United States

Department of

Agriculture

Forest Service

Southern

Research Station

General Technical

Report SRS-46

Existing Soil Carbon Models Do

Not Apply to Forested Wetlands

C.C. Trettin, B. Song, M.F. Jurgensen, and C. Li
The Authors

C.C. Trettin, Project Leader, USDA Forest Service, Southern Research Station,

Charleston, SC; B. Song, Assistant Professor, Belle W. Baruch Institute of Coastal

Ecology and Forest Science, Clemson University, Georgetown, SC; M.F.

Jurgensen, Professor, Michigan Technological University, School of Forestry and

Wood Products, Houghton, MI; and C. Li, Research Professor, University of New

Hampshire, Complex Systems Research Center, Durham, NE, respectively.

Cover: Bottomland hardwoods along the Edisto River ACE Basin, South Carolina.

Photo by Bill Lea

September 2001

Southern Research Station

P.O. Box 2680

Asheville, NC 28802
1

Abstract

When assessing the biological, geological, and chemical cycling of

nutrients and elements—or when assessing carbon dynamics with respect

to global change—modeling and simulation are necessary. Although

wetlands occupy a relatively small proportion of Earth’s terrestrial surface

(< 3 percent), they contain a disproportionate share of the terrestrial carbon

pool (15 to 22 percent). Models that do not accurately represent wetland

soil processes cannot, therefore, provide reasonable simulations. We

evaluated 12 widely used soil C models to determine their applicability to

wetland ecosystems: CANDY, CENTURY, DAISY, DNDC, ITE, MBLGEM, NCSOIL, QSOIL, ROTHC, SOMM, VVV, and WMEM. Only three

(CENTURY, DNDC, and WMEM) allow for anaerobic conditions; none

contains components for anoxia, ground water hydrology, multiple organic

and physical soil layers, or a daily time-step, all of which are necessary

when modeling soil C in wet soils. Accordingly for any land area that

includes wetlands, none of the individual models would produce

reasonable simulations based on soil processes. We present a wetland soil

C model framework based on desired attributes, the DNDC model, and

components of the CENTURY and WMEM models. Our proposed

synthesis would be appropriate when considering soil C dynamics at

multiple spatial scales and where the land area considered includes both

wetland and upland ecosystems.

Keywords: Carbon, model, organic matter, soil, wetland.

Introduction

Anaerobic conditions in wetland soils alter the rate of

organic matter decomposition, often yielding soils that have

significant accumulations of organic matter, i.e., histosols.

Organic matter turnover and accumulation in wetlands has

significant affects on hydrology, biogeochemical cycling,

and plant community dynamics. Organic matter dynamics

in wetlands are also thought to have a major role in global

carbon cycling, containing an estimated 15 to 22 percent of

the terrestrial soil carbon (soil C) and being a major source

of atmospheric methane (Eswaran and others 1995, Gorham

1991). The carbon balance in wetlands is sensitive to

environmental and land management factors. For example,

soil C pools may be significantly reduced following some

forest management practices such as water management

and site preparation (Trettin and others 1995). In contrast,

other management considerations, e.g., restoration, are

being suggested to realize the carbon sequestration potential

of wetland soils as an approach to help mitigate carbon

dioxide emissions. Despite the recognition that soil C in

wetlands is important at multiple scales, from internal

ecosystem dynamics to global climate change, little

Existing Soil Carbon Models

Do Not Apply to Forested Wetlands

C.C. Trettin, B. Song, M.F. Jurgensen, and C. Li

attention has been given to the models now used, either

implicitly or explicitly, to assess ecosystem functions and

responses to management, restoration, or other

anthropogenic effects. Models that contain wetland processes

are essential for assessing management and conservation

options for this important ecological resource.

In response to issues such as global warming, climate

change, acid deposition, and the effects of forest

management, numerous carbon and nutrient cycling models

have been developed over the past 20 years. However, those

efforts focused on upland forests, grasslands, and agricultural

ecosystems. Comparisons of carbon and or nitrogen process

among upland models have been thoroughly reported using

data sets from long-term upland studies (De Willigen 1991;

McGill 1996; Otter-Nacke and Kuhlman 1991; Pan and

others 1996, in press; Parton 1996; Ryan and others 1996a,

1996b; Shao and Henderson-Sellers, in press; Smith and

others 1997; VEMAP Members 1995). Findings using those

models adequately characterize long-term soil C changes,

and there is relatively good coherence among the models.

