-1 y-1 ( UBA 2004). Because harvesting is not 

permitted in GSMNP, Nu is 0 kg ha-1 y-1. Denitrification 

was negligible in these upland forest stands (Wells et 

al. 1988); Nde is 0 kg ha-1 y-1. [N]crit w as set to 0.2 mg 

N L-1. Nse was determined as described above for CL 

(S + N).

Uncertainty in Critical Loads Calculations

Sources of uncertainty in these critical loads 

calculations come from measured and modeled 

parameters. The terms for deposition, soil mineral 

weathering, nutrient sequestration, and leaching—

especially the acceptable ANC leaching—all introduce 

uncertainty into these calculations (Hall et al. 2001b). 

The mineral soil weathering rate has a particularly high 

uncertainty (Hodson and Langan 1999). Hall et al. 

(2001b), Hodson and Langan (1999), and Reinds et al. 

(2008) discuss in detail the sources of uncertainty in 

critical load calculations.

Methods for Dynamic Modeling Using 

the Very Simple Dynamic Model

We used the VSD model to compare the effects 

of multiple deposition scenarios on soil solution 

NO3−, ANC, Al, Al:Bc, pH, and soil base saturation 

between 1860 and 2150. The VSD model uses mass 

balance, equilibrium, and flux equations to simulate 

soil solution chemistry over time. Required input 

parameters include soil chemical and physical 

properties, cation weathering and uptake, and 

deposition and climate data. The model uses single 

time-steps, does not include sulfate adsorption, and 

assumes that initial concentrations are in equilibrium 

with inputs. We used modeled historical deposition 

data from the Southern Appalachian Mountain 

Initiative project (Sullivan et al. 2001 and B.J. Cosby, 

personal communication3) to estimate historical 

deposition at these sites (see Pardo and Duarte 2007). 

Observed soil C pool and C:N ratio values are required 

to calibrate initial C pool and C:N ratio values; 

observed base saturation is required to calibrate the 

exchange constants for Al:Bc exchange (log K Al:Bc) 

Table 3.—Parameters used to calculate critical loads for nutrient N

Parameter NO3 ̄ Value Comments

Acceptable nitrate 

concentration

[N]le(acc) 0.2 mg N L-1 From UBA (2004)

Acceptable soil N 

immobilization

Ni 0.5 kg N ha-1 y-1 A range from 0 to 1 kg N ha-1 y-1 is typically used

Biomass N removal Nu 0 kg N ha-1 y-1 No harvesting is permitted in the park

Net N sequestration Nse 0–5 kg N ha-1 y-1 Varies with elevation

Denitrification Nde 0 kg N ha-1 y-1 Denitrification is assumed to be negligible in 

these upland forest soils

3 B. Jack Cosby, Professor, Department of Environmental 

Sciences, University of Virginia.
8

and H+:Bc exchange (log K H+:Bc). The VSD is a simple 

model, and these are the only calibrations made before 

the actual simulations were run. Posch et al. (2003) and 

Posch and Reinds (2009) describe the model fully.

In this analysis we focus on four deposition scenarios 

(Table 4) based on planned or hypothetical emissions 

control strategies; these span the range of responses 

of the 11 deposition scenarios we evaluated with the 

VSD model (Pardo and Duarte 2007). Scenario 1 

holds 1999 deposition rates constant into the future. 

Scenario 3 uses the relative reduction factors for total 

S (SO4

2

−, SO2) and total N (NO3−, NH4

+) deposition 

from the U.S. Environmental Protection Agency (U.S. 

EPA)- Clean Air Interstate Rule (CAIR) based on 36- 

km Community Multiscale Air Quality modeled runs 

using 2001 as the base deposition year and 2015 as 

the target year. Scenario 5 has 80 percent reductions 

in total S and NO3− and a 9 percent increase in NH4

+

spread evenly over 2002 to 2015. Scenario 11b uses 

scenario 3 reductions through 2015, followed by 90 

percent reductions in total S and NO3− and 80 percent 

reduction in NH4

+ to 2050. These scenarios allow us to 

model future trajectory of the ecosystems under current 

and reduced atmospheric inputs or evaluate rate of 

relative recovery.

Very Simple Dynamic Model Input Parameters

Posch and Reinds (2009) describe the required VSD 

input parameters; Table 2 summarizes these parameters 

for the GSMNP sites. Pardo and Duarte (2007) provide 

further information about applying the VSD model at 

GSMNP.

Soil Input Parameters. Base cation weathering was 

determined as described above for CL (S + N). 

Additional soil input parameters for VSD include 

depth, bulk density, moisture, cation exchange 

capacity, base saturation, carbon pool, C:N, partial 

pressure of carbon dioxide (PCO2) in soil solution, 

and organic acids concentration (cRCOO). All except 

PCO2, cRCOO, and soil moisture are depth-weighted 

means calculated from measured values at each site. 

For the Upper and Lower Spruce-Fir and Beech Gap 

sites, soil data came from the IFS study (Johnson 

and Lindberg 1992). For the Mixed Hardwoods site, 

soil data came from pedon data used in creating the 

Natural Resources Conservation Service order 2 soil 

map for GSMNP (Khiel and Thomas 2007). We used 

the relationships from Brady (1990) to estimate soil 

moisture (field capacity) based on reported soil texture; 

we did not use the measured data, because these data 

indicate soil moisture at the time of sampling and do 

not necessarily represent typical field capacity. Because 

we did not have measured PCO2 in soil solution (as a 

multiple of PCO2 [atm] in air) for any of the sites, the 

VSD default value of 17 was used. We used B-horizon 

total organic carbon (TOC) values from Noland Divide 

(0.2636815 mol C m-3; Van Miegroet, unpublished 

data4) in place of dissolved organic carbon values to 

determine the cRCOO and multiplied these values 

by the average carboxyl content (10 μeq mg-1 organic 

carbon) from Oliver et al. (1983). We selected the 

Oliver model to model the dissociation of organic 

acids.

