Riparian Buffer Width, 

Vegetative Cover, and 

Nitrogen Removal 

Effectiveness: 

A Review of Current Science and 

Regulations

EPA/600/R-05/118

October 2005

Riparian Buffer Width, Vegetative Cover, 

and Nitrogen Removal Effectiveness:

A Review of Current Science and Regulations

Paul M. Mayer 

Steven K. Reynolds, Jr.

Timothy J. Canfield

U.S. Environmental Protection Agency 

Office of Research and Development 

National Risk Management Research Laboratory

Ada, Oklahoma 74820

Marshall D. McCutchen

East Central University

Ada, Oklahoma 74820

Project Officer

Paul M. Mayer

Ground Water and Ecosystems Restoration Division

National Risk Management Research Laboratory

Ada, Oklahoma 74820

National Risk Management Research Laboratory

Office of Research and Development

U.S. Environmental Protection Agency

Cincinnati, Ohio 45268
ii

Notice

The U.S. Environmental Protection Agency through its Office of Research 

and Development funded, managed, and collaborated in the research described 

here through in-house efforts. It has been subjected to the Agency’s peer and 

administrative review and has been approved for publication as an EPA document. 

Mention of trade names or commercial products does not constitute endorsement 

or recommendation for use.

All research projects making conclusions or recommendations based on environmental data and funded by the U.S. Environmental Protection Agency are 

required to participate in the Agency Quality Assurance Program. This project did 

not involve the collection or use of environmental data and, as such, did not require 

a Quality Assurance Project Plan.
iii

Foreword

The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation’s land, air, and water 

resources. Under a mandate of national environmental laws, the Agency strives to formulate and implement actions 

leading to a compatible balance between human activities and the ability of natural systems to support and nurture 

life. To meet this mandate, EPA’s research program is providing data and technical support for solving environmental problems today and building a science knowledge base necessary to manage our ecological resources wisely, 

understand how pollutants affect our health, and prevent or reduce environmental risks in the future.

The National Risk Management Research Laboratory is the Agency’s center for investigation of technological and 

management approaches for preventing and reducing risks from pollution that threatens human health and the 

environment. The focus of the Laboratory’s research program is on methods and their cost-effectiveness for prevention and control of pollution to air, land, water, and subsurface resources; protection of water quality in public 

water systems; remediation of contaminated sites, sediments and ground water; prevention and control of indoor 

air pollution; and restoration of ecosystems. NRMRL collaborates with both public and private sector partners to 

foster technologies that reduce the cost of compliance and to anticipate emerging problems. NRMRL’s research 

provides solutions to environmental problems by: developing and promoting technologies that protect and improve 

the environment; advancing scientific and engineering information to support regulatory and policy decisions; and 

providing the technical support and information transfer to ensure implementation of environmental regulations and 

strategies at the national, state, and community levels.

This publication has been produced as part of the Laboratory’s strategic long-term research plan. It is published 

and made available by EPA’s Office of Research and Development to assist the user community and to link researchers with their clients.

The goal of this report is to synthesize the existing scientific literature on the effectiveness of riparian buffers to improve water quality through their inherent ability to process and remove excess anthropogenic nitrogen from surface 

and ground waters. Due to this ability, riparian buffers often are employed as an environmental management tool 

by resource management agencies. Despite significant research effort toward understanding the ecological functions of riparian buffers, there remains no consensus for what constitutes optimal riparian buffer design or proper 

buffer width to achieve maximum nitrogen removal effectiveness. This report does not provide a one-size-fits-all 

recommendation for such a design or width but rather attempts to identify generalizations and trends extracted from 

published literature that will aid managers in making decisions about establishing, maintaining, or restoring riparian 

buffers in watersheds of concern. Although, buffer width stands out as one factor influencing the capacity for buffers 

to remove nitrogen, numerous other factors described herein play significant roles that must be understood before 

employing riparian buffers as part of a comprehensive watershed management plan. 

Stephen G. Schmelling, Director

 Ground Water and Ecosystems Restoration Division

 National Risk Management Research Laboratory
iv

Synopsis and Abstract

Synopsis

1) Riparian buffers are vegetated zones adjacent to streams and wetlands that represent a best management practice (BMP) for controlling nitrogen entering water bodies. 

2) Current research indicates that riparian buffers of various vegetation types are effective at reducing 

nitrogen levels in groundwater and streams.

3) Buffer width is only one factor controlling nitrogen removal effectiveness.

4) Subsurface removal of nitrogen in riparian buffers is often high, especially where conditions promote 

microbial denitrification.

5) Riparian buffers are a single component of comprehensive watershed management plans, which must 

also include point source and non-point source control of nitrogen.

Abstract

Mayer, P.M., S.K. Reynolds, M.D. McCutchen, and T.J. Canfield. Riparian buffer width, vegetative cover, and nitrogen 

removal effectiveness: A review of current science and regulations. EPA/600/R-05/118. Cincinnati, OH, U.S. 

Environmental Protection Agency, 2006. 

Riparian zones, the vegetated region adjacent to streams and wetlands, are thought to be effective at intercepting 

and controlling nitrogen loads entering water bodies. Buffer width may be positively related to nitrogen removal effectiveness by influencing nitrogen retention through plant sequestration or removal through microbial denitrification. 

We surveyed peer-reviewed scientific literature containing data on riparian buffers and nitrogen concentration in 

streams and groundwater of riparian zones to identify causation and trends in the relationship between buffer width 

and nitrogen removal capacity. We also examined Federal and State regulations regarding riparian buffer widths to 

determine if such legislation reflects the current scientific understanding of buffer effectiveness.

Nitrogen removal effectiveness varied widely among riparian zones studied. Subsurface removal of nitrogen was 

efficient but did not appear to be related to buffer width. Surface removal of nitrogen was partly related to buffer 

width and was generally inefficient, removing only a small fraction of the total nitrogen flowing through soil surface 

layers. While some narrow buffers (1-15 m) removed significant proportions of nitrogen, narrow buffers actually 

contributed to nitrogen loads in riparian zones in some cases. Wider buffers (>50 m) more consistently removed 

significant portions of nitrogen entering a riparian zone. Buffers of various vegetation types were equally effective 

at removing nitrogen in the subsurface but not in surface flow. The general lack of vegetation type or buffer width 

effects on nitrogen removal, especially in the subsurface, suggests that soil type, watershed hydrology (e.g., soil 

saturation, groundwater flow paths, etc.), and subsurface biogeochemistry (organic carbon supply, high nitrate inputs) 

may be more important factors dictating nitrogen concentrations due to their influence on denitrification.

State and Federal guidelines for buffer width also varied widely but were generally consistent with the peer-reviewed 

literature on effective buffer width, recommending or mandating buffers ~7-100 m wide. Proper design, placement, 

and protection of buffers are critical to buffer effectiveness. To maintain maximum effectiveness, buffer integrity 

should be protected against soil compaction, loss of vegetation, and stream incision. Maintaining buffers around 

stream headwaters will likely be most effective at maintaining overall watershed water quality while restoring degraded riparian zones, and stream channels may improve nitrogen removal capacity. Riparian buffers are a “best 

management practice” (BMP) that should be used in conjunction with a comprehensive watershed management 

plan that includes control and reduction of point and non-point sources of nitrogen from atmospheric, terrestrial, 

and aquatic inputs.

