Evaluation of tillage, rotation, cover crop and nitrogen rate effects on nitrogen cycling in a corn-soybean rotation syste

Field experiments were conducted at Mead, NE between 1990-1993 to evaluate the effects of tillage, rotation, cover crops and N rates on crop yields, surface residue cover and N cycling in a corn-soybean rotation system. Winter rye was planted following soybean in corn-soybean rotation under two tillage systems viz., no-till and spring disk with a nested N variable of five N rates, 0, 50, 100, 150 and 300 kg N ha$sp{-1}$. In 1991, a $sp{15}$N labelled residue study was conducted by establishing a series of contiguous microplots in each rotation/cover crop with or without fertilizer N under disk tillage and cross labelling different N sources (crop residue, fertilizer and soil) with N-15 isotope.;Rye dry matter yield averaged 1.35 Mg ha$sp{-1}$ during a good year for rye production which gave adequate residue cover in corn-soybean system and recycled an average of 45 kg N ha$sp{-1}$ to corn. On average, rotation of corn with soybean (w & w/o rye) led to 36% and 26% increase, respectively, in grain and stover yield of corn. Corn grain yield following soybean with a rye cover crop was reduced by 8.6% compared to rotation w/o rye. Nitrogen application to corn beyond 50 kg N ha$sp{-1}$ did not increase the grain yield of corn. Soybean grain yield and N uptake averaged 2.9 Mg ha$sp{-1}$ and 206 kg ha$sp{-1}$, respectively, over all years. A net reduction in residual soil nitrate (RSN) level occurred when rye followed soybean in corn-soybean system. However, a net accumulation of RSN occurred following corn after soybean/rye. The lability of the residues was in the order of rye $>$ soybean $>$ corn. The quantity of corn N uptake from different N sources was on the order of soil $>$ fertilizer $>$ soybean $>$ rye and corn. In corn following corn fertilizer N constituted a greater proportion of total N uptake than corn-soybean rotation system. A net N loss of 5-10% from the corn plant at harvest occurred where soybean and rye were residue sources. Soybean contributed the most N to potentially mineralizable N (PMN) and inorganic N pool during the growing season, compared to other residues. Rye residue enhanced the net N mineralization of soybean residue. Nitrogen deficit averaged 30% at the end of the growing season.;Nitrogen credit should be given to rye to account for its N contribution to corn. Diminished response to N fertilization for corn in rotation resulted from large amounts of N available from soybean and rye for corn N uptake.