Deposition and Climate Input Parameters for Very 

Simple Dynamic Model Calculations. Deposition and 

climate input parameters for VSD calculations include 

hydrologic flux (runoff) and atmospheric deposition 

(S, N, base cation, chloride) rates. The modeled 

hydrologic flux rate for the Upper and Lower SpruceFir and Beech Gap sites came from the IFS study (Vose 

and Swank 1992); the value for the Mixed Hardwood 

site came from a map of mean annual runoff for the 

Northeastern, Southeastern, and Mid-Atlantic United 

States (Krug et al. 1990). Deposition rates for the 

Table 4.—Deposition scenarios used for critical loads dynamic modeling in GSMNP

Scenario Total S NO3 ̄ NH4

+ Deposition reductions

1 No change No change No change Current deposition (1999–2004 mean)

3 –48% –56% +5% Deposition reductions evenly distributed from 2002 to 2015 

(U.S. EPA-CAIR)

5 –80% –80% +9% Deposition reductions evenly distributed from 2002 to 2015

11b –90% –90% –80% Scenario 3 reductions were used through 2015; the 

remainder of the deposition reductions are evenly 

distributed from 2015 to 2050

4 Helga Van Miegroet, Professor Emerita, Wildland Soils 

and Biogeochemistry, Utah State University.
9

high-elevation sites are based on mean measured wet 

deposition data collected using an Aerochemetrics 

wet-only collector in an open clearing near the Lower 

Spruce-Fir site from 1999 to 2004 (Cai et al. 2010 and 

B. Robinson, personal communication1). The volumeweighted mean annual wet deposition values were 

then scaled, as described below, to estimate total (wet 

+ dry + cloud) deposition (Table 1). Total deposition 

was measured during the IFS for the period 1986–89 

(Johnson and Lindberg 1992).

For the Upper and Lower Spruce-Fir sites, we used the 

scaling factor of 4.4:1 for total: wet N deposition based 

on the ratios of total: wet N deposition from the IFS as 

reported by Van Miegroet et al. (2001) to estimate total 

(wet + dry + cloud) N deposition. For S deposition, 

we used a scaling factor of 3.7:1 based on the ratio 

of total: wet deposition reported in the IFS (Johnson 

and Lindberg 1992). Base cation deposition at the 

Upper and Lower Spruce-Fir sites was determined as 

described above for critical loads S + N calculations. 

For the Beech Gap site, we used the ratio from the 

year 2000 of total deposition at the Beech Gap site: 

total deposition at the Upper Spruce-Fir site, 0.50 as 

reported by Weathers et al. (2006, and K. Weathers, 

personal communication2), to estimate total deposition 

of N, S, and BC. For the Mixed Hardwood site, we 

used measurements of wet deposition data (S, N, BC, 

chloride) from the NADP site at Elkmont (NADP site # 

TN-11) and estimates of dry deposition (S and N only) 

from the CASTNET site at Look Rock. We estimated 

total BCdep as described above for critical load S + N 

calculations.

Cation Exchange Input Parameters. We used the 

Gaines-Thomas relationship, which has been broadly 

used in critical loads calculations in the United States 

(Chen et al. 2004, Cosby et al. 2001, NEC/ECP 2001) 

to model the exchange between the solid phase and 

the soil solution for Al3+, protons (H+) and Bc (= 

Ca + Mg + K). Observed soil C pool and C:N ratio 

values are required to calibrate initial C pool and C:N 

ratio values; observed base saturation is required to 

calibrate the exchange constants for Al:Bc exchange 

(log K Al:Bc) and H+:Bc exchange (log K H+:Bc) (see 

Posch and Reinds 2009). The gibbsite equilibrium 

constant (logKAlox (mol2 l-2)

1-expAl) was set to 8.77 for 

all three sites, and the exponent [Al] term (gibbsite 

equilibrium) was 3, indicating a +3 charge on the Al 

ion in solution.

Nitrogen Input Parameters. Nitrogen cycling in VSD 

is also limited by minimum and maximum C:N ratios 

in soil. The VSD model uses measured C:N and the 

C pool in soil to calculate the N pool and determine 

the fraction of N that is immobilized (the balance is 

leached). Because the measured C:N ratios at these 

sites (Johnson and Lindberg 1992) were below the VSD 

minimum of 15, a default value of 15 was used, which 

underestimates the N pool but does not change the 

model function (all excess N is leached).

RESULTS

Critical Loads of Acidity and Nutrient 

Nitrogen and Exceedance

In the steady-state mass-balance calculations of 

critical loads, we compared four chemical criteria 

used to calculate ANCle, a key part of the CL (S + N)

calculation: Al concentration, Al:Bc ratio, pH in soil 

solution, and base saturation of soil. The critical 

threshold of Al:Bc = 0.1 resulted in the lowest critical 

load for acidity compared to other critical thresholds 

for most sites (Figure 2A), ranging from low of 1,252 eq 

ha-1 y-1 for the Mixed Hardwood site to a high of 3,682 

eq ha-1 y-1 for the Lower Spruce-Fir site. The chemical 

criterion of no decrease in base saturation yielded 

similar results except for the Upper Spruce-Fir site 

where the critical load for no change in base saturation 

was higher. Using the solution Al concentration critical 

threshold (0.2 meq L-1) yielded critical loads similar 

to those using the upper Al:Bc threshold (1.0), except 

at the Lower Spruce-Fir site, where it was lower. The 

critical threshold of pH = 4.2 consistently resulted in 

the highest critical loads values, from 4,965 eq ha-1 y-1

for the Mixed Hardwood site to 8,785 eq ha-1 y-1 for the 

Lower Spruce-Fir site.

Exceedance of the critical load can be used to evaluate 

the susceptibility of a site to S and N deposition 

(exceedance = actual deposition – critical load). Using 

the most conservative critical thresholds (Al:Bc = 

0.1 and no decrease in base saturation), deposition 

exceeded CL (S + N) at all the sites except Mixed 

Hardwood (Figure 2B). Using the critical thresholds 

for Al concentration and Al:Bc = 1, the deposition was 

approximately equal to the critical load at the Upper 

Spruce-Fir site; the CL (S + N) was not exceeded for the 

remaining sites. At none of the sites was the CL (S + N) 

exceeded using the critical threshold of pH = 4.2.
10

0

1000

2000

3000

4000

Upper Spruce-Fir Lower Spruce-Fir Beech Gap Mixed Hardwood

CL (eq ha-1 yr -1)

CLnutN CL (S+N)

A

-1000

0

1000

2000

3000

Upper Spruce-FirLower Spruce-Fir Beech Gap Mixed Hardwood

Exceedance (eq ha-1 yr -1)

Exceedance of CLnutN Exceedance of CL (S+N)

B

0

2000

4000

6000

8000

10000

Upper Spruce-Fir Lower Spruce-Fir Beech Gap Mixed Hardwood

CL(S+N) eq ha-1 yr -1

Al:Bc = 0.1 mol mol-1

Base saturation (no decrease)