Keywords: attenuation, buffer strip, denitrification, groundwater, nitrate, nitrogen, stream, riparian buffer, surface 

water, watershed, vegetated filter strip
v

Contents

Foreword .........� iii

Synopsis....................................................................................... ................................. iv

Abstract........................................................................................... ................................. iv

Acknowledgments............................................................................... ............................ ix

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

Literature Survey Methods................................................................. .................................2

Results......................................................................................... ..................................3

Synthesis of Published Reviews on Buffer Effectiveness........................ .............................3

Detailed Insight into the Peer-Reviewed Literature about Buffer Effectiveness ....................... 3

Vegetated buffers around wetlands ....................................................................................3

Forested buffers ..................................................................................................................3

Grasslands .........................................................................................................................4

Meta-Analysis of the Peer-Reviewed Literature about Buffer Effectiveness........ ....................4

Methods.......................................................................... ......................................4

Results............................................................................ ......................................5

Discussion....................................................................... ....................................13

Federal Regulations and Recommendations......................................... .........................13

United States Code (USC) ...............................................................................................13

Code of Federal Regulations (CFR) .................................................................................13

State and Provincial Regulations and Recommendations.......................... .......................14

Other Aspects of Buffer Effectiveness......................................................... .................14

Buffer Restoration, Planning, and Design..................................................... ................15 

Summary and Conclusions....................................................................... .......................17

Literature Cited.................................................................................... ..........................19

Appendices ....� 25

Appendix 1 .............................................................................................................................25

Appendix 2 ..............................................................................................................................26
vi
vii

Figures

1. Relationship of nitrogen removal effectiveness to riparian buffer width ...................................9

2. Nitrogen removal effectiveness in riparian buffers by flow path.............. ..............................10

3. Relationship of nitrogen removal effectiveness to riparian buffer width by 

flow path ............................................................................. ....................................10

4. Nitrogen removal effectiveness in riparian buffers by buffer vegetation type .........................11

5. Relationship of nitrogen removal effectiveness to riparian buffer width by 

riparian vegetation type............................................................. ..................................11

6. Nitrogen removal effectiveness in riparian buffers by buffer vegetation type 

and water flow path................................................................. ...................................12

7. Relationship of nitrogen removal effectiveness to nitrogen load:buffer 

width ratio.......................................................................... .......................................12
viii

Tables

1. Summary Table of Riparian Buffer Effectiveness at Removing Nitrogen by 

Vegetative Cover, Hydrologic Flow Path, Buffer Width and Soil Type ............................. ...6

2. Mean and Percent Effectiveness of Riparian Buffers at Removing Nitrogen ..........................9

Appendix 

1. Summary Table of United States Code Referring to "Riparian," "Buffer," 

"Vegetated," and "Filter Strip" ....................................................... ...............................25

2. Summary Table of Code of Federal Regulations Referring to "Riparian," 

"Buffer," "Vegetated," and "Filter Strip" .............................................. ............................26
ix

Acknowledgments

We thank D. Niyogi, D. Walters, and S. Wenger for their constructive comments on 

this manuscript. T. Wiggins assisted in the search for literature and government 

regulations. K. Tynsky created the cover art. Cover photos were provided by T. 

Barthelmeh, K. Jewell, and J. Williams. The U.S. Environmental Protection Agency 

through its Office of Research and Development funded and managed the research 

described here through in-house efforts. It has been subjected to the Agency’s 

peer and administrative review and has been approved for publication as an EPA 

document. 
x
1

Introduction

 The U.S. Environmental Protection Agency (U.S. EPA) considers nitrogen one of the top stressors in aquatic ecosystems (U.S. EPA 2002a). Though nitrogen is an important nutrient for all organisms, excess nitrogen is a pollutant 

that causes eutrophication in surface water and contaminates groundwater (Carpenter et al. 1998). Streams receive 

chronic nitrogen inputs from upland sources such as fertilizers, animal wastes, leaking sewer lines, atmospheric deposition, and runoff from highways (Carpenter et al. 1998, Swackhamer et al. 2004). Subsequent eutrophication leads 

to environmental impacts such as toxic algal blooms, oxygen depletion, fish kills, and loss of biodiversity (Vitousek et 

al. 1997). Nitrate nitrogen (NO3

-

) also is a drinking water pollutant, especially dangerous to infants <6 months old who 

are at risk of methanoglobin-induced anemia or blue baby syndrome in which nitrate (converted to nitrite in the body) 

inhibits oxygen uptake, potentially leading to brain damage, or death (Welch 1991). The allowable level of nitrogen in 

drinking water for children ≤6 months old is 10 ppm (mg/1) as nitrate nitrogen (U.S. EPA 2002b). 

Nitrogen enters aquatic ecosystems in one in of several forms including nitrate nitrogen (e.g. fertilizers), particulate 

nitrogen (e.g. litter fall from trees), ammonium (e.g. sewage and animal waste), and nitrous oxides from fossil fuel combustion (Schlesinger 1997). The means of entry into a system may differ for each type of nitrogen. For example, nitrous 

oxides enter by atmospheric deposition, nitrate often enters through groundwater, and particulate nitrogen follows terrestrial routes. Nitrogen is transformed by biological processes including uptake by plants and microbial denitrification, 

a process where anaerobic bacteria transform nitrate nitrogen to N2, a gas phase of nitrogen (Schlesinger 1997). Only 

denitrification removes nitrogen from a system, whereas, nitrogen uptake by plants eventually returns nitrogen to the 

system through senescence and microbial decay. Nitrate nitrogen is of concern as an environmental stressor because 

it is biologically reactive, poses a human health risk, and often is found in groundwater. 

Riparian buffers are thought to be an effective, sustainable means of buffering aquatic ecosystems against nutrient 

stressors such as nitrogen (Phillips 1989a) and thus are considered a best management practice (BMP) by State and 

Federal resource agencies (i.e., USDA-NRCS Environmental Quality Incentive Program, Conservation Programs Manual 

Part 515.91). Riparian buffers attenuate nitrogen through plant uptake, microbial immobilization and denitrification, soil 

storage, and groundwater mixing (Lowrance et al. 1997). The effectiveness of a buffer will depend upon its ability to 

intercept nitrogen in its various forms traveling along surface or subsurface pathways. 

Often buffers are defined operationally as the zone of vegetation adjacent to streams, rivers, creeks, or wetlands (i.e., 

Lee et al. 2004). For this paper, riparian buffer, riparian zone, buffer strip, filter strip, and vegetated filter strip are terms 

used synonymously. However, these terms may be defined differently depending on the application and agency in 

question. Regardless of terminology, the extent to which riparian buffers attenuate nitrogen and improve stream water 

quality is thought to be at least partly a function of buffer width (Vidon and Hill 2004), by some estimates, accounting 

for 81% of a buffer’s nitrogen removal effectiveness (Phillips 1989a). Intuitively, larger and wider riparian buffers should 

transform and remove more nitrogen from the watershed. Therefore, numerous State and Federal resource agencies 

have guidelines recommending buffers of minimum width to protect stream ecosystems from nutrient inputs (Belt et al. 

1992, Christensen 2000, Lee et al. 2004.). Despite this trend toward regulation of riparian buffer widths, the specific 

mechanisms responsible for removing nitrogen within buffers are not thoroughly understood. Furthermore, what is 

known is not widely distributed to those who might be able to utilize the information to manage and restore riparian 

buffers to maintain water quality (Hickey and Doran 2004). An urgent need exists for guidance on proper and effective 

use of buffers to maintain water quality.