However, many of the models have been applied at scales

that transcend upland boundaries and in landscapes

dominated by wetlands. The derived estimates may be

suspect if the model does not represent inherent ecosystem

processes that control carbon dynamics, e.g., anoxia,

alternating hydroperiods, and complex interactions of soil

chemistry and abiotic processes that are inherent to wetland

soils.

We were concerned that current soil C models do not

adequately represent the carbon cycle in wetlands, and that

applying them to wetlands or on a landscape scale would be

inappropriate. We evaluated widely used soil C process

models to determine if they would be applicable to wetland

ecosystems. Rather than conducting evaluations using

wetland data sets, we evaluated model structure and function

to determine if critical wetland ecosystem components and

processes were included in them. If so, we could justify

using wetland data sets. If not, an alternative model

framework would be appropriate. The latter proved to be the

case. The soil C model we suggest includes surface and

ground water hydrology, as well as redox soil chemistry, and

would facilitate assessment of the role of wetlands in carbon

cycling at the landscape or regional scale. These capabilities

are particularly useful in assessments that consider the

opposing carbon functions of wetlands (as a carbon sink and
2

ITE, and VVV) using 12 data sets from long-term

experiments. They found that ROTHC, CANDY, DNDC,

CENTURY, DAISY, and NCSOIL have better fits than the

SOMM, ITE, and VVV models when applied to upland

soils. Accordingly, most of the selected models have proved

useful in assessing soil C dynamics. Among the selected

models, CENTURY is representative of the carbon process

models that are used at large scales (from regional to global)

in global change assessments (Pan and others 1996,

VEMAP Members 1995). BIOME2, BioGeochemistry

Cycles (BIOME-BGC), CENTURY, Dynamic Global

Phytogeography Model (DOLY), Mapped Atmosphere-Plant

Soil System (MAPSS), and Terrestrial Ecosystem Model

(TEM). Each of these large-scale models has been used for

distinct vegetation types ranging from arid shrublands,

grasslands, savannas, forests, and tundra.

The structure and attributes of these 12 models were

examined in light of important ecosystem variables that

influence soil C processes and site conditions in wetlands.

Attributes were: (1) environmental variables that control

organic matter turnover, (2) temporal and spatial scales, (3)

hydrology, (4) soil profile characterization, (5) soil physical

properties, and (6) soil chemical properties. The following

tabulation shows each attribute with respect to specific

conditions important to modeling wetlands (each model was

evaluated to determine the extent to which it had those

desired attributes):

Model attribute Specific factors

Environmental variables Climatic variables: air

temperature, precipitation.

Soil abiotic variables within

soil layers: temperature,

moisture.

Scale Temporal scale, resolution of

hourly to daily needed.

Hydrology Vertical water movement;

ground water input; flooding.

Soil profile characteristics Presence of multiple soils

layers. Organic subsoil

layers.

Soil physical properties Clay content, and important

factor affecting carbon.

Soil chemical properties Redox processes within soil

layers, which affect carbon

and nitrogen pathways.

methane source), factors that are considered in climatechange research. A wetland soil C model would also

necessarily integrate other important biogeochemical

processes, e.g., nitrogen cycles, and facilitate the assessment

of various management activities on wetland sustainability

and ecosystem restoration. A model specific to wetlands also

would be useful in identifying critical research needs.

Approach

Comparison of Current Soil C Models

Jenkinson (1990) grouped soil C models into four

categories: (a) a single, homogeneous compartment model,

which is the simplest form describing the soil C

decomposition rate using a linear equation; (b) a twocompartment model, where organic matter from plant

residue is split into two compartments, one decomposing

much slower than the other; (c) the noncompartmental decay

model, which assumes that decomposition occurs on a

continuum (Bosatta and Ågren 1985) (because of its

mathematical complexity, this approach has not been

broadly applied); and (d) multicompartmental models,

which divide carbon cycling processes into various

compartments, each with decomposition-rate constants that

are multiplied by one or more rate modifiers to reflect biotic

and abiotic factors, i.e., environmental constraints.

Multicompartmental models are the most commonly used,

especially when simulating long-term dynamics, i.e., years

to centuries.

