In-Situ Nitrogen Mineralization for Corn and Soybean in Poorly Drained Soils with and without Tile-Drainage.

Mineralization provides substantial amounts of N, but quantification of when and how much N is produced in agricultural fields is lacking. Our objective was to determine how much total inorganic N (TIN) is produced during a growing season in corn (Zea mays L.) and soybean (Glycine max L. Merr.) in rotation under different N and soil drainage management. Controlled drainage gages installed in 2011 provided drained or undrained conditions. The top 15 cm of drained plots contained 1.4 g kg^-1 total organic-N and 19.7 g kg^-1 total organic-C. The undrained plots contained 1.6 and 23.8 g kg^-1 total organic-N and -C, respectively. Plots received pre-plant 0 and 134 kg N ha^-1 for corn and 0 and 45 kg N ha^-1 for soybean. In situ incubation was done in 2014 by inserting PVC tubes (5 cm diameter, 15-cm deep) in the soil and capped. Mineralization and nitrification was measured by the difference in ammonium and nitrate concentrations at the start of the incubation (from samples collected near the incubation tube) and concentrations inside the tube after incubation. This process was repeated approximately every 14 days. The season-long cumulative TIN (ammonium plus nitrate) produced in soybean receiving no N fertilizer was 4 and 17 kg TIN ha^-1 for drained and undrained conditions, respectively and in soybean receiving N fertilizer 25 and 98 kg TIN ha^-1 for drained and undrained conditions, respectively. For corn receiving no N fertilizer 15 and 33 kg TIN ha^-1 were produced for drained and undrained conditions, respectively, and for N-fertilized plots 111 and 360 kg TIN ha^-1 for drained and undrained conditions, respectively. Overall there was a 3.5 fold increase in TIN produced in undrained relative to drained conditions and N fertilization produced a 9 and a 6 fold increase in TIN production in corn and soybean, respectively.