Long-Term Agroecosystem Research in the Central Mississippi River Basin: Hydrogeologic Controls and Crop Management Influence on Nitrates in Loess and Fractured Glacial Till

Nitrogen from agriculture is known to be a primary source of groundwater NO3-N. Research was conducted in a northeastern Missouri watershed to assess the impact of cropping systems on NO3-N for a loess and fractured glacial till aquifer underlying claypan soils. Three cropped fields with 10 yr of similar management were each instrumented with 20 to 25 monitoring wells, 3 to 15 m in depth, in 1991 to 1992. Wells were sampled and analyzed for NO3-N at least annually from 1991 to 2004. Initial NO3-N concentrations were variable, ranging from undetectable to >24 mg L-1 but averaged 7.0 mg L-1. Groundwater NO3-N was significantly higher in Field 3, probably the result of concurrent applications of manure and N fertilizer before 1980. Overall changes in NO3-N levels in Fields 1 and 2 were generally small; however, NO3-N levels for Field 3 have decreased an average of 0.28 mg L-1 yr(-1). Excessive loading of N into the matrix of the glacial till may have had a long-term impact on NO3-N for this field. Despite the presence of dissolved O-2 in the aquifer, evidence of denitrification in some upper-landscape groundwater wells was found. The greatest decreases in NO3-N concentration occurred as groundwater moved through an in-field tree line or through a riparian zone. While overall conclusions were complicated by the long-term impact of past management, the capacity of the till to buffer changes, hydrogeologic variability found among wells, and the activity of biological processes, we conclude that cropping practices during this study did not increase glacial till NO3-N.