We selected multicompartmental models because they are

best suited for studying mechanisms controlling soil C

processes, and they have been most broadly applied. We

identified 12 soil C models from the literature and used

them in this study. The models were: Carbon-NitrogenDynamics (CANDY) (Franko 1995), CENTURY (Parton

and others 1987), DAISY (Hansen and others 1991),

DeNitrification and DeComposition (DNDC) (Li and others

1994), Institute of Terrestrial Ecology (ITE) (Thornley and

Verberne 1989), Marine Biological Laboratory General

Ecosystem Model (MBL-GEM) (Rastetter and others 1991),

NCSOIL (Molina and others 1983), QSOIL (Bosatta and

Ågren 1985), Rothamsted C Model (ROTHC) (Jenkinson

and others 1987), Soil Organic Matter Model (SOMM)

(Chertov 1990), VanVeen/Verberne (VVV) (Verberne 1992),

and Wetland Methane Emission Model (WMEM) (Cao

1996). We selected these models because they have been

widely used and are well documented in the literature. Smith

and others (1997) compared 9 soil C models (CANDY,

CENTURY, DNDC, DAISY, NCSOIL, ROTHC, SOMM,
3

Results and Discussion

Review of Existing Carbon Models

Design attributes—The model structures reflected various

levels of detail and represented a range of ecosystem

processes. However, all structures except WMEM

represented upland ecosystems exclusively and, therefore,

are not appropriate for wetland ecosystems. Model

comparison results are presented in table 1. Anaerobic

condition, a critical component in wetland carbon cycles,

was a factor in only three of the models. In the CENTURY

model, anaerobic conditions are divided into three classes

and used as a multiplier, between 0 and 1, to represent rates

of decomposition. The DNDC and WMEM models also

consider anaerobic conditions to represent decomposition,

and methane emissions as well. None of the 12 models

considered ground water, the critical controlling variable;

and only 2 considered vertical water movement. Six

differentiated among vertical soil layers, another important

soil characteristic; but none provided for organic subsurface

horizons. Model time-step ranged from hourly to centurial,

the latter being beyond the range to reasonably model

wetland dynamics. Accordingly, only CENTURY, DNDC,

and WMEM contained attributes necessary when

considering wetlands.

The CENTURY model has been widely used to assess

carbon cycling in various ecosystem studies, especially

relative to global climate change. The compartments in this

model are fairly simple, but they retain ecological relevance.

The model includes a water budget submodel and uses soil

temperature and moisture at various depths in the soil

profile to regulate decomposition. It is one of only two

models (the other one is MBL-GEM) that include lignin to

N ratios of organic matter substrates as an indicator of

substrate quality; the other models rely on the C to N ratio.

The time increment of the model is 1 month. The

CENTURY model does not include detailed nitrogen

mineralization processes, nitrification, denitrification, or

methane emission.

The DNDC model was designed to assess organic matter

decomposition and denitrification in fields (Li and others

1996); however, the model recently has been used

successfully in upland forests (Li and others 2000, Stange

and others 2000). The DNDC includes multiple soil layers,

moisture and temperature controls on organic matter

turnover, provisions for methane emission, and a nitrogen

submodel. The time increment in the DNDC model can be

as short as 1 hour, which is an advantage when considering

redox chemistry in response to alternating hydroperiods.

Organic matter includes litter, microbial, and humad pools;

each has multiple compartments that are tracked within each

soil layer, including the forest floor. The DNDC has a soil

Table 1—Comparison of 6 factors critical to soil carbon cycling in wetlands among 12 models

Environmental Anaerobic

variable status Scale Hydrology conditions Soil texture Soil profile

Vertical soil Ground water Redox Different

Model Dynamic Constant Hours Months Centuries water flow flow process Clay content One layer layers

CANDY X X X X

CENTURY X X X X X X

DAISY X X X X

DNDC X X X X X X X

ITE X X X

MBL–GEM X X X X

NCSOIL X X X

QSOIL X X X

ROTHC X X X X

SOMM X X X

VVV X X X X

WMEM X X X X X
4

water submodel that provides for vertical soil water

movement and DOC transport.

The WMEM model was designed to simulate methane

(CH4) emissions from rice soils; it subsequently has been

used for other ecosystems. The WMEM is the only model

specifically developed for wetlands, but because it was

designed to measure only one compound, it does not contain

attributes necessary for a generalizable wetland soil C

model. Specific limitations include—that the soil water

balance is driven by precipitation and does not include

ground water, the soil is considered as only a single layer,

and the time-step is too long to consider transient redox

conditions.