The purpose of this document is to identify causation and trends in the relationship between buffer width and nitrogen 

removal capacity extracted from peer-reviewed studies with empirical data on buffer effectiveness. Our secondary objective 

was to survey the State and Federal regulations and guidelines regarding riparian buffers to determine if buffer widths 

required under current law are consistent with effective buffer widths identified from the peer-reviewed literature.
2

Literature Survey Methods

We employed database search engines (e.g., Cambridge Abstracts, Google Scholar, etc.) and existing bibliographies 

(e.g., Correll 2003) to locate riparian buffer zone literature. We used search terms singly or combination including: riparian, buffer, width, filter strip, vegetated filters, nitrogen, etc. We summarized the conclusions from comprehensive and 

regional literature syntheses and the results from peer-reviewed research papers that contained original data quantifying 

the effects of riparian buffer width on nitrogen attenuation. We also surveyed Federal agency documents and previously 

published reviews of buffer width literature for opinions and recommendations on minimum effective buffer width. Papers 

that did not relate nitrogen removal to buffer width were not included in the results. Data presented in proceedings and 

other non-peer-reviewed sources were generally not included here except as part of generalizations presented in other 

literature reviews because methods therein could not be verified. We placed greater emphasis on studies that quantified 

a rate of nitrogen removal calculated per unit distance or per unit area. Such data may provide a quantifiable estimate 

of buffer effectiveness and aid in establishing minimum buffer widths based on removal effectiveness.

We also surveyed relevant Federal and State regulation codes, peer-reviewed literature reviews of government guidelines, 

and recommendations by government agencies that were not part of regulatory legislation. We attempted to locate 

Federal regulations and laws concerning riparian buffers by searching the web versions of the United States Code, 

Public and Private Laws, and the Code of Federal Regulations. Various agency websites were searched, including the 

websites of the United States Department of Agriculture (USDA), the United States Forest Service (USFS), the United 

States Army Corps of Engineers (USACE), the Bureau of Land Management (BLM), and the Government Accountability 

Office (GAO). We spoke directly with agency officials to clarify findings and to aid in the search for other regulations. 

Federal and State regulations and recommendations were compared to the previous literature-based results on riparian 

zones to determine the degree of consistency between them.
3

Results

Synthesis of Published Reviews on Buffer Effectiveness

We found 14 comprehensive and regional reviews of riparian buffer literature, most of which contained generalizations 

and recommendations based on both peer-reviewed and non-peer-reviewed research. In general, riparian forest vegetation and wetlands have been demonstrated as effective nutrient filters, particularly those between ~10-50 m wide 

(Belt et al. 1992, Johnson and Ryba 1992, Castelle et al. 1994, Fennessy and Cronk 1997, Fischer and Fischenich 

2000, Christensen 2000). Narrower riparian buffers (5-6 m) may still reduce subsurface nitrate flows by up to ~80% 

(Muscutt et al. 1993, Parkyn 2004). However, extensive experimental support for buffer zones <10 m, like those used 

extensively on many farms, is lacking (Hickey and Doran 2004). Furthermore, riparian buffer zones >30 m were recommended for fully effective subsurface nutrient reduction (Muscutt et al. 1993, Wenger 1999). According to Wenger and 

Fowler (2000), “The most effective buffers are at least 30 meters, or 100 feet, wide composed of native forest, and are 

applied to all streams, including very small ones.” The use of riparian buffers to filter nutrients from surface flow was 

not recommended by Barling and Moore (1994) because dissolved nitrate was not significantly reduced.

Groundwater flow paths, soil characteristics (i.e., moisture storage, hydraulic conductivity, roughness, and slope), seasonal, and climate may significantly impact the rate and magnitude of subsurface nitrate removal. Groundwater flow 

above shallow, impermeable soil layers maximizes water residence time and contact with plant roots and organic-rich 

soils, thereby increasing the potential for nitrate removal by plant uptake and microbial activity (Hill 1996, Christensen 

2000). Considerably less nitrate removal per unit distance occurred where local or regional groundwater flowed at 

deeper depths or through organically-poor soil (Hill 1996). Where groundwater bypassed the root zone and surface 

soil layers, the retention of nitrogen was minimal (Lowrance et al. 1997).

Detailed Insight into the Peer-Reviewed Literature about Buffer Effectiveness

Vegetated buffers around wetlands

Wetland buffer zones were highly variable in their effectiveness, removing from 12-80% of surface water nitrogen (Yates 

and Sheridan 1983, Brüsch and Nilsson 1993). However, much faster nitrate reductions can occur in the groundwater 

of wetlands where, in some cases, >95% of nitrate can be removed within 1 m (Burns and Nguyen 2002). Brüsch 

and Nilsson (1993) documented temperature and seasonal components to nitrate reductions in surface runoff across 

a 15-25 m wide peat wetland. Average nitrate reduction was 73.7% in the summer, 12.2% during the first winter, but 

~38% during the second winter season due to higher temperatures. Despite seasonal variance in mean surface runoff 

nitrate concentration of 15 to 50 ppm, nitrate concentration in an adjacent stream did not exceed 5 ppm throughout the 

study. Seasonal patterns, but with higher percentage of nitrate reduction (>90%), also were noted in a 200-m wide reed 

and alder wetland within a river channel scar (Fustec et al. 1991).

Wetland buffers on soils with limited organic matter (i.e., sand or gravel) tended to show lower capacity to remove 

nitrogen. Cooper (1990) found that, while subsurface nitrate removal from highly organic, saturated soils was ~90%, 

removal from within mineral colluvial soils was much less effective. Clausen et al. (2000) observed a 52-76% reduction 

in subsurface nitrate concentrations (95% of all nitrate loss) across a 5-m “poorly to very poorly drained alluvium wetland.” Hanson et al. (1994) and Vellidis et al. (2003) observed similar reductions in nitrate (59% and 78%, respectively) 

from sandy, forested wetlands (31 and 38 m wide, respectively). Under “severely suboptimal conditions” in forested 

wetlands (i.e., sparsely vegetated, poorly drained, bottomland soils), riparian buffer widths <100 m were estimated to 

be 90% efficient at removing nutrients from agricultural runoff. However, under less severe conditions, buffer widths 

of 40-80 m on poorly drained soils and 15-60 m on well-drained soils were estimated to remove most nutrient runoff 

passing through a forested wetland, riparian zone (Phillips 1989b).

Forested buffers 

The attenuation of nitrogen from groundwater flow can be rapid in forested riparian buffer zones. Schoonover and Willard (2003) found that 10-m forested buffers reduced groundwater nitrate concentration by 61%. Another study found a 

buffer averaging 38-m wide reduced nitrate concentration by 78% and ammonium by 52% (Vellidis et al. 2003). Others 
4

have documented more than 85% nitrate removal within the first 5 m of a buffer and 90-99% removal within 10-50 m 

of a buffer (Jacobs and Gilliam 1985, Lowrance 1992, Cey et al. 1999). As with wetland riparian buffers, most of the N 

transformation (~75%) occurred within the subsurface flow (Peterjohn and Correll 1984, Osbourne and Kovacic 1993). 

Furthermore, mature forests were 2-5 times more effective than “managed” (i.e., clearcut or selectively thinned) forests 

(Lynch et al. 1985, Hubbard and Lowrance 1997). Kuusemets et al. (2001) estimated that 85% of total nitrogen was 

retained in a heavily polluted 51-m wide riparian buffer, whereas only 40% of total nitrogen was retained in a buffer 

31 m wide. Riparian buffers 100 m and 200 m wide in North Carolina removed from 67%-100% of groundwater nitrate 

entering the stream (Spruill 2004).

Effectiveness of nitrogen removal in forested riparian zones can vary widely due to characteristics unrelated to width. 

