Biological nitrogen fixation by soybean and fate of applied 15N-fertilizer in succeeding wheat under conventional tillage and conservation agriculture practices

Four field experiments were conducted to investigate biological N-2 fixation (BNF) by irrigated soybean under conservation agriculture (CA) as compared with conventional tillage when crop residue (CR) is retained on the soil surface, and the fate of N-15-labelled fertilizer in succeeding wheat in the semi-arid subtropical soil. Comparable amounts of BNF by soybean were obtained using N-15 isotope dilution and N-15 natural abundance methods, suggesting that the latter, a less costly method could be employed to estimate BNF. Soybean could fix 61-125 kg N ha(-1) (52-85% of total N uptake), depending upon tillage and CR management. Significant increases in BNF by soybean were recorded when CR was retained on the soil surface of CA plots presumably due to better activity of rhizobia because of the relatively cooler rhizosphere environment. Recovery of applied fertilizer N in the soil-plant system at harvest of the wheat crop showed that 36-47% of it was utilized by the crop, 37-49% was left in the soil profile and 5-27% was lost (unrecovered fertilizer N). The recovery of fertilizer N in the soil profile revealed that the majority of it was present in the first 15 cm (54-61%), although downward movement of fertilizer N was also evident up to 120 cm soil depth. These results illustrate enormous benefits of CA practices with CR retained on soil surface on BNF in soybean, and similar patterns in N uptake and translocation from vegetative parts to grain and utilization of applied fertilizer N by wheat in both tillage systems.