Riparian Management of Subsurface Nutrient Losses Within Conventional Farms Under Differing Geologies

Abstract:

With greater food demands, agricultural inputs are predicted to increase. This is relevant to one of the great needs in farm re-design: nutrient retention. The purpose of this study was to address whether incorporating prairie tallgrass strips on farms has the potential to mitigate nutrient transport through subsurface flow paths in both clayey and sandy textured agricultural systems. It was hypothesized that prairie tallgrass strips down-gradient of crop fields (e.g. corn) will reduce effects of nutrient leaching to groundwater through their greater rooting depths and dense rooting network, which facilitate nutrient uptake. To test this hypothesis, water samples (both surface and subsurface water, including pore water and groundwater) were collected monthly (July 2018 – October 2018; April 2019 – March 2020) from corn fields and adjacent prairie tallgrass strips on two farms (i.e. one clay setting and one sandy setting). The samples were analyzed for dissolved nitrate and orthophosphate. It was determined that soil and climate (precipitation and month/year) were the greatest drivers of subsurface nitrogen and phosphorus transport. Prairie tallgrass strips had little effect (5% explained variation) on groundwater nitrogen and phosphorus over the 2019 growing season likely due to the above average rainfall (260 mm above the 30-year average). 2018 was much drier, and in sandy soil, during peak biological uptake in 2018 (July – August), incorporating tallgrass prairie strips of buffer lengths: 2 m, 4 m, and 8 m, downgradient of the crop field, reduced nitrate concentrations by 56%, 58%, and 75%, respectively, relative to the values in the groundwater at the cropped edge. With soil type and climatic conditions being the primary drivers of nutrient transport in agricultural systems, prairie strips can be further enhanced by including prairie species with diverse phenologies in order to capture nitrogen and phosphorus leaching earlier in the spring where most nutrient loading occurs to aquatic systems.