Extreme nitrogen loading (Lowrance et al. 1997) and increased hydraulic conductivity of the soil (Pinay and Decamps 

1988, Pinay et al. 1993, Sabater et al. 2003) decrease effectiveness of forested riparian buffer zones. In some cases, 

these conditions can result in a net increase in nitrate concentrations in the groundwater (Sabater et al. 2003, Groffman 

et al. 2003) and can double the necessary width of the riparian zone for effective nutrient removal (Kuusemets et al. 

2001). Spruill (2000) observed no difference in deep, “old” (>20 yr) groundwater underneath riparian zones with and 

without forested 30-m buffers, but 65-70% nitrate removal in shallow, “young” (<20 yr) groundwater through “reduction 

or denitrification.” Effects of buffer width and length were mixed in a New Zealand study of forested buffers. However, 

oldest, longest (longitudinal), and widest (lateral) buffers had the greatest total nitrogen reductions (Parkyn et al. 2003).

Saturated conditions led to removal of all nitrate within the first 30 m of forested riparian buffers in France (Pinay and 

Decamps 1988).

Grasslands

Grassed buffers or filter strips used alone or in conjunction with woody vegetation also can be effective at removing 

nitrogen. A 7.1-m grass buffer removed 80% of the total nitrogen and 62% of nitrate. Addition of a 9.2-m woody buffer to the grass buffer (total 16.3 m) increased effectiveness by 20%, removing 94% of the total nitrogen and 85% of 

the nitrate in runoff. However, effectiveness of the buffers in this study was negatively related to intensity of rainfall 

events (Lee et al. 2003). Giant cane (Arundinaria gigantea) reduced nitrate levels 90% in the first 3.3 m of the buffer, 

and 99% over 10 m, an effectiveness promoted by saturated conditions from upwelling groundwater (Schoonover and 

Williard 2003). In a study of seven herbaceous and herbaceous/forested riparian buffers in Canada, 90% removal of 

nitrate occurred 15 to 37 m into the riparian buffer depending on soil types and depth of the confining layer (Vidon and 

Hill 2004). Conversion of a portion of a corn field (Zea mays L.) to a riparian buffer of fine leaf fescue (Festuca spp.) 

decreased overland flow concentrations of total Kjeldahl nitrogen (TKN) by 70% and nitrate by 83% over the control 

and reduced nitrate concentrations in groundwater by 35%. Most (52%) of the nitrate decrease occurred within a 2.5-m 

wetland adjacent to the stream (Clausen et al. 2000).

In an Italian study, a 6-m wide grass/forest buffer (5-m grass + 1-m trees) reduced groundwater nitrate by >90% from 

maximum concentrations of ~25-28 ppm ( x – = 6.2) under the application field to a level always ≤2 ppm ( x – = 0.6) in 

groundwater (Borin and Bigon 2002). Grass buffers <5 m wide were ineffective in removing total nitrogen from surface 

runoff; those <10 m, but >5 m, wide were found to be 29-65% effective (Magette et al. 1989, Schmitt et al. 1999). Addition of a “forested” component to the grass buffer did not increase effectiveness (Schmitt et al. 1999). Grass filter 

strips 4.6 and 9.1 m wide reduced surface nitrate runoff from no-till cornfields by 27 and 57%, respectively. However, 

similar filter strips installed below animal feedlots were completely ineffective, yielding net gains in surface runoff nitrate 

concentrations (Dillaha et al. 1988, 1989).

Meta-Analysis of the Peer-Reviewed Literature about Buffer Effectiveness

Methods

Few peer-reviewed studies experimentally quantify nitrogen removal based specifically on riparian zone width. Rather, 

most studies measure nitrogen depletion at various locations throughout the riparian zone and/or in relation to biotic and 

abiotic variables such as vegetation or soil type. The lack of standardized field tests to quantify nitrogen removal based 

on buffer width sometimes makes comparisons among studies difficult. However, generalizations can be made based 

on the trends among 40 studies yielding 66 buffer width-effectiveness relationships (Table 1). In order to facilitate these 

generalizations and analyses, we grouped studies by vegetative cover type (i.e., wetland, forested, grassland) and by 

hydrologic flow conditions (e.g., surface vs. subsurface), factors that may influence nutrient attenuation in riparian buffer zones. We calculated nitrogen removal effectiveness (%) as 1) the % difference in nitrogen concentration between 

the influent into and effluent out of the riparian buffer, 2) % difference in nitrogen concentration between the terminus 

of the control buffer and that of the test buffer, or 3) if recalculation was impossible based on available data, the values 

presented by the authors were used directly (Table 1). These effectiveness data were plotted against buffer width, and 

linear and non-linear regression models were fitted to the data to reveal patterns of nitrogen removal based on width, 

buffer type, and hydrology. Though nitrate (NO3

-

) was the form of nitrogen most often measured among studies, we did 
5

not distinguish among nitrogen forms when calculating effectiveness. All buffers included in studies for which efficiencies 

could be calculated were included in the meta-analyses as independent data points. We then examined the relationship 

between loads (influents) and efficiencies to determine if thresholds existed for N removal. For studies that reported 

the actual influent nitrogen concentrations, we calculated the ratio of nitrogen load to buffer width as a measure of the 

level of impact to buffers and then used that ratio as the independent variable and nitrogen removal effectiveness as 

the dependent variable in a linear regression model to estimate buffer thresholds for nitrogen removal. Analyses were 

performed with SYSTAT version 11 (SPSS 2004).

Results

We found that nitrogen removal effectiveness varied widely among studies (Table 1) but overall, buffers were effective at removing large proportions of the nitrogen found in water flowing through riparian ecosystems (mean % ± 1SE: 

74.2 ± 4.0; Table 2). A small but significant proportion of the variance in removal of nitrogen was explained by buffer 

width (R2 = 0.14, N = 66; Figure 1). That is, wider buffers removed more nitrogen, but other factors also must have 

affected effectiveness. Additionally, greater consistency of nitrogen removal was evident with increasing buffer width 

(Figure 1). For example, nitrogen removal effectiveness in buffers <50 m wide was more variable than those >50 m 

where nearly all buffers exhibited about a 75% removal effectiveness (Figure 1). Thus, wider buffers are more likely 

to be efficient zones of nitrogen removal, whereas, narrower buffers may not always remove significant portions of 

nitrogen. Based on our non-linear regression model, 50%, 75%, and 90% removal efficiencies would occur in buffers 

approximately 3 m, 28 m, and 112 m wide, respectively (Figure 1, Table 2). 

We found that nitrogen removal effectiveness also differed by flow pattern. Subsurface removal of nitrogen was 

much more efficient than surface removal (mean % ± 1SE: subsurface 89.6 ± 1.8, N = 48; surface 33.3 ± 7.7, N = 18; 

t = 10.1, P < 0.001; Figure 2). Furthermore, subsurface removal of nitrogen did not appear to be related to buffer width 

(R2 = 0.02, N = 48; Figure 3), whereas, a small but significant proportion of the variance in surface removal of nitrogen 

was explained by buffer width (R2 = 0.29, N = 18; Figure 3). That is, wider buffers removed more nitrogen in surface 

runoff. While some narrow buffers (1-15 m) removed significant proportions of nitrogen, three studies found that narrow 

buffers actually contributed nitrogen to riparian zones (i.e. had negative effectiveness values; Table 1). Such cases are 

likely to be short-term events due to nitrification or high rainfall events that lead to rapid inputs of particulate nitrogen. 

Based on our non-linear regression model, 50%, 75%, and 90% nitrogen removal efficiencies in surface flow would 

occur in buffers approximately 34, 118, and 247 m wide, respectively (Figure 3, Table 2).