Al = 0.2 meq/L

Al:Bc = 1.0 mol mol-1

pH = 4.2

A

-5500

-4500

-3500

-2500

-1500

-500

500

1500

2500

Upper Spruce-Fir Lower Spruce-Fir Beech Gap Mixed Hardwood

Exceedance (S+N) eq ha-1 yr -1

Al:Bc = 0.1 mol mol-1

Base saturation (no decrease)

Al = 0.2 meq/L

Al:Bc = 1.0 mol mol-1 

pH = 4.2

B

0

2000

4000

6000

8000

10000

Upper Spruce-Fir Lower Spruce-Fir Beech Gap Mixed Hardwood

CL(S+N) eq ha-1 yr -1

Al:Bc = 0.1 mol mol-1

Base saturation (no decrease)

Al = 0.2 meq/L

Al:Bc = 1.0 mol mol-1

pH = 4.2

A

-5500

-4500

-3500

-2500

-1500

-500

500

1500

2500

Upper Spruce-Fir Lower Spruce-Fir Beech Gap Mixed Hardwood

Exceedance (S+N) eq ha-1 yr -1

Al:Bc = 0.1 mol mol-1

Base saturation (no decrease)

Al = 0.2 meq/L

Al:Bc = 1.0 mol mol-1 

pH = 4.2

B

The CLnutN is typically very low at sites without 

significant timber harvesting. Indeed, for these four 

sites within GSMNP, CLnutN ranged from 200 to 500 eq 

ha-1 y-1 (2.8–7 kg N ha-1 y-1) (Figure 3A), significantly 

lower than CL (S + N), irrespective of calculation 

Figure 2B.—Exceedance by site. The exceedance is a useful tool for 

assessing the extent of risk to ecosystems under current and future 

deposition scenarios. Exceedance = actual deposition – critical load.

Figure 3B.—Exceedance CLnutN and CL (S+N) using Al:Bc = 0.1.

method. The CLnutN was slightly higher for the Upper 

Spruce-Fir site than for the other sites, driven by the 

larger disturbance-induced regrowth and Nse. In all 

cases, the CLnutN was exceeded (Figure 3B).

Figure 2A.—Critical load for (S+N) by site. Critical loads were 

calculated using four chemical criteria: Al concentration (0.2 meq L-1), 

Al:Bc ratio (0.1 and 1.0 mol mol-1), and pH (4.2) in soil solution, and 

base saturation of soil (no decrease). Using the critical thresholds 

of Al:Bc = 0.1 and no decrease in base saturation yielded the lowest 

critical load for acidity.

Figure 3A.—CLnutN and CL (S+N) using Al:Bc = 0.1.
11

Dynamic Modeling of Soil Solution 

Trends Over Time

The VSD model allows us to predict the response of 

various ecosystem parameters (soil solution ANC, 

NO3− concentration, etc.) over time to different 

deposition scenarios (Figures 4–9). The Upper SpruceFir site, with low base saturation and moderate 

weathering rate, had the lowest pH (Figure 8) and 

initial ANC (Figure 5). ANC, NO3−, and Al:Bc (Figures 

4, 5, and 7) responded dramatically to increases in 

deposition and usually fell on the detrimental side of 

critical thresholds (lower for ANC; higher for NO3−, 

and Al:Bc) under scenarios 1, 3, and 5. At the Lower 

Spruce-Fir site, with moderately low base saturation 

and a high weathering rate, most soil solution 

parameters fell on the detrimental side of thresholds 

under scenario 1 but generally remained acceptable for 

scenarios 3, 5, and 11b, except for NO3− leaching and 

the upper ANC threshold of 100 μeq L-1. Ecosystem 

parameters were generally within thresholds for 

the Beech Gap and Mixed Hardwood sites, which 

are characterized by moderate weathering and, 

respectively, high and moderate initial base saturation, 

except for NO3− leaching, the upper ANC threshold 

of 100 μeq L-1, and base saturation for scenarios 1 and 

3. Pardo and Duarte (2007) provide the results of all 

deposition scenarios for all sites and all parameters.

Nitrate Concentration and Flux

Modeled NO3− concentrations of soil solutions 

at all sites were at or near zero in 1860. Nitrate 

concentrations rose steadily in the 20th century, 

reaching a peak in the mid-1970s, with a high value 

of 210 μeq L-1 at the Upper Spruce-Fir site. Reported 

soil solution NO3− concentrations were about 80 μeq 

L-1 f o r the Lower Spruce-Fir site and 136 μeq L-1 for 

the Upper Spruce-Fir site (Van Miegroet et al. 1992). 

At all sites for scenarios 1, 3, and 5, the modeled NO3−

concentration in soil solution exceeded the critical 

NO3− concentration threshold, [N]crit, of 0.2 mg N L-1

(14 μeq L-1). Deposition reductions of 90 percent for 

SO4

2

− and NO3− and reductions of 80 percent for NH4

+

(scenario 11b) reduced modeled NO3− concentration 

below the critical NO3− concentration (Figure 4) for 

the Beech Gap, Mixed Hardwood, and Upper SpruceFir sites, but not for the Lower Spruce-Fir site.

Figure 4.—VSD model results for soil solution NO3 ̄ for each of the four sites: Upper Spruce-Fir, Lower Spruce-Fir, Beech Gap, and Mixed Hardwood. 

Scenario 1 = no deposition change; scenario 3 = –48 percent S, –56 percent NO3 ̄, +5 percent NH4

+ deposition change; scenario 5 = –80 percent S, 

–80 percent NO3 ̄, +9 percent NH4

+ deposition change; scenario 11b = –90 percent S, –90 percent NO3 ̄, –80 percent NH4+ deposition change.

