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An Overview of the Effectiveness of Agricultural Conservation Practices for Water Quality Improvement
Yongping Yuan1,*, Ruth S. Book2, Kyle R. Mankin3, Lydia Koropeckyj-Cox1, Laura Christianson4, Tiffany Messer5, Reid Christianson4
Published in Journal of the ASABE 65(2): 419-426 (doi: 10.13031/ja.14503). 2022 American Society of Agricultural and Biological Engineers.
1Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina, USA.
Submitted for review on 25 February 2021 as manuscript number NRES 14503; approved for publication as an Invited Review Article and as part of the Agricultural Conservation Practice Effectiveness Collection by the Natural Resources & Environmental Systems Community of ASABE on 25 January 2022.
Mention of company or trade names is for description only and does not imply endorsement by the USDA. The USDA is an equal opportunity provider and employer.
- Denitrifying bioreactor and nutrient management effectively reduced nitrate-N loads from subsurface drainage.
- Constructed wetland was effective in removing total N, nitrate-N, and total P from wetland influent water.
- Filter strip was the most effective practice in reducing sediment, total N, and total P loads from surface runoff.
- Cover crop was effective in reducing sediment and total P loads from surface runoff.
- More research is needed on conservation practice effectiveness in reducing dissolved P loss from agricultural fields.
Abstract. This article introduces a Special Collection of literature reviews documenting the performance and cost-effectiveness of six agricultural conservation practices (ACPs): conservation crop rotation, cover crop, filter strip, nutrient management, denitrifying bioreactor, and constructed wetland. The overall objectives of the Special Collection are to: (1) review published studies on ACP effectiveness in reducing nutrient and sediment losses from agricultural fields; (2) compare, integrate, and synthesize the results from those studies to obtain a systematic understanding of the mitigation efficacy of each ACP in a consistent format across the selected ACPs; and (3) assemble cost analyses and obtain general insights on performance-based costs of the ACPs. The specific objectives of this introductory article are to summarize key information from each of the six review articles and develop a comparative understanding of the performance and cost-effectiveness of the six ACPs. Among the selected ACPs, denitrifying bioreactor, constructed wetland, cover crop, crop rotation, and nutrient management were all effective in reducing nitrate-N loads in subsurface drainage, with performance effectiveness in load reduction ranging from 23% to 40%. A corn-soybean rotation (relative to continuous corn) was the most cost-effective among the selected ACPs and can reduce nitrate-N load at a net benefit of about USD $5 per kg nitrate-N compared to continuous corn. Filter strip was most effective in reducing sediment, total nitrogen (N), and total phosphorus (P) loads from surface runoff and can be effective in reducing nitrate-N and dissolved P. Cover crop was also effective in reducing sediment and total P loads. Studies of the selected ACPs for their performance effectiveness for dissolved P are limited, and results varied among the ACPs included; thus, more research is needed relative to ACP effectiveness in reducing dissolved P loss, particularly in subsurface flow. Finally, although each review article included cost-analysis information, more data and analyses are needed to better understand the cost-effectiveness of ACPs and their ecological benefits.
Keywords.Constructed wetland, Cost-effectiveness, Cover crop, Crop rotation, Filter strip, Nutrient management, Denitrifying bioreactor, Reduction effectiveness.
This article introduces a Special Collection on agricultural conservation practice (ACP) effectiveness and serves as an introductory article for the collection.
Water bodies and coastal areas around the world are threatened by increases in sediment and nutrient loads, which adversely affect drinking water sources, aquatic species, and other ecologic functions and services (Haycock and Muscutt, 1995; Verhoeven et al., 2006). More specifically, increased nutrients from upstream watersheds have increased algal blooms. For example, the city of Toledo, Ohio, shut down its drinking water supply in response to harmful algal blooms detected in Lake Erie in the summer of 2014, which affected more than half a million residents. Another example is the Gulf of Mexico, where increased occurrences of seasonal hypoxia have been linked with increased nutrient fluxes from the Mississippi River basin (NSTC, 2000; USEPA, 2015; Alexander et al., 2008; Rabalais et al., 2001; Dale et al., 2007).
Many agricultural conservation practices (ACPs) have been implemented to reduce nutrient and sediment losses from farm fields, and in turn reduce negative impacts on downstream water bodies and their ecological functions. The term “best management practices” (BMPs) is often used as a synonym for ACPs, but it can be misleading because an ACP is only “best” when it is appropriate for a site. When discussing conservation practice effectiveness in general, ACP is the preferred term. For more than three decades, significant government investment has been provided to implement ACPs intended to reduce nutrient and sediment losses from upstream watersheds and in turn reduce negative impacts on downstream water bodies and their ecological functions. Despite these efforts, continued eutrophication, hypoxia, and harmful algal blooms persist in the nation’s waters (Fang et al., 2019). Additional information and research are needed to achieve a better understanding of the comparative effectiveness of ACPs so that improved environmental outcomes can be achieved with the limited funding available. This article synthesizes available information on the effectiveness ACPs in reducing nonpoint-source pollution and their relative costs. By presenting available data and highlighted comparisons, such information facilitates more effective ACP policies and program implementation, both in performance and cost.
This Special Collection provides a comprehensive review and evaluation of the performance and cost-effectiveness of six ACPs on nutrient and sediment reduction (table 1). The included ACPs can be referenced in the USDA-NRCS National Handbook of Conservation Practices (USDA-NRCS, 2019).
Table 1. Summary of agricultural conservation practices reviewed in this Special Collection. Agricultural Conservation Practice USDA-NRCS Code Special Collection Reference Supplemental Material[a] Conservation crop rotation 328 Koropeckyj-Cox et al. (2021) 14017 Cover crop 340 Christianson et al. (2021a) 14028 Filter strip 393 Douglas-Mankin et al. (2021) 14169 Nutrient management 590 Liu et al. (2021) 14078 Denitrifying bioreactor 605 Christianson et al. (2021b) --- Constructed wetland 656 Messer et al. (2021) ---
[a] As applicable, supplemental material and appendices are available at asabe.figshare.com. The supplemental material for each article can be found by searching figshare for the number indicated in this column.
The objectives of this introductory article are to: (1) summarize key information from the six review articles in the Special Collection, each of which summarize published studies on the effectiveness of a specific ACP, and (2) develop a comparative understanding of ACP performance effectiveness and cost-effectiveness. Specifically, each article:
- Compares, integrates, and synthesizes results from studies conducted in different experimental settings and site conditions to obtain a systematic understanding of mitigation efficacy.
- Performs cost analyses and obtains general insights on performance-based costs of ACPs in reducing nutrient and sediment losses from agricultural fields.
- Develops recommendations for cost-effective ACPs to be considered for prioritization when funding agencies are developing their programs.
- Collects defensible scientific data to support and update the technical content in the USDA-NRCS conservation practice standards and to help the USDA-NRCS and other organizations document the magnitude of the nutrient reduction efforts in watersheds nationwide.
ACP Performance Effectiveness
Measures of Performance Effectiveness
Performance effectiveness is commonly calculated using the percent mass load and/or concentration reduction compared to a baseline (e.g., Smith et al., 2019; Chaubey et al., 2010). Ideally, both percent mass load and concentration should be evaluated because mass loads are intuitive and helpful for understanding the contributions of different sources to meeting federal total maximum daily load (TMDL) goals within a watershed, while nutrient concentrations are useful for assessing water quality criteria from human health and ecotoxicity perspectives. Because the ultimate TMDL goal of mass load reduction is to lower the pollutant concentrations to meet the water quality criteria for human health and ecotoxicity, the two approaches are directly related.
To calculate loads, data on stream discharge and pollutant concentrations, collected at the same measurement site, are required; however, this can be challenging due to the complexity of measuring flow volume. Thus, influent and effluent nutrient concentrations are usually reported for the constructed wetland instead (Messer et al., 2021). Furthermore, using concentration, especially flow-weighted concentration, which removes the impacts of different flow volumes resulting from variability in precipitation patterns at both spatial and temporal scales, is an advantage in assessing effectiveness (Liu et al., 2021). Other studies have acknowledged the challenges of quantifying mass loads for effectiveness assessments (Christianson and Harmel, 2015; Ni et al., 2020). For example, Christianson and Harmel (2015) separated their dataset into “wet” (>850 mm) and “dry” (<820 mm) years based on the approximate mean and median precipitation to reduce the impact of precipitation on the effectiveness assessment.