We also found that nitrogen removal effectiveness varied by buffer vegetation type (N = 66; F = 4.8, P = 0.002; Figure 4 

and Table 2). Grass buffers were significantly less effective than forest buffers at removing nitrogen (P = 0.001, Bonferroni adjustment), whereas, other buffers were equally effective (Figure 4). 

Forested and wetland buffers showed no relationship between buffer width and nitrogen removal effectiveness (Figure 5). 

However, grass buffer effectiveness increased with buffer width in a non-linear fashion (Figure 5). Grass and grass/forest 

buffers were not always effective at removing nitrogen and, in three cases where buffers were <10 m, actually added 

to nitrogen loads (Figure 5). Based on the non-linear model results, we calculated the approximate buffer widths by 

vegetative types necessary to achieve 50%, 75%, and 90% effectiveness (Table 2). Nitrogen removal efficiencies of 

50%, 75%, and 90% were predicted for grass buffers approximately 16, 47, and 90 m wide and for grass/forest buffers 

approximately 5, 20, and 47 m wide, respectively. Given the low R2 values, buffer widths could not be predicted for 

effective nitrogen removal for grass or grass/forest buffers (Table 2). Note also, that the relationship between buffer 

width and effectiveness for forested wetlands was negative (Figure 5), suggesting that narrow forested wetland buffers 

are more effective than wide buffers. This non-intuitive result is likely due to the small sample size and not a cause and 

effect relationship. Therefore, buffer widths were not predicted for this vegetation type. Subsurface removal of nitrogen 

was generally high regardless of vegetation type, whereas, surface removal was less efficient and more variable among 

all buffer vegetation types (Figure 6).

In a similar meta-analysis with a more limited data set but fitting the same non-linear model as here, Oberts and Plevan 

(2001) found that nitrate nitrogen retention in wetland buffers was positively related to buffer width (R2 values ranged 

from 0.35 – 0.45). Nitrogen removal efficiencies of 65-75% and 80-90% were predicted for wetland buffers 15 m and 

30 m wide, respectively, depending on whether nitrate nitrogen was measured in surface or subsurface flow.

Finally, we found evidence for a threshold of nitrogen removal in buffers based on the nitrogen load entering the buffer. 

We calculated a ratio of nitrogen influent (pmm) to buffer width (m) for all studies that quantified influent loads and then 

fitted a linear model with nitrogen removal effectiveness as the dependent variable. Nitrogen removal effectiveness 

declined as the nitrogen load to width ratio increased (R2 = 0.11, P = 0.02, N = 55; Figure 7). That is, buffer effectiveness declined when nitrogen loads were high relative to buffer width. However, five studies showed low or no nitrogen 

removal effectiveness even when nitrogen loads were small relative to buffer width (Figure 7), a pattern due to the ineffectiveness of nitrogen removal in surface flows. Thus, these data were highly variable but imply a threshold for nitrogen 

removal in buffers suggesting that reducing buffer width will risk nitrogen contamination to watersheds.
6

Table 1. Summary Table of Riparian Buffer Effectiveness at Removing Nitrogen by Vegetative Cover, Hydrologic Flow Path, Buffer Width and Soil 

Type. ("nd" = not detected; "–" = data not provided by authors)

Vegetative 

Cover Type Flow Path Buffer Width N form Mean Influent (pmm) Mean Effluent (pmm) Effectiveness(%) Major Soil type(s) Study

grass surface 4.6 total N – – -15 sandy loam Magette et al. 1989grass surface 9.2 – – 35grass surface 7.5 total N 68 44 35 silty clay loam Schmitt et al. 1999grass surface 15 68 33 51grass surface 4.6 nitrate 1.86 2.37 -27 silt loam Dillaha et al. 1988grass surface 9.1 1.86 2.13 -15grass surface 4.6 nitrate – – 27 silt loam Dillaha et al. 1989grass surface 9.1 – – 57grass surface 91 total N 21.6 13.3 38 – Zirschky et al. 1989grass surface 27 nitrate 0.37 0.34 8 – Young et al. 1980grass surface 26 NH3 3.61 3.05 16 very fine sandy loam Schwer and Clausen 1989grass surface 26 TKN 48.9 11.76 76 very fine sandy loam Schwer and Clausen 1989grass subsurface 25 nitrate 15.5 6.2 60 coarse sand Vidon and Hill 2004bgrass subsurface 70 nitrate 1.55 0.32 80 fine sandy loam/silt loam Martin et al. 1999grass subsurface 39 nitrate 16.5 3 82 silty clay loam Osborne and Kovacic 1993grass subsurface 25 nitrate 12.15 1.92 84 peat/sand Hefting and de Klein 1998grass subsurface 16 nitrate 2.8 0.3 89 stony clay loam Haycock and Burt 1993grass subsurface 10 nitrate 7 0.3 96 entisols/histosols Hefting et al. 2003grass subsurface 100 nitrate 375 <5 98 – Prach and Rauch 1992grass subsurface 10 nitrate 7.54 0.05 99 silt loam Schoonover and Williard 2003grass subsurface 30 nitrate 44.7 0.45 99 sand/loamy sand Vidon and Hill 2004bgrass subsurface 50 nitrate 6.6 0.02 100 fine sandy loam Martin et al. 1999
7

Vegetative 

Cover Type Flow Path Buffer Width N form Mean Influent (pmm) Mean Effluent (pmm) Effectiveness(%) Major Soil type(s) Study

grassforest surface 7.5 total N 68 49 28 silty clay loam Schmitt et al. 1999 grassforest surface 15 total N 68 40 41grassforest subsurface 6 nitrate 6.17 0.56 91 loam/sandy loam Borin and Bigon 2002grassforest subsurface 70 nitrate 11.98 1.09 91 loamy sand Hubbard and Lowrance 1997grassforest subsurface 66 nitrate 5.8 0.17 97 gravel Vidon and Hill 2004bgrassforest subsurface 33 nitrate 5.7 0.11 98 sandy loam/loamy sand Vidon and Hill 2004bgrassforest subsurface 45 nitrate 17.8 0.18 99 peat Vidon and Hill 2004bgrassforest subsurface 70 nitrate 1.65 0.02 99 fine sandy loam/silt loam Martin et al. 1999

forest surface 30 nitrate 0.37 0.08 78 silt/stony loam Lynch et al. 1985forest surface 70 nitrate 4.45 0.94 79 fine sandy loam Peterjohn and Correll 1984forest subsurface 50 nitrate 26 11 58 entisols/histosols Hefting et al. 2003forest subsurface 200 nitrate 11 4 64 medium-coarse sand Spruill 2004forest subsurface 10 nitrate 6.29 1.15 82 silt loam Schoonover and Williard 2003forest subsurface 55 nitrate – – 83 – Lowrance et al. 1984forest subsurface 20 nitrate – – 83 – Schultz et al. 1995forest subsurface 85 nitrate 7.08 0.43 94 fine sandy loam Peterjohn and Correll 1984forest subsurface 204 nitrate 29.4 1.76 94 peat/sand/gravel Vidon and Hill 2004bforest subsurface 50 nitrate 13.52 0.81 94 loamy sand Lowrance 1992forest subsurface 60 nitrate 8 0.4 95 sand/gravel/clay Jordan et al. 1993forest subsurface 16 nitrate 16.5 0.75 95 silty clay loam Osborne and Kovacic 1993

