Decreasing nitrate-N loads to coastal ecosystems with innovative drainage management strategies in agricultural landscapes: An experimental approach

Innovative controlled drainage strategies in agricultural ditches such as spatially orientated low-grade weirs show promise to significantly improve nutrient (e.g., nitrate, NO3−-N) reductions by expanding the area available for biogeochemical transformations, as well as providing multiple sites for runoff retention. The overall objective of this study was to identify the contributions made by low-grade weirs to source NO3−-N concentrations and loads to downstream coastal ecosystems. This objective was achieved by assessing, from an experimental standpoint, the effectiveness of weirs in reducing NO3−-N concentrations and loads in replicated ditch systems in Jonesboro AR. Overall NO3−-N load reduction rates were approximately 2250 ± 718 and 1935 ± 452 mg/h for ditches with and without weirs, respectively, resulting in mean percent NO3−-N load reductions of 79 ± 7.5 and 73 ± 9% for ditches with and without weirs, respectively. Although NO3−-N concentration reductions were substantial in both systems, overall, for the duration of the experiment no significant treatment effect was detected. A stepwise linear regression and repeated measures ANOVA analyzed the relationship of time × treatment on nitrate concentration and load in ditch effluents. The regression model explained 31.1% of the variance in NO3−-N concentration, which indicated a highly significant relationship between NO3−-N concentration and time × treatment (F = 31.9, p < 0.001). NO3−-N reductions were significantly higher in weir treatments based on time (t = 120 min; F = 3.25; p = 0.042) as compared to systems without weirs. Low-grade weirs show promise in improving nutrient reductions in agricultural drainage ditches, by increasing residence time and reducing on a time step basis, outflow concentrations and loads to downstream systems.

► Two agricultural drainage systems are tested for nitrate-N reductions.
► Mean percent nitrate-N load reductions were similar.
► Hydraulic residence time is crucial for enhancing nitrate-N reduction.
► Significant relationship between nitrate-N concentration and time and drainage treatment.