For the articles in this Special Collection, either mass loads, concentrations, or both were reported, depending on data availability. Performance effectiveness of the selected ACPs was assessed in comparison with a baseline conventional practice or a baseline of no conservation practice. Table 2 summarizes the methods used in each ACP assessment, and details can be found in each original article.
Table 2. Summary of performance effectiveness calculations for six agricultural conservation practices. Agricultural Conservation Practice Comparison with Baseline Comments Conservation crop rotation Corn-soybean rotation
vs. continuous corn
Percent mass load reduction. Direct water quality (nitrate-N) comparisons
were only available for corn-soybean rotation in relation to continuous corn.
Cover crop Cover crop vs. no cover crop Percent mass load and percent concentration reduction. Filter strip Filter strip vs. no filter strip Percent mass load reduction. Nutrient management Reduced application rate
categories vs. very high
application rate category
Percent concentration reduction. Four application rate categories based
on the rate quartiles in the data: low (<134 kg N ha-1), moderate (134 to
167 kg N ha-1, high (167 to 200 kg N ha-1, and very high (>200 kg N ha-1)
Denitrifying bioreactor Outflow vs. inflow Percent mass load and percent concentration reduction, calculated as:
(inflow concentration/load – outflow concentration/load)
Constructed wetland Outflow vs. inflow Percent concentration reduction, calculated as:
(inflow concentration – outflow concentration)
Performance Effectiveness for Runoff Volume
Performance effectiveness for runoff volume was presented only by the filter strip review (Douglas-Mankin et al., 2021). Runoff reduction averaged 52% (n = 197) and was significantly related to filter strip width. Runoff reduction tended to increase with increasing width up to about 15 m, plateauing at about 70% reduction (Douglas-Mankin et al., 2021).
Performance Effectiveness for Sediment
The reviews of cover crop and filter strip included sediment reduction, and both ACPs are very effective in removing sediment load from runoff (table 3). The focus of the cover crop review (Christianson et al., 2021a) was mainly on N and P. Cover crop effectiveness in reducing sediment loss was briefly reviewed in relation to its effectiveness in reducing P loss, although much of the earlier cover crop research considered cover crop for erosion control. Most research was done using grass (n = 12); however, some research simply reported winter cover. With the low counts for all cover crops other than grass, and overlapping ranges for all categories, an aggregated mean estimate of 73% reduction is presented (table 3). Sediment mass load reduction for filter strip averaged 78% (range: -26% to 100%, n = 198) (Douglas-Mankin et al., 2021). Sediment reduction was significantly related to filter strip width and tended to increase with increasing width up to about 20 m, plateauing at about 80% to 90% reduction in mass load.
Some of the reviewed ACPs are not suitable for addressing sediment issues. A denitrifying bioreactor designed according to USDA-NRCS Conservation Practice Standard 605 generally is not intended for sediment removal (USDA-NRCS, 2015b); however, a bioreactor can provide excellent sediment removal if used in a high-solids/sediment application (Choudhury et al., 2016; Tanner et al., 2012; Christianson et al., 2016, 2021b) but at the expense of decreased design life for nitrate-N reduction.
Table 3. Performance effectiveness for sediment load reduction in surface runoff for selected agricultural conservation practices. Agricultural
Load Reduction (%) Min 25th
Max Cover crop Aggregated (n = 25) 0 54 73 95 100 Grass (n = 12) 90 94 96 100 100 Filter strip (n = 198) -26 68 78 95 100
Performance Effectiveness for Nitrate-N
All studies in the Special Collection reported performance effectiveness for nitrate-N (table 4) and summarized those results based on flow paths, i.e., subsurface drainage discharge, leachate from surface to subsurface, and surface runoff. For subsurface drainage discharge, mean load reductions ranged from 23% to 40%. Denitrifying bioreactor had the highest mean load reduction and could reduce nitrate-N load by up to 99%. In addition to denitrifying bioreactor, crop rotation and nutrient management are effective ACPs for removing nitrate-N from subsurface drainage discharge.
