How effective are slurry storage, cover or catch crops, woodland creation, controlled trafficking or break-up of compacted layers, and buffer strips as on-farm mitigation measures for delivering an improved water environment?
Louise M Donnison, Paul J Lewis, Barbara Smith (Game and Wildlife Conservation Trust) and Nicola P Randall (Lead Reviewer)
Over the last fifty years, European agriculture has become more intensive due to increased applications of fertilizers and agrochemicals to agricultural land. Currently 50% of the nitrates in European rivers are estimated to be from agricultural sources. In the UK, agricultural activities are estimated to contribute 70% of nitrates, 28% of phosphates and 76% of sediments measured in rivers. River waters of catchments dominated by agricultural land use can have elevated levels of pesticides and bacterial pathogens.
The aim of this systematic review was to assess the effectiveness of slurry storage, cover/catch crops, woodland creation, controlled trafficking/break-up of compacted layers and buffer strips, as on-farm mitigation measures, for delivering an improved water environment.
Methods and outline results
Electronic databases, the internet, and organisational websites were searched to find articles that investigated the impact of the on-farm mitigation measures on water quality. The searches identified 146, 941 records (excluding Google Scholar and web searches). The removal of duplicates and irrelevant articles from the search results left 718 records.
The 718 relevant articles were coded to create a searchable Microsoft Access database (systematic map) which describes the water quality research to date for the topic specific mitigation measures. All evidence was coded with country of study, mitigation and water quality measurement, if the information was missing then not clear was recorded. Additionally full text articles were coded for study design and those studies without confounding factors were coded for outcome. The database can be used to sort or filter on category and provide simple numerical counts.
The systematic map database was composed of mainly buffer strip (including trees) and cover/catch crops studies. The map also contained some slurry storage studies which were diverse and often at least 10 years old. There were only a few woodland creation studies in the map as most studies composed of trees were categorized under buffer strips, the studies that remained measured water quality after afforestation on former agricultural soil or planting of tress for biomass. Very little evidence was found for subsoiling (break up compacted soil) or controlled traffic on grassland.
There were 467 studies coded in the systematic map at full text (including studies with confounding factors) which were given a value for scientific rigour based on whether they were randomized, controlled, replicated (spatial or temporal), designed (manipulative, correlative or sampling) and conducted for longer than a year. These values can be used to provide a rudimentary indication of the type of research available for each mitigation.
There were 410 studies coded in the systematic map at full text (excluding studies with confounding factors) which were given a value for effectiveness in reducing N, P, sediment, pesticides or bacterial pathogens in water. These values were used to provide a rudimentary indication of the overall effectiveness of each intervention on specified outcomes, based on the available evidence.
A meta-analysis was conducted to assess the effectiveness of cover/catch crops in reducing nitrate leaching as compared to a fallow no vegetation control. The meta-analysis suggested a consistent positive effect in of cover/catch crop in reducing leaching Nitrate when compared to a fallow, and that there was no difference in effectiveness of cereals and brassicas for reducing Nitrate leaching. Only 10 studies were included in the meta-analysis due to difficulties in extracting data from primary studies.
Buffer strips (Grass and tree buffers)
Buffer strips composed of grass and/or trees are thought to improve water quality by physically trapping sediments and associated pollutants, and by immobilizing soluble nutrients through plant uptake or microbial degradation.
Average effectiveness values suggested that buffer strips were most effective for reducing sediment, followed by pesticides, N, P, and bacterial pathogens (in decreasing order), however these values should be interpreted within the limitations of the evidence. Pre-existing meta-analyses also found that buffer strips could be effective in improving water quality.
Buffer strips were the most commonly studied mitigation in the database (225 studies with data that enabled assessment of effectiveness of the intervention). Over half of the studies were manipulative (n=147), at least a third were controlled (n=104) and often fully replicated. Nearly half of the studies were conducted for longer than a year, but not many studies were randomized. Most of the buffer studies were field or plot based (n=187), often on loam soils (n=121).
