Cover Crops Reduce Nitrate Leaching in Agroecosystems:A Global Meta-Analysis.

Cover crops are well recognized as a tool to reduce NO leaching from agroecosystems. However, their effectiveness varies from site to site and year to year depending on soil, cash and cover crop management, and climate. We conducted a meta-analysis using 238 observations from 28 studies (i) to assess the overall effect of cover crops on NO leaching and subsequent crop yields, and (ii) to examine how soil, cash and cover crop management, and climate impact the effect of non-leguminous cover crops on NO leaching. There is a clear indication that nonleguminous cover crops can substantially reduce NO leaching into freshwater systems, on average by 56%. Nonlegume-legume cover crop mixtures reduced NO leaching as effectively as nonlegumes, but significantly more than legumes. The lack of variance information in most published literature prevents greater insight into the degree to which cover crops can improve water quality. Among the factors investigated, we identified cover crop planting dates, shoot biomass, and precipitation relative to long-term mean precipitation as potential drivers for the observed variability in nonleguminous cover crop effectiveness in reducing NO leaching. We found evidence indicating greater reduction in NO leaching with nonleguminous cover crops on coarse-textured soils and during years of low precipitation (<90% of the long-term normal). Earlier fall planting and greater nonleguminous shoot biomass further reduced NO leaching. Overall, this meta-analysis confirms many prior studies showing that nonleguminous cover crops are an effective way to reduce NO leaching and should be integrated into cropping systems to improve water quality.