For crop rotation, extended crop rotations such as corn-soybean-wheat and corn-oat-alfalfa-alfalfa are generally more effective in preventing N losses than more conventional cropping systems, such as continuous corn (CC) or corn-soybean (CS) rotations. However, only a few studies were available on extended crop rotations, and more studies are needed to improve our understanding. Direct comparisons were only available between CC and CS rotations; changing from CC to CS reduces nitrate-N load from subsurface drainage discharge by 33% on average (63% maximum). Although the simple CS rotation has become ubiquitous in some parts of the U.S. and is often dismissed as an ACP, CS consists of a series of crops in the same field over a given rotation cycle or time period (USDA-NRCS, 2015a). Here, CS is used to represent the crop rotation ACP because of the availability of direct comparison studies.
Regarding nutrient management, changing fertilizer application rate from very high to high reduces mean nitrate-N load in subsurface drainage discharge by 29% (table 4). Furthermore, constructed wetland is effective in reducing nitrate-N concentrations in subsurface drainage discharge (mean 30%, maximum 92%) and in surface runoff (mean 45%, maximum 100%) based on wetland effluent and inflow concentrations. The effectiveness of cover crop on nitrate-N removal depends on the cover crop type, with mean load reductions ranging from 23% for cereal rye to 40% for mixes with grass, based on data from subsurface drainage discharge and leachate from surface to subsurface. For surface runoff, filter strip reduces mean loads by 34% (100% maximum) and wetland reduced mean concentration by 45% (100% maximum).
Table 4. Performance effectiveness for nitrate-N load reduction (%) and/or concentration reduction (%) in subsurface drainage discharge, in leachate from surface to subsurface, and in surface runoff for selected agricultural conservation practices. Agricultural Conservation Practice[a] Load Reduction[b] Minimum 25th Quartile Mean 75th Quartile Maximum Crop rotation impact on subsurface drainage (n = 7) -75% (---) --- 33% (---) --- 63% (---)
Denitrifying bioreactor impact on subsurface drainage(n = 15 for load, n = 18 for concentration)[c]
9% (10%) 16% (41%) 40% (58%) 55% (70%) 99% (99%) Cover crop impact on subsurface drainage and leachate
Cereal rye (n = 58 for load, n = 34 for concentration)
-64% (-10%) -4% (8%) 23% (33%) 56% (55%) 100% (94%)
Grass including cereal rye (n = 72 for load, n = 48 for concentration)
-64% (-10%) 0% (12%) 26% (34%) 55% (52%) 100% (94%)
Mixed with grass (n = 11 for load, n = 9 for concentration)
-13% (-9%) 21% (27%) 40% (42%) 63% (69%) 74% (72%) Filter strip impact on surface runoff (n = 103) -1130% (---) 25% (---) 34% (---) 78% (---) 100% (---) Constructed wetland (for cropland treatment) Impact on subsurface drainage (n = 11) --- (-29%) --- --- (30%) --- --- (92%) Impact on surface runoff (n = 28) --- (11%) --- --- (45%) --- --- (100%) Nutrient management impact on subsurface drainage (43 studies, 557 site-years in total) Low (<134 kg N ha-1) --- --- 28% (38%) --- --- Moderate (134 to 167 kg N ha-1) --- --- 28% (22%) --- --- High (167 to 200 kg N ha-1) --- --- 29% (14%) --- --- Very high (>200 kg N ha-1) --- --- 0 (0) --- ---
[a] Continuous corn was used as the baseline for studies that directly compared practices. The number of studies (n ) used to calculate the effectiveness is shown in parentheses.
[b] Numbers shown in parentheses are concentration values; “---” indicates no data.
[c] Data from appendix of Christianson et al. (2021b) in this Special Collection (15 site-years for load reductions, and 18 site-years for concentration reductions). Load reductions may not include untreated bypass flow.
Performance Effectiveness for Total N
Performance effectiveness data for total N (table 5) were available for cover crop, filter strip, and constructed wetland. For denitrifying bioreactor, much of the total N load in subsurface drainage discharge is nitrate-N. Thus, total N reductions for denitrifying bioreactor are not reported here but would be generally consistent with table 4 for nitrate-N. The effectiveness of a cover crop on total N removal depends on the cover crop type, with efficacies of 34% for cereal rye and 5% for legumes based on data from the Chesapeake Assessment Scenario Tool (CAST, 2017) and reported by Christianson et al. (2021a). For surface runoff, filter strip reduces mean total N loads by 57% (98% maximum), and the reduction tends to increase with increasing width up to about 20 m, peaking at about 80% reduction (Douglas-Mankin et al., 2021).
