Development and analysis of the measured annual nutrient loads from agricultural environments (manage) drainage nutrient concentration database

Agricultural systems are the primary contributor of non-point source nitrogen (N) and phosphorus (P) pollution to ground and surface waters in North America. It is imperative to understand the mechanisms responsible for nutrient transport from this land use leading to the eutrophication of surface waters and the hypoxic zone in the Gulf of Mexico. It is also vital to understand the effects of management practices in drained systems, as subsurface drains are important pathways connecting agricultural landscapes with water bodies. In this study, annual nutrient concentrations were obtained from literature and compiled into a major database to assess factors leading to nutrient loss through subsurface drainage systems. First, a literature review was conducted on over 400 drainage nutrient related studies, of which 79 were chosen to compile 1564 site-years of annual agricultural drainage N and P concentrations. Then, using this new Drain Concentration table in the MANAGE (Measured Annual loads from AGricultural Environments) database, the effects of common agricultural management factors (e.g. crop rotation, nutrient management, tillage) on annual drainage N and P drainage concentrations were analyzed. Across the database, there was a 69% probability that a site-year would exceed an annual nitrate (NO3-N) concentration of 10 mg N/L, the USEPA Maximum Contaminant Level (MCL) for drinking water, but only a 27% probability a reported site-year would exceed 0.0375 mg dissolved reactive phosphorus (DRP)/L, which is a critical total P concentration to avoid for freshwater eutrophication. While a surprising number of reported annual means were arithmetic rather than flow-weighted averages (27 vs. 71%), reporting of arithmetic means in peer-reviewed literature has decreased over time. The highest annual flow-weighted mean NO3-N concentrations were from corn, corn and soybean grown within the same plot, and soybean cropping site-years (13.98, 13.53, and 12.09 mg N/L, respectively). However, crop selection was shown not to be a significant predictor for annual average drainage DRP concentrations. On average, nitrogen application rates for corn below 75 kg N/ha did not reduce annual NO3-N concentrations compared to rates greater than 75 kg N/ha. There was no significant difference in annual drainage nitrate concentrations from site-years reporting a split nitrogen application versus site-years without a split nitrogen application. Stepwise regression using twenty-nine first and second order variables and two-way interactions was performed on NO3-N and DRP concentrations in corn site-years. Regression analysis of NO3-N concentrations had an overall model R2 of 0.59 (n=254). Based on partial R2’s, N application rate had the greatest effect on NO3-N concentrations (flow-weighted and arithmetic) in corn site-years followed by fertilizer timing, tillage type, the interaction between tillage and split N application, the interaction between tillage and annual drain discharge, and the method of fertilizer application. Regression analysis of DRP concentrations had an overall R2 of 0.94, and although the model was much less robust due to a very small sample size (n=47), fertilizer timing was most correlated with annual DRP concentrations. Findings indicate there is still much to be learned as to the effects of management practices on nutrient transport in artificially drained systems. The MANAGE database will continue to evolve and remain a resource for new exploratory efforts to better understand and mitigate nutrient losses from agricultural systems.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste