Nitrogen is one of the key drivers of global agricultural production. Finding ways to meet its requirement through conservation agriculture (CA), exploring biological N₂ fixation (BNF) and improving plant use efficiency of fertilizer N have become extremely important tasks. Four field experiments with irrigated soybean (Glycine max L.) and wheat (Triticum aestivum L.) were conducted to estimate BNF by soybean under conventional-till (CT) and CA using ¹⁵N techniques, and to investigate the fate of ¹⁵N-fertilizer applied to succeeding wheat. Comparable amount of BNF by soybean was obtained using ¹⁵N isotope dilution and ¹⁵N natural abundance methods, 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 led up to 36% lower estimation of BNF than with the use of spontaneous weeds (Eleusine aegytiacum L., Euphorbia hirta L. and , Cynodon dactylon L.), which have similar growing size, and rooting depth as the soybean. With the use of these weeds as reference plants, the irrigated soybean in the semi-arid subtropical soils could biologically fix N₂ ranging from 81-125 kg ha^-1 (68-85% of total N uptake), depending upon tillage and crop residue (CR) management. Significant increases in BNF by soybean were recorded where CR was retained on soil surface of CA presumably due to better activity of rhizobia because of the relatively cooler rhizosphere environment. Recovery of applied ¹⁵N in the soil-plant system at the harvest of wheat showed that 36-47% was utilized by crop, 37-49% was left in the soil profile and 5-27% was lost, which was estimated as unrecovered ¹⁵N. Utilization of ¹⁵N was significantly lower when (a) rate of fertilizer N was increased from the recommended rate of 120 kg N ha^-1 to 150 kg N ha^-1 in both CT and CA without CR, and (b) crop residue was incorporated in CT, or retained on the soil surface in CA. The recovery of ¹⁵N in soil profile at the harvest of wheat revealed that majority of the residual fertilizer N was present in 0-15 cm (54-61%), but downward movement of ¹⁵N below 0-15 cm soil surface layer was evident in soil profile up to 120 cm soil depth.