Limitations of the evidence base
Studies were often at a field scale which may not capture the effects of preferential flow paths or buffer strip placement on buffer strip performance. Studies were often on loam or unknown soil types, which may not capture the effect of soil particle size on buffer strip performance. Studies often assessed effectiveness over short periods of time, which may not capture changes in buffer strip effectiveness over time. Buffer strip effectiveness may depend on experimental factors such as vegetation types, but this was not investigated. Only a third of the studies had data for all four seasons, yet season may have an impact on effectiveness due to seasonal differences in plant growth and nutrient uptake.
61% of buffer strip studies investigated the effectiveness of buffers for reducing N of buffer studies, (n=139).
Authors indicated that buffer strips are generally effective for reducing at least one type of N (72% of buffer studies measuring N, n=100), but that this varied for different forms. Authors indicated that buffers strips were more effective at reducing Total-N (74% of buffer studies measuring Total-N, n= 29) and nitrate-N (67% of buffer studies measuring nitrate, n= 80), than ammonium-N (50% of studies measuring ammonium, n=23).
44% of buffer strip studies investigated the effectiveness of buffers for reducing sediments (n=98). Authors indicated that buffer strips are generally effective for reducing sediments (87% of buffer studies measuring sediments, n=85).
42% of buffer strip studies investigated the effectiveness of buffers for reducing P (n=94).
Authors indicated that buffer strips could be effective for reducing at least one type of P (65% of studies measuring P, n=61) but that this varied for different forms of P. Buffers strips appeared to be more effective at reducing total-P (73% of buffer studies measuring total- P, n= 46), than orthophosphate-P (55% of buffer studies measuring orthophosphate, n=23) or soluble-P (26% of buffer studies measuring soluble P, n=5).
17% of buffer strip studies investigated the effectiveness of buffers for reducing pesticides (n=38), often using atrazine (68% of buffer studies measuring pesticide, n=26) or metolachlor (32% of buffer studies measuring pesticide, n=12).
Authors indicated that buffer strips are generally effective for reducing at least one of the 38 pesticides measured (71% of studies measuring pesticide, n=27).
Bacterial pathogen counts
Only 8% of buffer strip studies investigated the effectiveness of buffers for reducing bacterial pathogen counts (n=19). 63% of studies measuring bacterial pathogen counts were effective at reducing at least one of the bacterial pathogen count measurements (n=12).
Fast-growing cover or catch crops, planted over the winter months are designed to improve water quality by protecting the soil against erosion thereby minimizing the risk of runoff, and reducing the risk that nutrients are leached from the root zone.
The Evidence indicated that cover crops are most effective at reducing leaching of N and of sediments into water courses.
Cover/catch crops were the second most commonly studied mitigation (n=132 studies scored for effectiveness). Most studies were manipulative (n=125), controlled (n=115), fully replicated and conducted for longer than a year and sometimes randomized. 84% of cover/catch crop studies were field or plot based (n=111) often on loam soils (54% of cover/catch crop studies, n=71).
Limitations of the evidence base
Studies were mainly sampled at a field scale. The one study that made measurements within a river system over 17 years, did not find an agreement between field and river data. Cover catch crop studies were often conducted on loam or unknown soil types, which may not capture differences between soil types and nutrient leaching (e.g. sandy soils). Only a quarter of the studies assessed effectiveness across all 4 seasons. Although some studies were of long duration (up to 30 years), the effect of stopping cover/catch cropping on effectiveness was not studied that often, one study suggested that nutrients caught by cover catch crops can be leached in subsequent years if no cover/catch crop is planted. Climatic data was often difficult to extract from studies, however some studies reported year to year variation in effectiveness depending upon the date when autumn rains started.
86% of cover/catch crop studies investigated the effectiveness of cover/catch crops for reducing N (n=114), mainly measured as nitrate (95% of studies measuring N, n=108).
72% of cover/catch crop studies were reported by authors to be generally effective for reducing at least one form of N (n=82).
A meta-analysis on a subset of data (n=10), suggested that cover/catch crops are effective at reducing N compared to a fallow control (Z = 7.869, P = <0.001), but that there was significant variation between the studies (Q = 131.31, df =10, P = <.001).
