Quantification of N 2 fixation and annual N benefit from N 2 fixation in soybean accrued to the soil under soybean‐wheat continuous rotation

A computational exercise was undertaken to quantify the percent N derived from atmosphere %Ndfa) in soybean and consequent N benefit from biological N 2 ‐fixation process annually accrued to the soil by the soybean crop using average annual N‐input/‐output balance sheet from a 7 yr old soybean‐wheat continuous rotational experiment on a Typic Haplustert. The experiment was conducted with 16 treatments comprised of combinations of four annual rates of farmyard manure (FYM ≅ 0, 4, 8, and 16 t ha –1 ) and four annual rates of fertilizer N (≅ 0, 72.5, 145, and 230 kg N ha –1 ) applications. The estimated N contributed through residual biomass of soybean (RBN S ) consisting of leaf fall, root, nodules, and rhizodeposition varied in the ranges of 7.02–16.94, 11.65–28.83, 3.31–8.91, and 11.3–23.8 kg N ha –1 yr –1 , respectively. A linear relationship was observed between RBN S and harvested biomass N (HBN S ) of soybean in the form of RBN S = 0.461 × HBN S – 20.67 (r = 0.989, P < 0.01), indicating that for each 100 kg N assimilated by the harvested biomass of soybean, 25.4 kg N was added to the soil through residual biomass. The Ndfa values ranged between 13% and 81% depending upon the annual rates of application of fertilizer N and FYM. As per the main effects, the %Ndfa declined from 76.4 to 26.0 with the increase in annual fertilizer‐N application from 0 to 230 kg N ha –1 , whereas %Ndfa increased from 40.8 to 65.8 with the increase in FYM rates from 0 to 16 t ha –1 , respectively. The N benefit from biological N 2 fixation accrued to the soil through residual biomass of soybean ranged from 7.6 to 53.7 kg N ha –1 yr –1 . The treatments having %Ndfa values higher than 78 showed considerable annual contribution of N from N 2 fixation to the soil which were sufficient enough to offset the quantity of N removed from the soil ( i.e. , native soil N / FYM‐N / fertilizer‐N) with harvested biomass of soybean.