Model output attributes—Carbon output variables for 10

of the models are shown in table 2; the QSOIL and MBLGEM models were not included because output descriptions

were incomplete. Each of the 10 models simulates the total

residue mass in the forest floor and soil organic matter

content. Five models divide residue, microbial, and soil

pools into two functional groups according to their turnover

rate: labile and resistant. The CANDY, ITE, NCSOIL,

SOMM, and WMEM did not divide residue into labile and

resistant components, while CANDY, ITE, SOMM, and

WMEM did not include microbial pools. Five models

considered inert soil organic matter. Only two models

(CENTURY and DNDC) included a soil leaching

component. Five of the ten models calculated CO2 emissions, and two (DNDC and WMEM) simulate methane

production. The DNDC and CENTURY models give the

largest number of output categories. The DNDC and

NCSOIL can generate daily outputs, but most models

generate monthly-to-yearly outputs. Among these 10

models, only CENTURY and DNDC provide the requisite

outputs to adequately characterize wetland soil C dynamics.

Eight models include nitrogen cycling components (table 3).

Most nitrogen pools were calculated based on the C to N

ratio. Thus, the output variables for nitrogen pools are

similar to the ones in carbon pools. All models that simulate

the nitrogen process include the output of nitrogen from the

total dead residue, and nitrogen in soil organic matter. All

models also calculate total mineral nitrogen, and four

models consider two forms of mineral nitrogen (NH4 and

NO3). Only DNDC measures nitrogen gas fluxes.

Framework for a Wetland Model

Based on this review of structural design and attributes, it is

evident that the 12 models do not adequately account for

anoxia, alternating hydroperiods, complex interactions of

soil chemistry and abiotic factors, and time-steps that are

important to wetland soil processes. Development of a

wetland soil C model that includes anoxia, surface and

ground water flows, and soil chemistry would allow

assessment of the role wetlands have in carbon cycling at

the landscape or regional scale. This capability will be

necessary to assess sustainability, the opposing radiative

functions of wetlands as a carbon sink and methane source,

carbon sequestration potential, and interactions with land

management practices.

Using a wetland ecosystem concept and the basic

framework of DNDC, with selected attributes of the

CENTURY and WMEM models, we developed a conceptual

model for soil C dynamics that incorporates important

controlling processes for organic matter decomposition in

wetlands. Improvements over the existing models include:

(1) anaerobic conditions for different soil layers are taken

into account; (2) redox potential is used to control the

decomposition and methane emission factor; (3) it allows

for surface and ground water flows that may go up, instead

of only down as in current water budget models; (4) it

recognizes multiple soil layers including a surface organic

layer, and the potential for subsurface layers to be either

organic or inorganic; and (5) methane flux, nitrification, and

denitrification are controlled by redox and soil moisture.

The carbon model is presented schematically in figure 1. It

includes the attributes mentioned.

The model soil is considered to have multiple layers that

may be either mineral or organic horizons. Hydrologic

control is the principal factor that differentiates uplands

from wetlands. Accordingly, the soil water submodel is the

critical component of the wetland soil C model (fig. 2).

Similarly, because carbon dynamics are inextricably linked

to nitrogen, a nitrogen component must reflect the complex

dynamics in both aerated and reduced conditions (fig. 3).

We intend these components to be organized into the general

modeling framework of DNDC, which provides for

climatic, vegetation, and management inputs and controls

(Li 1996). The merit of this model framework is that it

includes the primary factors that control carbon cycling in

wetlands. Conceptually, it is valid for both peatlands and

mineral soil wetlands. Another important attribute is that it

is based on existing models, thereby alleviating many

developmental hurdles.

Summary

Modeling soil C dynamics is a critical process in assessing

biogeochemical cycling of nutrients and elements, and in
5

Residue - carbona Soil - carbon

Total dead

(labile + Microbial biomass Total SOM (labile Gas

Labile Resistant resistant) Resistant Passive + resistant + Temporal output

Model residue residue carbon Labile Resistant Total Labile SOM SOM SOM passive) DOC CO2 CH4 Scale range

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Grams per square meter - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Table 2—Carbon output variables for 10 selected models

CANDY X X X X X Month - years

CENTURY X X X X X X X X X X X X Year - century

DAISY X X X X X X X X X X Month - years

DNDC X X X X X X X X X X X X X Day - years

ITE X X Month - years

NCSOIL X X X X X X X Day - years

ROTHC X X X X X X X X Year - century

SOMM X X X X Year - century

VVV X X X X X X X X X X Year - century

WMEM X X X X Year - century

CANDY X X X X X X X X X X Month - years

CENTURY X X X X X X X X X X X X Year - century

DAISY X X X X X X X X X X Month - years

DNDC X X X X X X X X X X X X X X X Day - years

ITE X X X X X Month - years

NCSOIL X X X X X X X X X X Day - years

ROTHC Year - century

SOMM X X X X X Year - century

VVV X X X X X X X X X X X Year - century

WMEM Year - century

Total dead Microbial biomass Gas Temporal

Labile Resistant (labile + Labile Resistant Passive Total Total Gas (NH3, output scale

Model residue residue resistant) Labile Resistant Total SOM SOM SOM SOM NH NO mineral DON N2, N O) range 2

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Grams per square meter - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Table 3—Nitrogen output variables for 10 selected models

Residue Soil - nitrogen

SOM = soil organic matter; DON = dissolved organic matter.
6

Figure 1—Proposed wetland soil carbon model.