0

40

80

120

160

200

1850 1900 1950 2000 2050 2100 2150

Soil Solution NO3

- (μeq/L)

Year

Upper Spruce-Fir

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold Measured NO3

0

40

80

120

160

200

1850 1900 1950 2000 2050 2100 2150

Soil Solution NO3

- (μeq/L)

Year

Lower Spruce-Fir

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold Measured NO3

0

40

80

120

160

200

1850 1900 1950 2000 2050 2100 2150

Soil Solution NO3

- (μeq/L)

Year

Beech Gap

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold

0

40

80

120

160

200

1850 1900 1950 2000 2050 2100 2150

Soil Solution NO3

- (μeq/L)

Year

Mixed Hardwood

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold
12

Acid Neutralizing Capability

There are several critical thresholds for ANC in soil 

solution. The most conservative is ANC = 100 μeq 

L-1, which minimizes the risk of any soil solution 

acidification; the least conservative is ANC = 0 μeq L-1, 

which is the minimum ANC value that would protect 

against chronic acidification. Because of concerns 

about episodic stream acidification, selecting a higher 

critical ANC value of 20 or 50 μeq L-1 is prudent 

(Driscoll et al. 2001). Initial modeled ANC values 

varied widely by site (Figure 5). The modeled soil 

solution ANC value at the Upper Spruce-Fir site was 

–25 μeq L-1 in 1860; Lower Spruce-Fir ANC was greater 

than 100 μeq L-1. ANC values at all sites decreased in 

the 20th century, remaining below 0 μeq L-1 ANC at 

the Upper Spruce-Fir site under all scenarios, although 

ANC returned to preindustrial levels under scenario 

11b. At the Lower Spruce-Fir site, scenarios 3, 5, and 

11b resulted in modeled ANC of at least 50 μeq L-1 in 

2150; scenario 1 (no change in deposition) resulted in 

a modeled ANC close to –100 μeq L-1. At the Beech 

Gap and Mixed Hardwood sites, ANC values remained 

above 0 μeq L-1 for all scenarios. Scenarios 5 and 11b 

resulted in increasing ANC over time; scenarios 1 and 

3 resulted in continued decreases in ANC over time.

Aluminum Concentration

Modeled soil solution Al values were below the critical 

threshold of 200 μeq L-1 (0.2 meq L-1) at all sites in the 

year 2015 (Figure 6) and increased with time under 

scenario 1; however, these values approached the 

critical threshold only at the Upper Spruce-Fir site. At 

the Lower Spruce-Fir site, modeled Al concentrations 

increased dramatically but remained below critical 

thresholds with deposition scenario 1. All other 

deposition reduction scenarios resulted in decreases 

in modeled Al concentration across sites from a peak 

in the 1970s; modeled Al concentrations in beech and 

mixed hardwood sites all approach zero in 2150.

Figure 5.—VSD model results for ANC for each of the four sites: Upper Spruce-Fir, Lower Spruce-Fir, Beech Gap, and Mixed Hardwood. Scenario 1 = 

no deposition change; scenario 3 = –48 percent S, –56 percent NO3 ̄, +5 percent NH4

+ deposition change; scenario 5 = –80 percent S, –80 percent 

NO3 ̄, +9 percent NH4

+ deposition change; scenario 11b = –90 percent S, –90 percent NO3 ̄, –80 percent NH4+ deposition change.

-350

-300

-250

-200

-150

-100

-50

0

50

100

150

1850 1900 1950 2000 2050 2100 2150

Soil Solution ANC (μeq/L)

Year

Upper Spruce-Fir

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold Measured ANC

-350

-300

-250

-200

-150

-100

-50

0

50

100

150

1850 1900 1950 2000 2050 2100 2150

Soil Solution ANC (μeq/L)

Year

Lower Spruce-Fir

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold Measured ANC

0

10

20

30

40

50

60

70

80

90

100

1850 1900 1950 2000 2050 2100 2150

Soil Solution ANC (μeq/L)

Year

Beech Gap

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold

0

10

20

30

40

50

60

70

80

90

100

1850 1900 1950 2000 2050 2100 2150

Soil Solution ANC (μeq/L)

Year

Mixed Hardwood

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critcal threshold
13

Figure 6.—VSD model results for soil solution Al for each of the four sites: Upper Spruce-Fir, Lower Spruce-Fir, Beech Gap, and Mixed Hardwood. 

Scenario 1 = no deposition change; scenario 3 = –48 percent S, –56 percent NO3 ̄, +5 percent NH4

+ deposition change; scenario 5 = –80 percent 

S, –80 percent NO3 ̄, +9 percent NH4

+ deposition change; scenario 11b = –90 percent S, –90 percent NO3 ̄, –80 percent NH4+ deposition change. 

Soil solution Al for Lower Spruce-Fir scenario 1 reaches 0.065 μeq L-1 by 2150.

Aluminum:Base Cation Values

At all sites except the Upper Spruce-Fir Site, Al:Bc 

ratios throughout the modeling run were below the 

critical thresholds of both 0.1 mol mol-1 and 1.0 mol 

mol-1 for deposition reduction scenarios; the Al:Bc 

ratio increased dramatically at the Lower SpruceFir site under scenario 1 and exceeded the critical 

threshold of 0.1 mol mol-1 (Figure 7). At the Upper 

Spruce-Fir site, modeled Al:Bc values in 1860 exceeded 

the critical threshold of 1.0 mol mol-1. At the Upper 

Spruce-Fir site, Al:Bc values decreased over time with 

all deposition reduction scenarios and resulted in an 

Al:Bc ratio below the critical threshold of 0.1 mol mol-1

under scenario 11b reductions.

pH

The modeled soil solution pH values were higher 

than the critical pH threshold of 4.2 at all sites for 

all deposition scenarios (Figure 8), and modeled soil 

solution pH increased with deposition reductions. 

Modeled pH values at the Lower Spruce-Fir site 

decreased significantly under scenario 1.

Base Saturation

Soil base saturation decreased over time under scenario 

1 at all sites (Figure 9). At the Upper Spruce-Fir site, 

base saturation increased with all deposition reduction 

scenarios. At the Lower Spruce-Fir, Beech Gap, and 

Mixed Hardwood sites, base saturation decreased 

slightly under scenario 3 and increased slightly under 

scenarios 5 and 11b.

0

50

100

150

200

250

300

1850 1900 1950 2000 2050 2100 2150

Soil Solution Al (μeq/L)

Year

Lower Spruce-Fir

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1850 1900 1950 2000 2050 2100 2150

Soil Solution Al (μeq/L)

Year

Beech Gap

Dep scenario 1 Dep scenario 3 Dep scenario 5 Dep scenario 11b

0.00

0.02

0.04

0.06

0.08

0.10

0.12

1850 1900 1950 2000 2050 2100 2150

Soil Solution Al (μeq/L)

Year

Mixed Hardwood

Dep scenario 1 Dep scenario 3 Dep scenario 5 Dep scenario 11b

0

50

100

150

200

250

300

1850 1900 1950 2000 2050 2100 2150

Soil Solution Al (μeq/L)

Year

Upper Spruce-Fir

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold
14

Figure 7.—VSD model results for Al:Bc for each of the four sites: Upper Spruce-Fir, Lower Spruce-Fir, Beech Gap, and Mixed Hardwood. Scenario 1 = 

no deposition change; scenario 3 = -48 percent S, –56 percent NO3 ̄, +5 percent NH4

+ deposition change; scenario 5 = –80 percent S, –80 percent 

NO3 ̄, +9 percent NH4

+ deposition change; scenario 11b = –90 percent S, –90 percent NO3 ̄, –80 percent NH4+ deposition change.