Only 14% of cover/catch crop studies investigated the effectiveness of cover/catch crops for reducing sediments (n=19). Authors indicated that cover/catch crops were generally effective at reducing sediment in 68% of the studies (n=13).
10% of cover/catch crop studies investigated the effectiveness of cover/catch crops for reducing P (n=14). Of these 14 studies, only 3 were effective at reducing any type of P.
Slurry storage and altering timing of slurry application to crops can impact on water quality by ensuring that slurry applications are timed to improve uptake of nutrients by crops.
This review did not directly address the question, ‘does alteration of slurry timing impact on water quality?’, but instead investigated the value of slurry storage for improving water quality. The evidence was diverse, being mainly composed of studies that measured slurry leakage, or die-off of pathogens in slurry during storage, but a few studies investigating the timing of slurry applications to match plant uptake were found. A separate study (a rapid evidence assessment) has been commissioned to specifically investigate the impact of altering timing of slurry application on water quality.
With regard to the question asked in this Systematic Review, the value of slurry storage, the evidence was variable, but indicated that storage can reduce levels of bacterial pathogens in slurry. A disproportionate amount of studies had confounding factors, particularly at a catchment level and were excluded from effectiveness assessment. 42 studies were found that could be included in an assessment of the effectiveness of slurry storage. Under half were manipulative (n=18), with only a third of the studies controlled (n=13), studies were often not always fully replicated, often of short duration and not randomized.
Limitations of the evidence base
Many of the studies were more than 15 years old, and some referred to slurry storage using earth lined stores which may not meet current legislation. Much of the evidence for N and P was based on detection of slurry leakage rather than water quality which makes it difficult to compare the results for slurry storage to other mitigation measures. Many studies were not of the highest scientific rigour, and often did not have pre-slurry storage baseline data. Some authors suggested that results for leakage may have been due to experimental error e.g. slurry stores being completely emptied, resulting in clay soils cracking. One author had concerns that it was not possible to identify if the slurry had leaked as part of the initial sealing or much later when to storage was operational. Most studies were conducted for less than 2 years therefore the effect over time e.g. age of slurry storage may not have been accurately assessed. Only 10 studies investigated the effect of P.
71% of slurry storage studies investigated the effectiveness of slurry storage for reducing N (n=30).
Authors indicated that slurry storage was often not effective for reducing or preventing leakage of N for at least one form of N (17% of slurry storage studies that measured N were effective, n=7).
Bacterial pathogen counts
45% of slurry storage studies investigated the effectiveness of slurry storage for reducing bacterial pathogen counts (n=19).
68% of studies found that slurry storage was generally effective for reducing bacterial pathogen counts in stored slurry for at least one form of bacterial pathogen count (n=13).
Only 24% of slurry storage studies investigated the effectiveness of slurry storage for reducing P (n=10). Only 2 of these studies found that slurry storage was effective for reducing any form of P or leakage of P.
Woodland creation (excluding tree buffer studies)
Woodland creation has the potential to improve water quality by improving water infiltration through soil, thereby reducing runoff and the risk of pollutants entering water sources. Woodland may also uptake nutrients, which would otherwise be lost to water sources.
Buffer strip studies with a tree component were not categorized under woodland creation, but were instead categorized as buffer studies. 48% of buffer studies had a tree component (n=107).
Other woodland creation studies were limited, as most research falls outside the direct scope of the fairly narrow focus of the question addressed here, and the total number of studies found (n=12) was lower than originally anticipated. The woodland studies included were quite diverse consisting of studies of afforestation on former agricultural land, or studies of trees grown for biomass. Effectiveness of woodland creation was difficult to assess due to variations in the type and design of studies and a relatively small sample size.
Some afforestation studies did not have a non-woodland control, but instead measured changes in water quality over different aged woodlands making it difficult to ascertain if woodland had improved water quality compared to agricultural land. Some biomass studies did not have a non-woodland control, but instead used a non-fertilized treatment as a control. Most of the woodland creation studies measured N (92% of woodland creation studies, n=11). Only 1 study measured sediments, bacterial pathogen counts or P (n=1).