Table 1. Continued.
8

Vegetative 

Cover Type Flow Path Buffer Width N form Mean Influent (pmm) Mean Effluent (pmm) Effectiveness(%) Major Soil type(s) Study

forest subsurface 16 nitrate 6.6 0.3 95 stony clay loam Haycock and Pinay 1993forest subsurface 15 nitrate – – 96 – Hubbard and Sheridan 1989forest subsurface 165 nitrate 30.8 1 97 peat/sand Hill et al. 2000forest subsurface 50 nitrate 6.26 0.15 98 peat/sand Hefting and de Klein 1998forest subsurface 220 nitrate 10.8 0.22 98 peat/loamy sand Vidon and Hill 2004bforest subsurface 50 nitrate 7.45 0.1 99 loamy sand Jacobs and Gilliam 1985forest subsurface 10 nitrate 13 0.1 99 silt loam Cey et al. 1999forest subsurface 100 nitrate 5.6 0.02 100 sandy clay/coarse sand Spruill 2004forest subsurface 30 nitrate 1.32 nd 100 silt clay Pinay and Decamps 1988forest subsurface 100 nitrate 12 nd 100 silt/plant debris/sand Spruill 2004

forestwetland surface – nitrate 0.34 0.07 81 sand Yates and Sheridan 1983forestwetland subsurface 31 nitrate 62.7 25.9 59 sand Hanson et al. 1994forestwetland subsurface 38 nitrate 30.6 6.7 78 sandy loam Vellidis et al. 2003forestwetland subsurface 14.6 nitrate – – 84 sandy mixed mesic Simmons et al. 1992forestwetland subsurface 5.8 nitrate – – 87 sandy mixed mesic Simmons et al. 1992forestwetland subsurface 5.8 nitrate – – 90 sandy mixed mesic Simmons et al. 1992forestwetland subsurface 6.6 nitrate – – 97 loamy mixed mesic Simmons et al. 1992forestwetland subsurface 30 nitrate 1.06 nd 100 clay loam Pinay et al. 1993

wetland surface 20

nitrate

57 50 12

peat/sand Brüsch and Nilsson 1993 wetland surface 20 57 15 74

wetland subsurface 5

nitrate

6.56 1.55 76

stony silt loam Clausen et al. 2000 wetland subsurface 5 3 1.44 52

wetland subsurface 1 nitrate 1 – 96 clay loam/clay Burns and Nguyen 2002wetland subsurface 200 nitrate 10.5 0.5 95 silt/sand/gravel Fustec et al. 1991wetland subsurface 40 nitrate 77.48 0.31 100 fine to coarse sand Puckett et al. 2002

Table 1. Continued.
9

Figure 1. Relationship of nitrogen removal effectiveness to riparian buffer width. All studies combined. Lines indicate 

probable 50%, 75%, and 90% nitrogen removal efficiencies based on the fitted non-linear model. 

Table 2. Mean and Percent Effectiveness of Riparian Buffers at Removing Nitrogen. Buffer Widths Necessary to 

Achieve a Given Percent Effectiveness (50%, 75%, 90%) are Approximate Values Predicted by the NonLinear Model, y=a*ln(x)+b. Effectiveness was not predicted (np) for Models with R2 Values <0.2 

Flow Path or 

Vegetative cover 

type

N

Mean nitrogen 

removal 

effectiveness 

(%)

1SE

Relationship to buffer 

width

Approximate buffer 

width (m) by 

predicted effectiveness

50% 75% 90% Model R2

All studies 66 74.2 4.0 y = 10.5*ln(x) + 40.5 0.137 3 28 112

Surface flow 18 33.3 7.7 y = 20.2*ln(x) – 21.3 0.292 34 118 247

Subsurface flow 48 89.6 1.8 y = 1.4*ln(x) + 84.9 0.016 np np np

Forest 22 90.0 2.5 y = -0.7*ln(x) + 92.5 0.003 np np np

Forested Wetland 7 85.0 5.2 y = -7.3*ln(x) + 104.3 0.203 np np np

Grass 22 53.3 8.7 y = 23.0*ln(x) – 13.6 0.277 16 47 90

Grass/forest 8 80.5 10.2 y = 18.1*ln(x) + 20.4 0.407 5 20 47

Wetland 7 72.3 11.9 y = 3.0*ln(x) + 68.9 0.005 np np np
10

Figure 2. Nitrogen removal effectiveness in riparian buffers by flow path. The center vertical line of the box and whisker 

plot marks the median of the sample. The length of each box shows the range within which the central 50% 

of the values fall. Box edges indicate the first and third quartiles. Whiskers show the range of observed 

values that fall within the midrange of the data. Asterisks indicate outside values.

Figure 3. Relationship of nitrogen removal effectiveness to riparian buffer width by flow path. Lines indicate probable 

50%, 75%, and 90% nitrogen removal efficiencies in the surface flow path based on the fitted non-linear 

model.
11

Figure 5. Relationship of nitrogen removal effectiveness to riparian buffer width by riparian vegetation type. Curves 

are fitted to non-linear model: y = a*ln(x) + b

Figure 4. Nitrogen removal effectiveness in riparian buffers by buffer vegetation type. The center vertical line of the box 

and whisker plot marks the median of the sample. The length of each box shows the range within which the 

central 50% of the values fall. Box edges indicate the first and third quartiles. Whiskers show the range of 

observed values that fall within the midrange of the data. Asterisks indicate outside values. Boxes identified 

with the same letters are not significantly different (P > 0.05).
12

Figure 6. Nitrogen removal effectiveness in riparian buffers by buffer vegetation type and water flow path. The center 

vertical line of the box and whisker plot marks the median of the sample. The length of each box shows the 

range within which the central 50% of the values fall. Box edges indicate the first and third quartiles. Whiskers show the range of observed values that fall within the midrange of the data. Asterisks indicate outside 

values. 

Figure 7. Relationship of nitrogen removal effectiveness to nitrogen load:buffer width ratio. Fitted to linear model: 

y=a(x)+b. 
13

Discussion

Our meta-analysis suggests that nitrogen removal in the subsurface may be more directly influenced by soil type, watershed hydrology (e.g., soil saturation, groundwater flow paths, etc.), and subsurface biogeochemistry (organic carbon 

supply, high nitrate inputs) through cumulative effects on microbial denitrification activity than on buffer width per se. 

Surface flows bypass zones of denitrification and thus effectively remove nitrogen only when buffers are wide enough 

and have adequate vegetation cover to control erosion and filter movement of particulate forms of nitrogen. Grass 

buffers, for example, may be better at intercepting particulate nitrogen in the sediments of surface runoff by reducing 

channelized flow. 

Federal Regulations and Recommendations

United States Code (USC)

Riparian buffers are noted in at least 14 parts within the USC (Appendix 1). Under the auspices of the 1972 Federal 

Water Pollution Control Act or Clean Water Act (33USC1251 et seq. as amended through P.L. 107–303, November 27, 

2002), the U.S. EPA publishes lists of impaired waters for which Total Maximum Daily Loads (TMDL) are established that 

limit the amount of a pollutant, including nitrogen, that a water body can receive and remain compliant with State water 

quality standards (33USC1251.319). States are required to implement BMP’s, such as riparian buffers or vegetated 

filter strips, to achieve compliance. U.S. EPA provides generalized recommendations and funding for riparian buffers 

and filter strips as part of Comprehensive Nutrient Management Plans (U.S. EPA 2001, 2003). However, site-specific 

BMP approaches and implementation are the jurisdiction of the States.

No comprehensive Federal statutory laws exist directly dealing with riparian buffer width even though buffers are mentioned in the USC. In some cases, site-specific legislation has been enacted that mandates protection of riparian buffers. 

