Groundwater Nitrogen and Phosphorus Dynamics in a Claypan Watershed Under Crop Management.

Agricultural systems are often studied for widespread nitrogen (N) and phosphorus (P) contamination of surface and subsurface water resources. The objective of this study was to assess the spatial and temporal variations of N and P concentrations in groundwater under row crop management. Goodwater Creek Experimental Watershed (GCEW), Missouri in the claypan region has an extensive well network. Weekly samples were collected from three groundwater monitoring wells and three piezometers and analyzed for nitrate-N (NO₃-N), total N, orthophosphate (PO₄-P) and total P concentrations for 2015 to 2016. Nitrate concentrations were significantly greater at Romine north (RN>8.15 mg L^-1) and Romine west (RW>9.18 mg L^-1) (NO₃-N, p<0.001; TN, p<0.001) compared to Romine south (RS<0 mg L^-1) well. TN concentration also followed the same pattern but with elevated concentrations (RN and RW>8.5 mg L^-1, RS<0.17 mg L^-1). Significantly greater median PO₄-P (0.079 mg L^-1, p<0.001) and TP (0.096 mg L^-1, p<0.001) concentrations were observed at RS as compared to concentrations at RN and RW (<0.005 mg L^-1 PO₄-P and < 0.006 mg L^-1 TP). Median PO₄-P and TP concentrations were not significantly different (p=0.20) between RN and RW. Correlation patterns between PO₄-P and NO₃-N suggested that effective NO₃-N removal via denitrification at RS may have produced redox conditions which increased the release of PO₄-P to subsurface water. The monitoring of interflow with three piezometers showed backslope piezometer (PZ 6) contained greater NO₃-N, TN, PO₄-P and TP than summit (PZ 4) and footslope (PZ 7) piezometers., but with low concentrations. Results showed that preferential flow through the soil and hydraulic conductivity of the subsurface strata controlled NO₃-N transport in this claypan watershed and the PO₄-P concentrations may have mediated by redox conditions of the well.

Key words: Alluvial soil, Corn soybean, Denitrification, Nitrate, Orthophosphate