ABDF = f(temp, moist, redox)

Water = f(soil moisture, PPT, GW, ET, PS)

Temp = f(insolation, soil, moisture, water)

Redox = f(water, soil moisture, temp, SOM-H)

Lig = Lignin

L/N = Lignin/N

Txt = Soil texture

Peat = Decayed organic matter
7

Figure 2—Proposed hydrology submodel to the carbon model.
8

Figure 3—Proposed nitrogen submodel to the carbon model.

ABDF = f(temp, moist, redox)

Water = f(soil moisture, PPT, GW, ET, PS)

Temp = f(insolation, soil, moisture, water)

Redox = f(water, soil moisture, temp, SOM-H)

Lig = Lignin

L/N = Lignin/N

Txt = Soil texture

Peat = Decayed organic matter
9

assessing carbon dynamics with respect to global change

processes. Although wetlands occupy a small proportion of

the terrestrial surface of the Earth (< 3 percent), they are

reported to contain a disproportionate share of the terrestrial

carbon pool (15 to 22 percent). Accordingly, models that do

not accurately represent wetland soil processes cannot be

expected to provide reasonable simulations. We evaluated

12 widely used soil C models to determine if they could be

used when assessing wetland ecosystems. Only three of the

models contained provisions for anaerobic conditions; none

considered the necessary factors of anoxia, ground water

hydrology, multiple organic and physical soil layers, and a

daily time-step. Accordingly, simulations conducted using

these models cannot be expected to produce reasonable

simulations based on soil processes. Using desirable

attributes from three of the models, our framework for a

wetland soil C model will be appropriate when considering

soil C dynamics at multiple spatial scales.

Acknowledgments

This work was supported in part by grants from the U.S.

Department of Agriculture Forest Service, Southern Global

Change Program and the Forest Environment Program

within the National Council for the Paper Industry on Air

and Stream Improvement. Work at Michigan Technological

University was supported through Cooperative Agreement

SRS–33–CA–97–224 and SRS–33–CA–98–462,

“Assessment of Soil Carbon Models for Hydric Soils.” We

also gratefully acknowledge the reviews and constructive

comments provided by Felipe Sanchez and the anonymous

reviewers.

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Trettin, C.C.; Song, B.; Jurgensen, M.F.; Li, C. 2001. Existing soil carbon models do not apply to

forested wetlands. Gen. Tech. Rep. SRS–46. Asheville, NC: U.S. Department of Agriculture, Forest

Service, Southern Research Station. 10 p.

When assessing the biological, geological, and chemical cycling of nutrients and elements—or when

assessing carbon dynamics with respect to global change—modeling and simulation are necessary.

Although wetlands occupy a relatively small proportion of Earth’s terrestrial surface (< 3 percent), they

contain a disproportionate share of the terrestrial carbon pool (15 to 22 percent). Models that do not

accurately represent wetland soil processes cannot, therefore, provide reasonable simulations. We

evaluated 12 widely used soil C models to determine their applicability to wetland ecosystems:

CANDY, CENTURY, DAISY, DNDC, ITE, MBL-GEM, NCSOIL, QSOIL, ROTHC, SOMM, VVV,

and WMEM. Only three (CENTURY, DNDC, and WMEM) allow for anaerobic conditions; none

contains components for anoxia, ground water hydrology, multiple organic and physical soil layers, or a

daily time-step, all of which are necessary when modeling soil C in wet soils. Accordingly for any land

area that includes wetlands, none of the individual models would produce reasonable simulations based

on soil processes. We present a wetland soil C model framework based on desired attributes, the DNDC

model, and components of the CENTURY and WMEM models. Our proposed synthesis would be

appropriate when considering soil C dynamics at multiple spatial scales and where the land area

considered includes both wetland and upland ecosystems.

Keywords: Carbon, model, organic matter, soil, wetland.
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