Figure 8.—VSD model results for pH for each of the four sites: Upper Spruce-Fir, Lower Spruce-Fir, Beech Gap, and Mixed Hardwood. Scenario 1 = 

no deposition change; scenario 3 = –48 percent S, –56 percent NO3 ̄, +5 percent NH4

+ deposition change; scenario 5 = –80 percent S, –80 percent 

NO3 ̄, +9 percent NH4

+ deposition change; scenario 11b = –90 percent S, –90 percent NO3 ̄, –80 percent NH4+ deposition change.

4

4.5

5

5.5

6

1850 1900 1950 2000 2050 2100 2150

Soil solution pH

Year

Mixed Hardwood

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold

4

4.5

5

5.5

6

1850 1900 1950 2000 2050 2100 2150

Soil solution pH

Year

Beech Gap

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold

4

4.5

5

5.5

6

1850 1900 1950 2000 2050 2100 2150

Soil Solution pH

Year

Upper Spruce-Fir

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold Measured pH

4

4.5

5

5.5

6

1850 1900 1950 2000 2050 2100 2150

Soil solution pH

Year

Lower Spruce-Fir

Dep Scenario 1 Dep Scenario 3 Dep Scenario 5

Dep Scenario 11b Critical threshold Measured pH

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

1850 1900 1950 2000 2050 2100 2150

Soil solution Al:Bc

Year

Upper Spruce-Fir

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

1850 1900 1950 2000 2050 2100 2150

Soil solution Al:Bc

Year

Lower Spruce-Fir

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold

0.000

0.001

0.002

0.003

0.004

0.005

1850 1900 1950 2000 2050 2100 2150

Soil solution Al:Bc

Year

Beech Gap

Dep scenario 1 Dep scenario 3 Dep scenario 5 Dep scenario 11b

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005

1850 1900 1950 2000 2050 2100 2150

Soil solution Al:Bc

Year

Mixed Hardwood

Dep scenario 1 Dep scenario 3 Dep scenario 5 Dep scenario 11b
15

DISCUSSION

The critical loads calculated for these sites fall within 

the range reported for mountainous areas in the 

Northeastern United States. (Duarte et al. 2004, 2011, 

2013; NEG/ECP 2003). Miller (2005) reported that 

critical loads of acidity in New Hampshire and Vermont 

ranged from <250 eq ha-1 y-1 to >2500 eq ha-1 y-1; 

exceedances in New Hampshire and Vermont were 

250–2000 eq ha-1 y-1. In a study in Eastern Canada 

(Environment Canada 2004), Ouimet et al. (2006) 

reported mean critical loads of 519–2063 eq ha-1 y-1 

by province. Mean exceedance by province was 0–700 

eq ha-1 y-1 based on protecting 95 percent of forest 

area. Earlier assessments in this region suggest that 

the critical load for acidity has been exceeded for 

terrestrial and aquatic ecosystems (Fox et al. 1989; 

Sullivan and Cosby 2002, 2004; Sullivan et al. 2003). In 

an assessment of 66 streams in the Southern Blue Ridge 

province adjacent to GSMNP, most would not achieve 

an ANC >50–100 μeq L-1 over the long term (2100) 

(Sullivan et al. 2011a). Most streams could tolerate acid 

inputs equivalent to only 0–10 kg S ha-1 y-1 (0–624 eq 

ha-1 y-1) over the long term. Similarly, in their dynamic 

modeling assessments for streams of GSMNP using 

the PnET-BGC model, Zhou et al. (2015) found that 

the high-elevation Noland Divide Watershed, located 

in the Spruce-Fir site, could not attain ANC >0 under 

any future deposition scenario. Their target load for 

S + N of 400–1300 eq ha-1 y-1 (Zhou et al. 2015) falls 

within the range of critical load values we obtained for 

the Upper Spruce-Fir and Lower Spruce-Fir sites (Figure 

3), consistent with the Noland Divide Watershed 

straddling these two elevation bands. McDonnell et al. 

(2014) report extensive exceedance of the critical load 

for S deposition in GSMNP using a critical threshold 

of ANC = 50 μeq L-1; they report critical loads of 0–750 

eq ha-1 y-1. Oja and Arp (1998) reported CL (S + N) of 

593–922 eq ha-1 y-1 for the Beech Gap and the Upper 

and Lower Spruce-Fir sites (based on IFS data). They 

report CLnutN of 178–614 eq ha-1 y-1 for the Beech Gap 

and Upper and Lower Spruce-Fir sites. These estimates 

are somewhat lower than the values we report for CL 

(S + N) and very similar to the range that we report 

Figure 9.—VSD model results for base saturation for each of the four sites: Upper Spruce-Fir, Lower Spruce-Fir, Beech Gap, and Mixed Hardwood. 

Scenario 1 = no deposition change; scenario 3 = –48 percent S, –56 percent NO3 ̄, +5 percent NH4

+ deposition change; scenario 5 = –80 percent S, 

–80 percent NO3 ̄, +9 percent NH4

+ deposition change; scenario 11b = –90 percent S, –90 percent NO3 ̄, -80 percent NH4+ deposition change.

0%

5%

10%

15%

20%

25%

30%

1850 1900 1950 2000 2050 2100 2150

Soil Base Saturation

Year

Upper Spruce-Fir

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold

0%

5%

10%

15%

20%

25%

30%

1850 1900 1950 2000 2050 2100 2150

Soil Base Saturation

Year

Lower Spruce-Fir

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold

0%

5%

10%

15%

20%

25%

30%

1850 1900 1950 2000 2050 2100 2150

Soil Base Saturation

Year

Beech Gap

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold

0%

5%

10%

15%

20%

25%

30%

1850 1900 1950 2000 2050 2100 2150

Soil Base Saturation

Year

Mixed Hardwood

Dep scenario 1 Dep scenario 3 Dep scenario 5

Dep scenario 11b Critical threshold
16

for CLnutN. Empirical critical loads for nutrient N for 

forest ecosystems in the Eastern Forest ecoregion, which 

includes GSMNP, are > 3–8 kg ha-1 y-1 (200–600 eq ha-1

y-1) N (Gilliam et al. 2011, Pardo et al. 2011a), which 

are similar to the range we calculated in this study. 

