Nitrogen cycling and tile drainage nitrate loss in a corn/soybean watershed

Nitrogen (N) in surface waters has been linked to agricultural crop production, and more specifically, to NO3- exported by tile drainage. The objective of this study was to evaluate agricultural N pools and fluxes in a seed corn/soybean (Zea maize L./Glycine max L.) watershed (40 ha) to relate soil inorganic N pools with annual losses of NO3- in drainage tiles. During a 2-year period beginning in October 1993, soil samples in the top 50 cm located near the tile systems (predominantly Drummer silty clay loam, fine-silty, mixed mesic Typic Haplaquolls) were analyzed for microbial biomass C and N, inorganic N, and N mineralization rates. Water flow and NO3- concentrations were continuously measured in the three drainage tiles. Soil microbial biomass N ranged from 83 to 156 kg N ha(-1), and appeared more closely related to soil moisture than soil inorganic N pools. Soil inorganic N ranged from a low of 13 kg N ha(-1) during the soybean growing season to a high of 115 kg N ha(-1) after N fertilization. Following good growing seasons in 1993 and 1993, high crop uptake of N resulted in relatively small soil inorganic N pools of 40 and 24 kg N ha(-1), respectively, after crop harvest. In 1995, however, when poor growing conditions decreased crop N accumulation, 98 kg N ha(-1) remained in the watershed after harvest. Based on an average effective drainage area of 30 ha, 38 and 64 kg N ha(-1) leached out of the watershed through tile drainage for a total of 3.1 Mg N for the 1995 and 1996 water years, respectively. Tile N export from the watershed was greatest during high flow events when there concurrently existed large pools of soil inorganic N in the form of NO3-. Differences in annual N export for each tile were the result of a combination of factors including; timing and area of N fertilization, amount and distribution of precipitation, crop uptake of soil derived N, and inorganic N pools remaining after harvest. (C) 1998 Elsevier Science B.V.