Biological Nitrogen Fixation by Soybean and Fate of Applied ~(15)N-Fertilizer in Wheat under Conservation Agriculture

摘要

Nitrogen (N) is one of the key drivers of global agricultural production. Four field experiments with irrigated summer-grown soybean [Glycine max L.) and winter-grown wheat (Triticum aestivum L.) were conducted during 2006-08 to investigate whether biological n2 fixation (BNF) by soybean can be improved under conservation t agriculture (CA) where crop residues (CR) retained on the soil surface, as compared with conventional tillage (CT) and to assess the fate of ! Nfertilizer in subsequent wheat on Fatehpur loamy sand soil (Typic Haplustepf) using 15N labelled fertilizer. The BNF estimated using 15N isotope dilution and 15N natural abundance methods, were comparable suggesting that the latter method which does not require costly 15N-enriched fertilizer, could be employed to estimate BNF by legumes. Use of sorghum as a reference plant, grown in the same plot as soybean, led to up to 36 percent lower estimation of BNF than with the use of in situ spontaneous weeds {Eleusine aegytiacum L Euphorbia hirta L. and Cynodon dactylon L), which have a similar size and rooting depth as the soybean. The irrigated soybean in the semi-arid subtropical soils could biologically fix 81-125 kg-N-ha~(-1) (68-85 percent 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 presumably due to better rhizobia activity of caused by cooler rhizosphere environment. Recovery of applied 15N in the soil-plant system atharvest of the wheat crop showed that 36-47 percent of it was utilized by the crop, 37-49 percent was left in the soil profile and 5-27 percent was lost, which was estimated as unrecovered 15N. Utilization of ,5N was significantly lower when 25 percentmore fertilizer N was applied than recommended in both CT and CA without CR. It was also lower when CR was incorporated in CT, or retained on the soil surface in CA. The recovery of 15N in the soil profile at harvest of the wheat crop revealed that the majority of the residual fertilizer N was present in the first 15 cm (54-61 percent), although downward movement of 15N below this level of the soil surface layer was also evident in the soil profile up to a depth of 120 cm. These results illustrate the benefits of CA with CR retained on soil surface on BNF by soybean, and similar patterns in N uptake and translocation fromvegetative parts to grain and utilization of applied N by wheat in both tillage systems.