The consequences of this acidification of soil and surface 

water include documented adverse effects on fish and 

macroinvertebrate assemblages (Baldigo et al. 2018).

Critical Loads and Exceedances for 

Acidity (S + N)

The net input of base cations (BCdep + BCw – Bcu) 

is a measure of base cation availability within the 

ecosystem. Because Bcu is zero when no biomass is 

removed by harvesting, as is the case in GSMNP, the 

biggest drivers of the CL (S + N) were the BCw and the 

BCdep, and their relative importance differed by site. 

(The base cation sequestration [Bcse] term, used to 

account for the non-steady-state aggrading period in 

the Lower and Upper Spruce-Fir sites, is considerably 

smaller than BCdep). In some cases, the soil mineral 

weathering was significantly greater than the BCdep

inputs (Lower Spruce-Fir and Mixed Hardwood). At 

the Upper Spruce-Fir site, the BCdep was greater than 

the mineral soil weathering; at the Beech Gap site, they 

were of similar magnitude. The site with the lowest net 

base cation inputs had the lowest critical load (Mixed 

Hardwoods; using the chemical criteria Al:Bc = 0.1). As 

the net base cation input increased across sites so did 

the critical load.

Critical load and exceedance values varied greatly 

with chemical criteria. None of the critical loads using 

the criteria of Al = 0.2 meq L-1, pH = 4.2, and Al:Bc = 

1.0 were exceeded at any site in GSMNP, whereas the 

critical load was exceeded for most sites with Al:Bc = 

0.1—except Mixed Hardwoods—and for all sites with 

criteria of no change in base saturation. These results 

demonstrate the importance of selecting chemical 

criteria in modeling critical loads. Hall et al. (2001a) 

used various chemical criteria and critical thresholds 

to evaluate critical loads in the United Kingdom; their 

work and others (see Reinds et al. 2008, Watmough 

and Dillon 2003) highlight the need to choose the 

appropriate criteria and threshold to protect sensitive 

elements of a given ecosystem.

Because this study evaluates effects over a long 

period, we are in the unusual position of being able to 

comment on which critical load is best correlated with 

the forest condition. When the critical load is exceeded, 

detrimental effects may not yet be observable, because 

critical loads represent the long-term response of 

the ecosystem, and there may be a time lag before 

detrimental ecological effects are observable. In 

contrast, if the acid deposition is the cause of forest 

damage, then if one observes forest damage, there 

should be exceedance of the critical load. Hence, 

when measured values fall on the detrimental side of 

a critical threshold (higher for NO3−, Al, Al:Bc, and 

lower for pH, ANC, and base saturation), this indicates 

that the ecosystem is experiencing a harmful ecological 

effect and the critical load is exceeded. For the Upper 

Spruce-Fir and Lower Spruce-Fir sites, measured values 

fell on the detrimental side of critical thresholds of 

soil solution NO3−, ANC, and pH. These detrimental 

conditions indicate that the critical load is exceeded. 

The chemical criteria of Al = 0.2 meq L-1, pH = 4.2, and 

Al:Bc = 1.0 did not lead to exceedance of the critical 

load. However, the GSMNP ecosystem was damaged 

during the modeled period; therefore, these chemical 

criteria may not adequately protect the ecosystem 

from excess acidic deposition. The chemical criteria 

of no decrease in base saturation and Al:Bc = 0.1 may 

better protect sensitive elements of the ecosystem at the 

Upper Spruce-Fir and Lower Spruce-Fir sites.

Critical Loads and Exceedances for 

Nitrogen

As is often observed (Duarte al. 2011, 2013), the CLnutN 

is a small fraction of the CL (S + N), even when we use 

the most conservative critical threshold for CL (S + N) 

(Al:Bc = 0.1). The critical load function including 

the CLnutN (Figure 10) indicates which combinations 

of N and S deposition will protect an ecosystem 

from detrimental effects of both acidification and N 

saturation. The N deposition should not be greater 

than the CLnutN (Figure 10, dashed vertical lines), 

and the S deposition should be lower than the critical 

load function line at the point where the CLnutN line 

intersects it (Figure 10).

The standard critical loads protocol (UBA 2004) 

suggests a range of 0–1 kg ha-1 y-1 (0–71 eq ha-1 y-1) 

N for the acceptable soil N accumulation term (Ni); 

however, no consensus has yet been reached on longterm sustainable rates of soil N immobilization. We 

used the value of 0.5 kg ha-1 y-1 (36 eq ha-1 y-1), which 

is widely used for temperate forests (UBA 2004). Given 

the low values typically reported for CLnutN (in this 
17

study 2.8–7 kg ha-1 y-1, or 200–500 eq ha-1 y-1), there 

is some concern that the critical thresholds used in 

calculating the CLnutN are too low. Gundersen (1992) 

suggested a range for acceptable N leaching loss 

(Nle(acc)) for old growth stands of 4–5 kg ha-1 y-1 (290–

360 eq ha-1 y-1); the method we used gave us a value of 

about 3 kg ha-1 y-1 (210 eq ha-1 y-1). If we were to add 

0.5 kg ha-1 y-1 for additional Ni and 2 kg ha-1 y-1 (140 eq 

ha-1 y-1) for additional Nle(acc) (to reach the maximum 

acceptable leaching loss of 5 kg ha-1 y-1), we would 

calculate the highest possible CLnutN. Adding a total 

of about 2.5 kg ha-1 y-1 (180 eq ha-1 y-1) of allowable 

inputs would increase the CLnutN to 5.6–10 kg ha-1 y-1

(400–700 eq ha-1 y-1). In all cases, the CLnutN would 

still be exceeded and would be significantly lower than 

the CL (S + N), even if the CLnutN were increased by 

200 eq ha-1 y-1.

The critical nitrate concentration, [N]crit, is also a term 

that could be refined for the Eastern United States 

once data linking NO3− concentration with ecosystem 

responses are available. De Vries et al. (2007) used 

such data in Europe to refine [N]crit, based on observed 

detrimental responses by specific species.