Table 5. Performance effectiveness for total N load reduction (%) and/or concentration reduction (%) in subsurface drainage discharge, in leachate from surface to subsurface, and in surface runoff for selected agricultural conservation practices. Agricultural Conservation Practice Load Reduction[a] Minimum 25th Quartile Mean 75th Quartile Maximum Cover crop impact on subsurface drainage and leachate[b] Cereal rye --- --- 34% (---) --- --- Grass including cereal rye --- --- 25% (---) --- --- Mixed with grass --- --- 17% (---) --- --- Legumes --- --- 5% (---) --- --- Filter strip impact on surface runoff (n = 71) -6% (---) 41% (---) 57% (---) 80% (---) 98% (---) Constructed wetland (for cropland treatment) Impact on subsurface drainage (n = 11) --- (-20%) --- --- (24%) --- --- (77%) Impact on surface runoff (n = 28) --- (-127%) --- --- (14%) --- --- (97%)
[a] Numbers in parentheses are concentration values; “---” indicates no data.
[b] From CAST (2017) for normal planting time (after harvest of long-season crop such as soybean).
Based on constructed wetland studies, mean percent concentration reductions for total N ranged from 14% to 53% depending on the influent source (Messer et al., 2021). Wetlands receiving agricultural surface runoff had mean total N percent removals of 14%, while wetlands receiving subsurface drainage had mean percent removals of 24%, calculated based on wetland effluent and inflow concentrations. In contrast, subsurface flow constructed wetlands receiving livestock manure had an average mean total N removal of 53%, while surface flow constructed wetlands receiving livestock manure had mean total N removal of 43%. Wetlands with higher percent removals of total N were observed in systems receiving significantly high total N influent concentrations (Messer et al., 2021).
Performance Effectiveness for Dissolved P
Performance effectiveness data on dissolved P (table 6) were available for filter strip and constructed wetland. A denitrifying bioreactor can be a source or a sink of dissolved or total P. The mechanisms and consistency of P removal are unclear and are suggested for future research (see additional discussion in Christianson et al. (2021b). Few studies exist that report annual dissolved P reductions due to cover crop implementation. The impact of a cover crop on dissolved P in subsurface drainage was also highly variable even within the single four-year study presented here. Of the nine site-years meeting criteria for inclusion in Christianson et al. (2021a), four used cereal rye. Cereal rye showed a load reduction range of -35% to 93%, with an average of 29% (table 6). Filter strip reduced dissolved P loads in surface runoff by 47% on average (range: -108% to 100%, n = 95) (Douglas-Mankin et al., 2021), and constructed wetland reduced mean concentrations from surface runoff. The impact of constructed wetland on dissolved P in subsurface discharge was highly variable depending on the influent source, and the mean concentration reduction is a negative value (-31%), which means that, on average, the dissolved P concentration is higher in constructed wetland effluent than in the influent. Finally, one study (Koropeckyj-Cox et al., 2021) showed that corn-soybean rotation reduces dissolved P by 39% and corn-soybean-wheat rotation reduces dissolved P by 58%, but further research is needed to better understand the effectiveness of crop rotation in reducing dissolved P.
Table 6. Performance effectiveness for dissolved P load reduction (%) and/or concentration reduction (%) in subsurface drainage discharge, in leachate from surface to subsurface, and in surface runoff for selected agricultural conservation practices. Agricultural Conservation Practice Load Reduction[a] Minimum 25th Quartile Mean 75th Quartile Maximum Cover crop impact on subsurface drainage and leachate[b] Cereal rye -35% (19%) 6% (29%) 29% (43%) 53% (46%) 93% (88%) Filter strip impact on surface runoff (n = 71) -108% (---) 27% (---) 47% (---) 81% (---) 100% (---) Constructed wetland (for cropland treatment) Impact on subsurface drainage (n = 11) --- (-291%) --- --- (-19%) --- --- (78%) Impact on surface runoff (n = 28) --- (-138%) --- --- (24%) --- --- (100%)
[a] Numbers in parentheses are concentration values; “---” indicates no data.
