Tile Drainage of Agricultural Land and Its Impact on Nitrogen Mineralization.

Mineralization can provide significant amounts of N to crops, but quantification of when and how much N is produced in agricultural fields has been challenging. Our objective was to determine in situ N mineralization in corn (Zea mays L.) and soybean (Glycine max L. Merr.) under different N and soil drainage management (imposed three years earlier) using a sequential core sampling technique during two growing seasons. The experimental design was a split-split-plot arrangement in a randomized complete-block design with four replications, with drainage as whole plot (Drained and Undrained), crop as split plot (corn and soybean) and N rate as split-split plot (control (0N) and fertilized-N). Soil temperature, volumetric water content, soil mineral N was measured in 2014 and 2015. Drained soil had substantially less VWC. The soil lost 2.4 Mg C ha^-1 yr^-1 due to tile-drainage. Overall greater total organic carbon (TOC) and total nitrogen (TN) in the undrained soil resulted in 2.8 times greater total inorganic nitrogen (TIN) mineralized compared to the drained soil in the unfertilized control (0N) but the effect of drainage was not consistent across years when N was added, likely because fertilizer provided a high-N substrate to promote denitrification losses when soils had consistent high moisture content due to uniformly-distributed precipitation in one of the years. Across all variables, soils mineralized 2.8% of TN in soybean residue and 1.3% of TN in corn residue. Nitrogen fertilization increased mineralization rate, as high as 9.6 kg TIN ha^-1 day^-1, compared to <2.2 kg TIN ha^-1 day^-1 for 0N. Overall, TIN mineralized was 3.5 times greater with N fertilizer than the 0N, but fertilization made mineralization less predictable. The impact of fertilization on boosting mineralization under differential soil drainage needs further refinement if we are to improve decision-making tools for N application based on soil mineralization predictions.