Trends in Modeled Soil Solution 

Over Time

At all sites, deposition reductions improved the quality 

of soil solution: pH increased, and Al:Bc ratio, Al, and 

NO3− concentrations decreased (Table 5). In some 

cases, these improvements included crossing a critical 

threshold (e.g., a defined Al:Bc ratio). In other cases, 

for example, Al concentration and pH, the values 

were already on the “healthy” side of the threshold 

(above for pH, below for Al concentration), so further 

deposition reductions would provide additional 

protection against detrimental effects of acid deposition 

including those associated with episodic acidification. 

Similarly, any increase in base saturation protects 

against any future acidification associated with the 

removal of base cations. VSD does not model responses 

to episodic acidification, because critical loads are 

calculated over the long term and the VSD operates on 

an annual time step.

Reductions required to lower total (N + S) deposition 

below the critical load for Al:Bc = 0.1 mol mol-1 are 14 

percent at the Lower Spruce-Fir site, 23 percent at the 

0

16

32

48

64

14 28 42

0

1,000

2,000

3,000

4,000

500 1,000 2,500 3,000 3,500

S deposition (kg ha-1 yr -1)

N deposition (kg ha-1 yr-1)

S deposition (eq ha-1 yr -1)

1,500 2,000

Upper Spruce-Fir Upper Spruce-Fir CLnutN Upper Spruce-Fir deposition Lower Spruce-Fir

Lower Spruce-Fir CLnutN Lower Spruce-Fir deposition Beech Gap Beech Gap CLnutN

Beech Gap deposition Mixed Hardwood Mixed Hardwood CLnutN Mixed Hardwood deposition

N deposition (eq ha-1 yr-1)

Figure 10.—Critical load functions using the chemical criteria Al:Bc = 0.1 mol mol-1 with CLnutN marked. The critical load function is shown as 

a solid line for each site. The CLnutN is shown for each site with a dotted line. Deposition is shown with a filled circle; deposition for the Upper 

and Lower Spruce-Fir sites is the same. The N deposition should not exceed the CLnutN, and the S deposition should be lower than the critical 

load function line.
18

Beech Gap site, and 53 percent at the Upper Spruce-Fir 

site (Pardo and Duarte 2007). The U.S. EPA-CAIR 

deposition reduction level (scenario 3) represents 

reductions of approximately 40 percent of total N + S 

deposition, inadequate to reduce deposition below the 

critical load at the Upper Spruce-Fir site. This scenario 

would reduce the deposition below the critical load at 

the other three sites. Historical modeling shows that 

ANC values (Figure 5) and Al:Bc (Figure 7) at the 

Upper Spruce-Fir site fell on the detrimental side of the 

critical thresholds before anthropogenic deposition 

increased. Zhou et al. (2015) support this low ANC 

value at the Upper Spruce-Fir site. They report a 

modeled initial stream water ANC value of 1860 for the 

Nolan Divide Watershed of 28 μeq L-1, which falls 

between the values we report here of –25 μeq L-1 for 

Upper Spruce-Fir site and 100 μeq L-1 for the Lower 

Spruce-Fir site and is consistent with Nolan Divide 

Watershed draining both forest types. Historical site 

conditions should, if available, be considered when 

determining critical thresholds in critical loads 

modeling.

The VSD model does not include SO4

2

− adsorption, 

which introduces uncertainty about the timing of 

SO4

2

− losses: there is a time lag during periods of 

SO4

2

− adsorption and accelerated losses during periods 

of desorption. The net SO4

2

− retention observed at 

Noland Divide Watershed in GSMNP (Cai et al. 2010, 

2011a, 2011b; Nodvin et al. 1995) suggests that soil 

SO4

2

− capacity at this site was relatively low in the 

1980s (Harrison et al. 1989). Rapid movement of SO4

2

−

through the soil profile also suggests limited SO4

2

−

adsorption in the upper part of the soil profile (Cai 

et al. 2010). Recent soil column and field lysimeters 

studies (Cai et al. 2011a) suggest that although SO4

2

−

adsorption is considerable under current deposition 

(scenario 1), deposition reductions would trigger 

desorption of SO4

2

−. For streams in the Southern 

Blue Ridge province adjacent to GSMNP, Sullivan et 

al. (2011b) assumed reduced SO4

2– adsorption and 

increased mobilization of SO4

2

− from soils to streams. 

We do not include the dynamics of SO4

2

− adsorption 

and desorption in our steady-state critical load 

calculations, considering that over the long term, under 

steady-state conditions there should be no net storage 

of SO4

2

− on the soil exchanger. However, if adsorbed 

sulfate remains on the soil exchange complex over the 

long term, the acceptable amount of acid deposition 

would increase. In that case, the critical load that we 

report may be too low.

Table 5.—Summary of soil solution responses to deposition scenario modeling to 2150a

Site

Deposition

scenario

NO3 ̄

(μeq L-1)

ANC

(μeq L-1)

Al

(μeq L-1)

Al:Bc 

(mol mol-1) pH BS %

Upper

SpruceFir

1 +/- - + + +/- -

3 +/- + - - + +

5 +/- + - - + +

11b +/- + - - + +

Lower

SpruceFir

1 + - + + - -

3 + +/- - +/- +/- +/-

5 + + - - + +

11b +/- + - - + +

Beech 1 +/- - + + - -

Gap 3 +/- - - + - -

5 +/- + - - + +/-

11b +/- + - - + +

Mixed 1 + - + + - -

Hardwood 3 + - - + - -

5 + + - +/- +/- +/-

11b +/- + - - + +/- a Response symbols indicate increasing (+), decreasing (-), and stable (+/–) values. Shaded cells exceed critical 

thresholds. Critical thresholds: 14 μeq L-1 NO3 ̄, 50 μeq L-1 ANC, 200 μeq L-1 Al (0.2 meq L-1), 0.1 Al:Bc mol mol-1, 

pH 4.2, no decrease in base saturation.
19

Trends in Modeled Soil Solution Nitrate 

Over Time

Elevated NO3− concentration in soil solution is an 

indication of both N saturation and acidification. High 

NO3− leaching suggests a disruption of the internal N 

cycle, which is an early step in the progression toward 

N saturation (Aber et al. 1989, Stoddard 1994). The 

CLnutN is exceeded in all cases (Figure 3B), so it is not 

surprising that modeled NO3− concentrations in soil 

solution were greater than the critical threshold even 

when the CL (S + N) was not exceeded (Figure 3B).

Both the observed mean annual volume-weighted soil 

solution NO3− for the period 1985–88 (Van Miegroet 

et al. 1992) and for 1991–2006 at the Upper Spruce-Fir 

site (97–124 μeq L-1) (Cai et al. 2010) are well above the 

critical threshold but lower than the modeled value. 