[b] From CAST (2017) for normal planting time (after harvest of long-season crop like soybeans).
Performance Effectiveness for Total P
Performance effectiveness for total P (table 7) was available for cover crop, filter strip, and constructed wetland. There were no studies reporting annual reductions in total P for cover crop; however, state and regional assessments have estimated total P reductions due to cover crop (Christianson et al., 2021a). In the regional assessment presented in the Chesapeake Assessment Scenario Tool (CAST, 2017), cover crop showed 7%, 5%, and 3% total P load reductions for cereal rye/barley/wheat, various mixes, and legumes, respectively. In assessments in Arkansas, Illinois, Iowa, and Minnesota, cover crop was shown to have total P reductions of 30% (Christianson et al., 2021a). These differences can be attributed to the combination of literature and professional judgement used in the assessments.
Table 7. Performance effectiveness for total P load reduction (%) and/or concentration reduction (%) in subsurface drainage discharge, in leachate from surface to subsurface, and in surface runoff for selected agricultural conservation practices. Agricultural Conservation Practice Load Reduction[a] Minimum 25th Quartile Mean 75th Quartile Maximum Cover crop impact on subsurface drainage and leachate[b] Cereal rye --- --- 7% (---) --- --- Mixed with grass --- --- 5% (---) --- --- Legumes --- --- 3% (---) --- --- Grass, nutrient reduction strategies[c] --- --- 30% (---) --- --- Filter strip impact on surface runoff (n = 19) -15% (---) 46% (---) 63% (---) 87% (---) 100% (---) Constructed wetland (for cropland treatment) Impact on subsurface drainage (n = 11) --- (-256%) --- --- (-31%) --- --- (67%) Impact on surface runoff (n = 28) --- (-109%) --- --- (35%) --- --- (89%)
[a] Numbers in parentheses are concentration values; “---” indicates no data.
[b] From CAST (2017) for normal planting time (after harvest of long-season crop such as soybean).
[c] From the Arkansas, Illinois, Iowa, and Minnesota Hypoxia Task Force nutrient strategies. For more information see Christianson et al. (2018) and FTN Associates (2019).
Filter strip is very effective in reducing total P from surface runoff, with load reductions averaging 63% (range: -15% to 100%, n = 119) similar to total N (Douglas-Mankin et al., 2021). In addition, data from the literature showed that total P reduction tended to increase with increasing width up to about 20 m, peaking at about 100% reduction. The reported mean reduction in total P concentration in the constructed wetland study was 35% with a maximum of 89% for surface runoff, depending on the nutrient source (Messer et al., 2021). However, similar to dissolved P, constructed wetland is not effective in reducing total P from subsurface drainage (table 7).
Cost- Effectiveness Analysis for Selected ACPs
Knowing the costs and benefits of ACPs is essential for landowners, government agencies, and other stakeholders to make informed decisions related to ACP implementation. Cost-effectiveness analysis can be used to compare the relative costs and outcomes (effects) of different courses of action. In this section, we summarize how performance-based costs and their benefits were determined in each review article, if applicable.
Conservation Crop Rotation and Nutrient Management
The cost-benefit analysis methods for conservation crop rotation and nutrient management effectiveness were based on simple calculations of net revenue, i.e., subtracting the total costs of production from the gross revenues from crop sales for each management scenario (Koropeckyj-Cox et al., 2021; Liu et al., 2021). This is similar to the USDA-NRCS (2020) recommended method of comparing practice benefits to costs and estimating the net effects. For conservation crop rotation and nutrient management, statistics on crop prices, yields, production costs, and fertilizer prices were gathered primarily from the USDA National Agricultural Statistics Service (NASS) and the Iowa State University Cooperative Extension Service. Iowa was chosen as the main location for cost-effectiveness analyses due to the availability of specific production costs through Iowa State University’s Ag Decision Maker (Iowa, 2020) and the state’s impact on water quality in the Upper Mississippi River basin (David et al., 2010; Jones et al., 2018; Piske and Peterson, 2020; Saad and Robertson, 2020). After net revenue was calculated, the environmental benefits for each management scenario were estimated.