The United States Congress has passed laws requiring vegetation to be left undisturbed on the sides of stream and 

river banks during specific activities. For example, in 16 USC 539(d), the National Forest Timber Utilization Program 

(a.k.a. the 1990 Tongass Timber Reform Act):

“In order to assure protection of riparian habitat, the Secretary shall maintain a buffer zone of no less than 

one hundred feet in width on each side of all Class I streams in the Tongass National Forest, and on those 

Class II streams which flow directly into a Class I stream, within which commercial timber harvesting shall be 

prohibited...”

Timber harvesting from the National Forest System has also been regulated, in general, through the National Forest 

System Land and Resource Management Plan, which states, without providing strict guidelines, that harvesting plans 

must,

“insure that timber will be harvested from National Forest System lands only where soil, slope, or other watershed 

conditions will not be irreversibly damaged” and where “protection is provided for streams, streambanks, 

shorelines, lakes, wetlands, and other bodies of water from detrimental changes in water temperatures, blockages 

of water courses, and deposits of sediment, where harvests are likely to seriously and adversely affect water 

conditions or fish habitat” (16 USC 1604).

Riparian conservation has been cited within the USC as one of the purposes for the establishment of National Parks 

and as directives to the Secretary of the Interior (e.g. 16 USC Sec. 460). As all national parks must follow the dual 

policy of both multiple and sustained yield, several subsections within this section of USC address riparian zones of 

other national parks likewise, to “contribute to public enjoyment,” “protect important resource values,” etc. In all cases, 

the statutes are site-specific, and the riparian zones discussed are between 100 and 300 ft.

Code of Federal Regulations (CFR)

Riparian buffers are noted in at least 47 parts within the CFR (Appendix 2). The CFR places blanket statutory restrictions 

on certain industrial practices in riparian areas. For instance, 30 CFR 816.57 and 30 CFR 817.57 prohibit surface and 

underground mining activities within 100 ft of perennial or intermittent streams, and 36 CFR 228.108 prohibits mining 

operations within the National Forest System “in areas subject to mass soil movement, riparian areas, and wetlands.”

Voluntary participation programs such as the Conservation Reserve Program (CRP) and the Conservation Reserve 

Enhancement Program (CREP) provide landowners financial incentives to protect land and waterbodies through maintenance of buffers, wetlands, and by planting cover crops (7CFR1410). The CRP is administered through the U.S. 

Department of Agriculture’s Farm Service Agency (USDA-FSA) (7CFR, Chap. VII) with technical assistance provided 

by the National Resources Conservation Service (USDA-NRCS) (7CFR, Chap. VI). 

The USDA-FSA makes a distinction between filter strips and riparian buffers (7CFR1410.2). Filter strip is a “a strip or 

area of vegetation adjacent to a body of water the purpose of which is to remove nutrients, sediment, organic matter, 
14

pesticides, and other pollutants from surface runoff and subsurface flow by deposition, absorption, plant uptake, and 

other processes, thereby reducing pollution and protecting surface water and subsurface water quality and of a width 

determined appropriate for the purpose by the Deputy Administrator.” No minimum widths are specified for construction 

of filter strips (NRCS 2003; practice Code 393). 

Riparian buffer (NRCS 2003; practice Code 391) is “a strip or area of vegetation adjacent to a river or stream of sufficient width as determined by the Deputy Administrator to remove nutrients, sediment, organic matter, pesticides, and 

other pollutants from surface runoff and subsurface flow by deposition, absorption, plant uptake, and other processes, 

thereby reducing pollution and protecting surface water and subsurface water quality, which are also intended to provide shade to reduce water temperature for improved habitat for aquatic organisms and supply large woody debris for 

aquatic organisms and habitat for wildlife.” 

The importance of the distinction between filter strip and riparian buffer is in their implementation. For example, according to the CRP, riparian buffers should consist of three zones. Zone 1 starts at the top of the stream bank, is devoted to trees, and has a minimum width of 15 ft measured as perpendicular from the bank. Zone 2 is predominately 

composed of riparian trees and shrubs suitable to the site and has a minimum width of 20 ft. Zone 3 is only required 

for concentrated flow conditions and is devoted to native grasses and forbs (NRCS 2003). CRP limits enrollment of 

buffers constructed for water quality to those with less than a “maximum average width of 180 feet” and an absolute 

maximum width of 350 ft.

State and Provincial Regulations and Recommendations

State (USA) and Provincial (Canada) width guidelines for forested riparian buffers associated with timber harvesting 

were recently summarized by Lee et al. (2004). State width guidelines for buffers ranged from 15.5 – 24.2 m. Provincial 

guidelines generally required wider buffers (13.8 – 43.8 m). Widest buffers were recommended in northern Canada 

and narrowest in the southeastern U.S. In some areas, buffer widths were regulated as “one size fits all.” Elsewhere, 

width recommendations were modified using factors including size/permanence of waterbody, slope of surrounding 

terrain, and presence/absence of fish.

In general, Lee et al. (2004) found that State-level “blanket” regulations addressing nitrogen attenuation and riparian 

buffer zone widths were non-existent. Many states have no mandatory buffer regulations and almost none regulate agriculture where buffers may have greatest potential for attenuating nitrogen fertilizer or livestock. However, many states 

have documented standards related to sections 303(d) and 319 of the CWA that result in site-specific maintenance of 

riparian zones for watershed protection, including nutrient attenuation. For resource agencies that do not yet have such 

regulations and wish to develop standards, numerous models and existing riparian buffer ordinances are available to 

serve as templates (U.S. EPA, undated; SCDHEC, undated). Many local governing bodies at the county, municipal, or 

district level proved additional guidance or regulation regarding riparian buffers. Wenger and Fowler (2000) indicated 

that establishing and enforcing regulations for variable-width buffers contingent upon local land use, slope, soil type, etc. 

are most difficult and suggest, rather, a model, fixed-width buffer ordinance intended to be clear and enforceable.

Other Aspects of Buffer Effectiveness

Buffer width partly accounts for nitrogen removal effectiveness of riparian buffers. However, other factors may be equally 

or more important in determining buffer effectiveness such as vegetation type and depth of the root zone where plants 

can take up nitrogen (Asmussen et al. 1979, Cooper 1990). Nitrogen also is consumed by denitrifying bacteria which 

convert nitrate to inert dinitrogen gas (Korum 1992). Therefore, riparian zones are particularly effective at removing 

nitrate where groundwater conditions favor denitrification, such as saturated soils that maintain anaerobic sites (LeedsHarrison et al. 1999, Sloan et al. 1999, Sabater et al. 2003) and carbon supplies adequate for bacterial respiration in 

the subsurface (Hanson et al. 1994; Hill et al. 2000, 2004; Steinhart et al. 2001; Schade et al. 2001, 2002; Richardson 

et al. 2004). Therefore, narrow buffers may be effective if such groundwater characteristics promoting denitrification are 

present (Dillaha et al. 1989, Simmons et al. 1992) but, as our meta-analysis showed, wider buffers tended to be more 

effective. Streams with riparian zones that remain hydrologically connected with adjacent floodplains are more likely to 

function in ways that promote denitrification (Groffman et al. 2003; Groffman et al. 2005).