Because the model is a simplified one, especially with 

respect to N cycling and includes certain assumptions 

of equilibrium, there may be a time lag before modeled 

conditions occur in the ecosystem. Nonetheless, a 

considerable body of evidence suggests that the upper 

sites are indeed N saturated, in spite of the increased 

post-disturbance N uptake in the Upper Spruce-Fir 

site and the high N uptake in understory vegetation 

including regeneration (Barker et al. 2002; Moore et al. 

2007, 2008). Evidence of N saturation at the upper sites 

includes low soil C:N ratio, high nitrification rates, and 

long-term elevated stream water NO3− concentrations 

(Cai et al. 2012, Garten 2000, Johnson and Lindberg 

1992, Nodvin et al. 1995, Van Miegroet et al. 2001).

Deposition reductions of 90 percent for NO3− and 

60 percent for NH4

+ at the Upper Spruce-Fir site, 80 

percent for NH4

+ at the Beech Gap site, and 40 percent 

for NH4

+ at the Mixed Hardwood site are necessary to 

lower the total N deposition below the CLnutN (Pardo 

and Duarte 2007). None of the deposition reduction 

scenarios used in our model simulation lower the total 

N deposition below the CLnutN at the Lower Spruce-Fir 

site. The most stringent reduction scenario (11b) results 

in a reduction of total N deposition (NO3− + NH4

+) of 

~86 percent, short of the 89 percent total N deposition 

(NO3− + NH4

+) necessary. The Upper Spruce-Fir site 

can tolerate a higher level of N deposition because this 

site has been aggrading more rapidly, since the woolly 

adelgid infestation, than the Lower Spruce-Fir site 

(Barker et al. 2002, Van Miegroet et al. 2007).

CONCLUSIONS

By calculating critical loads and exceedances for four 

sites within the GSMNP and using different critical 

thresholds, we were able to evaluate which threshold 

is most likely to provide accurate results. The Al:Bc = 

0.1 mol mol-1 and no decrease in base saturation gave 

the most accurate assessment of the critical load based 

on long-term observation of site conditions. In spite 

of decreases in S emissions and deposition over the 

last few decades, recovery of these ecosystems would 

require very stringent reductions in deposition. The N 

deposition reductions necessary to lower deposition 

below the critical load for nutrient N range from 68 

percent to 90 percent of total N deposition; reductions 

required to lower total (N + S) deposition below the 

critical load range from 14 percent to 53 percent. These 

reductions would not, however, lower soil solution 

concentrations to below the critical thresholds before 

2150. The CAIR (U.S. EPA, n.d.) deposition reduction 

scenario represents reductions of only approximately 

40 percent of N + S and would therefore not be 

adequate to reduce deposition below the critical load at 

the most sensitive site. Continued emissions reductions 

associated with attaining National Ambient Air Quality 

Standards or visibility improvements in federally 

mandated class 1 areas may eliminate exceedance of 

critical loads at GSMNP.

Effects of elevated S and N deposition do not occur 

in isolation from other environmental stressors. 

Climate change is the primary environmental stressor, 

in many cases, and when coupled with disturbance 

(including pests) and elevated S and N deposition, 

may significantly aggravate detrimental effects on 

ecosystems (McDonnell et al. 2018; Porter et al. 2012, 

Wu and Driscoll 2010). We are not able to evaluate the 

extent to which susceptibility of the high-elevation 

Spruce-Fir sites to the balsam woolly adelgid may 

have increased because of the historical elevated N 

deposition. Certainly, numerous studies have reported 

linkages between increased foliar N and increased 

infestation of various pests (Pontius et al. 2006), 

including the balsam woolly adelgid (Carrow and 

Betts 1972). Similarly, a recent assessment by Porter 

et al. (2012) indicated that interactions between 

climate change and N deposition can have significant 

deleterious effects on biodiversity. Wu and Driscoll 

(2010) report that incorporating climate change—

increasing temperature, precipitation, carbon dioxide, 

and water use efficiency—into a dynamic model, 
20

PnEt-BGC, lowered the CL (S + N). These preliminary 

studies clearly illustrate the need for further 

investigation into the interaction of S and N deposition 

and other stressors to protect ecosystems in the future.

ACKNOWLEDGMENTS

The authors are grateful to Kathie Weathers (Cary 

Institute of Ecosystems Studies) for the information 

about atmospheric deposition in the Great Smoky 

Mountains National Park (GSMNP) and to Jack Cosby 

(University of Virginia) for the information about 

historical deposition. This manuscript would not have 

been possible without the data and interpretations 

provided by Mike Jenkins (GSMNP), Dale Johnson 

(University of Nevada), Steve Moore (GSMNP), Patrick 

Mulholland (Oak Ridge National Laboratory), Jim 

Renfro (GSMNP), and Bruce Robinson (University 

of Tennessee). We thank Bethel Steele (Cary Institute 

of Ecosystems Studies) for providing deposition data. 

Finally, we thank the members of our working group: 

Tamara Blett and Ellen Porter (National Park Service) 

and Jim Renfro (GSMNP). The Tennessee Valley 

Authority and the National Park Service provided 

funding for this project.

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and pH = 4.2. With deposition reductions of 48 percent S and 56 percent nitrate (NO3 ̄), neither the 

critical thresholds of acid neutralizing capacity (ANC) = 20 μeq L-1 nor a decrease in base saturation 

was achieved. With deposition reductions of 90 percent S and 90 percent NO3 ̄, ANC = 100 μeq L-1 was 

achieved at a single site. Historical ANC values affected a site’s ability to achieve ANC critical thresholds. 

The critical threshold for soil solution NO3 ̄ was exceeded for all but the most stringent deposition 

scenarios (–90 percent S and –90 percent NO3 ̄). Deposition reductions of 90 percent for NO3 ̄ and 80 

percent for ammonium (NH4

+) were not sufficient to lower deposition below the CLnutN at all sites. 

Data indicate that upper sites at GSMNP are N saturated; to protect these sites from acidification, the 

more protective chemical criteria of no decrease in base saturation and Al:Bc = 0.1 should be used 

when determining critical loads. When choosing chemical criteria in deposition reduction modeling, 

care should be taken to ensure that the criteria chosen will protect sensitive ecosystem elements.

KEY WORDS: critical load, acidic deposition, exceedance, nitrate leaching, nitrogen saturation

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