Mean annual load reductions of nitrate-N for the low (<134 kg N ha-1), moderate (134 to 167 kg N ha-1), and high (167 to 200 kg N ha-1) fertilizer rate categories were compared with the very high category (>200 kg N ha-1) (Liu et al., 2021). Similarly, using the median value of annualized net revenue for each fertilizer rate category, revenue reductions for the low, moderate, and high categories were also calculated and compared to the baseline (very high).
To evaluate the cost-effectiveness of cover crop, several efforts were evaluated, including peer-reviewed literature and state-level nutrient reduction strategies (Christianson et al., 2021a). Because cereal rye is still the primary cover crop, the costs associated with planting and management of cereal rye were used. Considering the regional differences in costs, as well as the region/method/strategy specific range in load reductions associated with cover crops, information on the cost of N and P reductions were limited to nutrient reduction strategies in two states (Illinois and Iowa).
To evaluate the cost-effectiveness of constructed wetland, life cycle economic cost data were extracted from seven studies in which wetland costs, including capital and operating costs, were reported relative to nutrient retention (Messer et al., 2021). To facilitate comparison among studies, all reported capital and recurring costs were converted to 2019 dollars, and annualized life cycle costs were determined using a common timeframe of 15 years (consistent with USDA-NRCS constructed wetland design life) and a discount rate of 3.4% (consistent with 2019 USDA discount rates).
Costs and cost-efficiencies for denitrifying bioreactor were reviewed and compared (Christianson et al., 2021b). Values in the literature included installation costs ($ installed), volumetric cost-efficiency ($ installed m-3 of bioreactor), area-based cost-efficiency ($ installed ha-1 drainage area treated), and annualized cost-efficiencies (annualized $ per mass N removed, $ kg-1 N year-1). Annualized cost-efficiencies are generally the only “apples to apples” method to compare upfront capital-intensive ACPs such as denitrifying bioreactor to annual practices such as cover crop. The reported annualized cost-effectiveness values considered planning horizons of 10 to 20 years and discount factors of 4% to 11%.
Beyond the opportunity costs associated with taking land out of production, other filter strip costs include site preparation, establishment costs including seed, and management costs (Douglas-Mankin et al., 2021). To fully evaluate the cost-effectiveness of these practices, ancillary environmental benefits from filter strip installation must also be quantified, such as hydrological sensitivity, soil erodibility, wildlife habitat, and water quality improvements. While these factors may be difficult to quantify monetarily, future research should work to quantify these factors, including not only water quality benefits but a broader range of ecosystem services.
Synthesizing Cost-Effectiveness Analysis for Selected ACPs
Based on data that directly compared a corn-soybean rotation to continuous corn, the mean annual nitrate-N loss from corn-soybean was much lower than from continuous corn. In addition, the annualized net revenue from the corn-soybean rotation was much higher than from continuous corn, yielding a net benefit (not cost) of $5 kg-1 nitrate-N. Limited data on extended rotations precluded a complete cost-effectiveness analysis for the crop rotation ACP.
Table 8. Cost-effectiveness of NO3-N reduction for selected ACPs. Agricultural
Annual Cost of Reduction
Mean Max Crop rotation[a] --- -5 --- Denitrifying
5,000 to 27,000 2.5 to 20 48 Nutrient
--- 0.8 to 3.8 (CS)
3.5 to 6.9 (CC)
[a]Change to corn-soybean from continuous corn. Negative cost of reduction indicates a benefit gained.
[b] Ranges of costs are due to different studies.
[c]Change to moderate or high N rate from very high. Ranges of costs are due to different management practices (CS = corn-soybean rotation, and CC = continuous corn).
For nutrient management, there was a “sweet spot” where the revenue from corn yields exceeded the cost of N fertilizer and where corn yields were not hindered by lower N fertilizer application rates. This “sweet spot” seemed to occur for corn receiving the moderate fertilizer rate in rotation with soybean. In general, it costs $4 to $7 to reduce 1 kg of nitrate-N from the very high fertilizer rate to either the high or moderate fertilizer rate for continuous corn, and $1 to $4 kg-1 nitrate-N for the corn-soybean rotation. Regarding denitrifying bioreactor, although the initial cost was relatively high ($5,000 to $27,000), this ACP appeared to be cost-effective, with a cost range of $2.50 to $20 kg-1 nitrate-N (table 8).