For maximum and long-term effectiveness, buffer integrity should be protected against a) soil compaction from vehicles, 

livestock, and impervious surfaces (e.g., pavement) that might inhibit infiltration or disrupt water flow patterns (Dillaha 

et al. 1989; NRC 2002), b) excessive leaf litter removal or alteration of the natural plant community (e.g., raking, tree 

thinning, introduction of invasive species) that might reduce carbon-rich organic matter from reaching the stream, and 

c) urbanization and other practices that might disconnect the stream channel from the flood plain (i.e., channelization, 

bank erosion, stream incision, and drain tiles) and thereby reduce the spatial and temporal extent of soil saturation 

(Paul and Meyer 2001, Groffman et al. 2003, Groffman et al. 2005). Buffer width may indirectly affect factors promoting 

denitrification. For example, narrow buffers that produce little vegetative biomass may not provide sufficient stocks of 

organic material for microbial denitrifiers. 
15

Buffer Restoration, Planning, and Design

Creating ordinances and zoning to protect existing buffers will likely be cheaper than creating new buffers or restoring degraded ones. However, restoring buffers may be a necessary component of watershed water quality protection 

(FISRWG 1998, NRC 2002). An engineering approach thought to maximize nutrient removal capacity of buffers involves 

a multiple vegetation species or plant zone system (Welch 1991, Schultz et al. 1995, NRCS 2003). This 3-zone strategy was originally intended for protecting streams against timber harvest or agricultural use and is characterized by a 

zone of grasses and forbs immediately next to the area of disturbance, a middle zone of shrubs, and a zone of trees 

nearest to the stream channel. In theory, sediments and nutrients in surface runoff flowing from agricultural fields or 

timbered areas are intercepted first by the grass zone, while nutrients entering deeper subsurface pathways are taken 

up by shrub and tree roots (NRC 2002).

Substantial evidence exists to emphasize the importance of maintaining riparian zones in upstream headwaters or 

backwaters regions, which can be areas of high nitrogen removal (Perry et al. 1999, Morrice et al. 2000, Peterson et 

al. 2001, Seitzinger et al. 2002, Richardson et al. 2004, Bernhardt et al. 2005a). For a 10th order stream, up to 90% of 

the cumulative stream length consists of ephemeral, first, and second order streams (NRC 2002). Thus, the largest 

proportion of annual stream nutrient load enters watersheds from the headwaters where the capacity to remove nitrogen 

is great, while less additional nitrogen processing occurs in the main channels of higher order streams (Richardson et 

al. 2004, Bernot and Dodds 2005).

Many stream restoration projects are conducted to re-establish geomorphic stability (Bernhardt et al. 2005b) using approaches that potentially alter nutrient and sediment dynamics (Steiger and Gurnell 2003) in ways that may promote 

conditions for denitrification such as increasing supply and burial of organic matter, reconnecting flood plains, and 

increasing hydraulic residence time (Groffman et al. 1996). Furthermore, removal of nitrate occurs within the stream 

channel after nutrients have moved through the riparian zone and entered the hyporheic zone (Peterson et al. 2001, 

Kemp and Dodds 2002) suggesting that, in addition to establishing riparian buffers, manipulation of stream channels to 

support denitrification also should be considered as a means to manage nutrients (Groffman et al. 2005).
16
17

Summary and Conclusions

Based on current studies, riparian buffers of various types are effective at reducing nitrogen in riparian zones, especially 

nitrogen flowing in the subsurface. Buffers generally are more effective where soil type, hydrology, and biogeochemistry are conducive to microbial denitrification and plant uptake. While some narrow buffers (1-15 m) removed nitrogen, 

wider buffers (>50 m) more consistently removed significant portions of nitrogen probably by providing more area for 

root uptake of nitrogen or more sites for denitrification.

On average, State guidelines (Lee et al. 2004) recommended buffer widths that corresponded well to the minimum 

effective buffer widths necessary to improve water quality only if conditions within buffers are conducive to denitrification. Federal regulations do not stipulate minimum buffer widths for nitrogen removal from streams. Rather, riparian 

buffers represent a suggested tool to protect stream water quality and/or for removing streams from impaired listing 

due to nitrogen pollution under 303(d) section of the Clean Water Act. Federally recommended buffer widths vary from 

~7-100 m, which encompass the width range of buffers expected to remove significant amounts of nitrogen.

Buffers extending along the length of both stream banks and in which there is prolonged contact time with the root zone 

will offer greater likelihood of nitrogen uptake by plants. Buffers will be most effective at controlling nitrogen through 

denitrification when 1) water flow (overland and subsurface) is evenly distributed and soil infiltration rates are high, 

2) anaerobic (saturated) conditions persist in the subsurface, and 3) sufficient organic carbon is present. Therefore, 

to maintain maximum effectiveness, buffer integrity should be protected against soil compaction, loss of vegetation, 

and stream incision. Maintaining buffers around stream headwaters will likely be most effective at maintaining overall 

watershed water quality while restoring degraded riparian zones, and stream channels may improve nitrogen removal 

capacity. However, because streams and riparian zones have limited capacity to process nitrogen, watershed nutrient 

management efforts also must include control and reduction of point and non-point sources of nitrogen from atmospheric, 

terrestrial, and aquatic inputs. In any case, riparian buffers are a “best management practice” (BMP) that should be 

used in conjunction with a comprehensive watershed management plan (U.S. EPA 1995, NRC 2002). Finally, riparian 

buffers are often protected to achieve multiple goals (e.g. sediment trapping, aesthetics, wildlife habitat), some of which 

may require wider buffers, specific vegetation types, and/or other special considerations (Castelle et al. 1994, Wenger 

1999, Fischer and Fischenich 2000).
18
19

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24
25

Appendices

Appendix 1. Summary Table of United States Code Referring to “Riparian”, “Buffer”, “Vegetated”, and “Filter Strip”. 

(Source: http://www.gpoaccess.gov)

United States Code

Title Chapter Part(s)

16 - Conservation 1 - National Parks, Military Parks, Monuments, and 

Seashores 460

2 - National Forests 539

6 - Game and Bird Preserves, Protection 689

36 - Forest and Rangeland Renewable Resources Planning 1604

41 - Cooperative Forestry Assistance 2103 2140

58 - Land and Wetland Conservation and Reserve Program 3831 3839

25 - Indians 11 - Irrigation of Alloted Lands 381

33 - Navigation and Navigable 

Waters

9 - Protection of Navigable Waters and or

Harbor and River Improvements Generally 465

11 - Bridges over Navigable Waters 500

36 - Water Resources Development 2336

42 – The Public Welfare 19 - Water Resources Planning 1962

43 - Public Lands 23 - Grants of Swamp and Overflowed Lands 994
26

Appendix 2. Summary Table of Code of Federal Regulations Referring to “Riparian”, “Buffer”, “Vegetated”, and “Filter 

Strip”. (Source: http://www.gpoaccess.gov)

Code of Federal Regulations

Title Chapter Part(s)

7 - Agriculture

VI - National Resources Conservation Service 601 610

VII - Farm Service Agency 718

XIV - Commodity Credit Corporation 1410 1467 1469

XVII - Rural Utilities Service 1767

XVIII - Rural Housing Service, Rural Business 

Cooperative, Rural Utilities Service, and 

Farm Service Agency

1940 1943 1955

10 - Energy I - Nuclear Regulatory Commission 51

18 - Conservation 

of Power and Water 

Resources

I – Federal Energy Regulatory Commission 5 380

XIII - Tennessee Valley Authority 1304

30 - Mineral Resources VII - Office of Surface

15 80 84 715

717 780 784 816

817

36 - Parks, Forests, and 

Public Property II - Forest Service 200 228 230 292

40 - Protection of the 

Environment I - Environmental Protection Agency 122 412

43 - Public Lands, Interior

II - Bureau of Land Management

2420 2450 3420 3800

3809 4100 4120 4130

III - Utah Reclamation Mitigation and 

Conservation Commission 10005

44 - Emergency 

Management and 

Assistance

I - Federal Emergency Management Agency 60 206 209

50 - Wildlife and Fisheries

I - U.S. Fish and Wildlfe Service 17 36 37

II - National Marine Fisheries Service 222 223 226
27
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