The cover crop review article focused on total N and total P. The cost data are based on considerations of cost outlays only, although new research is showing a net benefit (return) when considering long-term impacts on soil health and added resiliency (Abdollahi and Munkholm, 2014). Cover crop was found to be cost-effective in reducing total N, with an annual cost of $3.25 kg-1 N (table 9), while this ACP costs $47 kg-1 total P reduced (table 10). Different from cover crop, filter strip was very cost-effective in reducing both total N and total P, with $1 to $5 kg-1 total N and $2 to $16 kg-1 total P (tables 9 and 10). In addition, filter strip was more cost-effective on conventional tillage fields than on no-till fields. Finally, filter strip reduced sediment losses at a cost of $1.4 to $10.3 Mg-1 for conventional tillage fields and $12.5 to $18.6 Mg-1 for no-till fields (Douglas-Mankin et al., 2021).
For constructed wetland, life cycle costs ranged from $18 to $438 ha-1 contributing drainage area per year, with a mean of $162 ha-1 treatment area year-1. Nitrogen removal efficiencies ranged from $0.66 to $58 kg-1 N year-1 (mean: $15.7 kg-1 N year-1). Reported P removal efficiencies were even more variable, ranging from $0.1 to $1,923 kg-1 P year-1 (mean: $810 kg-1 P year-1). The variability in life cycle costs for wetlands can be partly explained by the variability in measured or assumed nutrient removal rates; for example, the N and P removal rates used by Lentz et al. (2013) spanned three orders of magnitude. Additional variability in reported costs is associated with, for example, the reliance of some systems on pumps. Differences in foregone land rental costs included in the analyses also contributed to the observed spread in the data.
Summary and Conclusion
This article introduces a Special Collection of six articles that review the effectiveness of ACPs for pollutant reduction from agricultural areas and summarizes and compares the key findings. The review articles focused on six ACPs: conservation crop rotation, cover crop, filter strip, nutrient management, denitrifying bioreactor, and constructed wetland. Each review article provided detailed information about an ACP, including its characteristics, location, and primary objectives, and summarized the ACP’s performance effectiveness for removal of pollutants, such as sediment, nitrogen, and phosphorus. Each review article also presented information on cost-effectiveness, based on either independent cost-analysis or literature review.
There are more studies in the literature on the effectiveness of these ACPs for N load reduction than for P load reduction. Each review article presented performance effectiveness for nitrate-N load reduction, which ranged from 23% to 40%, although higher reduction rates occurred for many of the ACPs in a given year. More research is needed to explicitly determine the causes of the annual variability in reduction rates and to determine how practice management (e.g., cover crop planting time) impacts annual reductions. Filter strip and cover crop are very effective in reducing total N loads. In fact, filter strip is the most effective ACP, with average performance effectiveness of 57%. Limited studies found that crop rotation was effective in reducing both dissolved P and total P loads; however, more studies are needed to validate the effectiveness of this ACP. Cover crop, filter strip, and constructed wetland showed mixed effectiveness for dissolved P load reduction, with filter strip ranging from -108% to 100% (n = 95). Constructed wetland is more effective in reducing dissolved P and total P from surface runoff than from subsurface drainage flow. Finally, filter strip is the most effective ACP for reducing total P on a load basis, with an average reduction of 63%.
Although crop rotation was shown to be the most cost-effective of the six ACPs for reducing nitrate-N load, with a net benefit of about $5 kg-1 nitrate-N reduction for corn-soybean rotation compared with continuous corn, the information is based only on data for corn-soybean rotation. Nutrient management showed a high cost-effectiveness of $1 to $7 kg-1 nitrate-N reduction, depending on the crop rotation. However, filter strip may be the most cost-effective ACP if considering the types of pollutants removed and the other ecosystem benefits provided. For all ACPs, more research is needed to better quantify their cost-effectiveness.
The USEPA, through its Office of Research and Development, funded and managed the research described here. It has been subjected to agency review and approved for publication. Although this article has been reviewed and approved for publication by the USEPA and USDA, the views expressed in this article are those of the authors and do not necessarily represent the views or policies of the agencies. The authors would like to thank Dr. Brent Johnson from the USEPA, the journal editors, and the anonymous reviewers for their technical review and valuable suggestions, which helped improve the manuscript.
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