Nitrogen fixation by soybean in commercial irrigated crops of central and southern New South Wales

Abstract Amounts of symbiotic N 2 fixation were monitored using the ureide-sap technique in 36 irrigated soybean crops [ Glycine max (L.) Merrill], from 26 commercial operations in the Coleambally irrigation area and in the Macquarie Valley of N.S.W. during the summer of 1990–1991. Crop N yields, N 2 fixation, and nodulation were measured twice during crop growth and estimates of the seasonal amounts of N 2 fixed were compared with amounts of seed N removed. For each crop, a net N-balance was calculated at harvest to determine whether there was a net gain or loss of N in soil as a result of soybean cropping. All crops under study were well nodulated; however, relative sap ureides collected indicated that the proportion of soybean N derived from N 2 fixation (P fix ) ranged between 21 and 94% at flowering, and 4 and 96% at seed-filling. Seasonal estimates of P fix and amounts of N 2 fixed ranged from 52 to 73% and 103 to 313 kg N ha −1 at Coleambally, and 13 to 64% and 44 to 238 kg N ha −1 in the Macquarie Valley, respectively. Following seed harvest these amounts of N 2 fixation resulted in N-balance determinations of between −33 to +69 kg N ha −1 at Coleambally, and −134 to +52 kg N ha −1 in the Macquarie Valley. The vegetative residues of 11 of the 33 soybean crops for which measurements continued until final harvest were estimated to have contributed more than 15 kg of fixed N ha −1 to soil, while N 2 fixation was insufficient to satisfy seed N requirements in 12 of the remaining crops. The average seasonal P fix determinations were not greatly different at Coleambally between crops considered to be in net positive N-balance (65%) and in net negative balance (59%), and the final result was more dependent upon soil type, and the average amounts of crop N accumulated and seed N removed than reliance upon N 2 fixation for growth per se . However, in the Macquarie Valley, average crop N yields and seed N removal were similar across all soil types, and there was a distinct relationship between average P fix values and net N-balance (59 cf. 32% for crops in positive and negative balance, respectively). Crops with the highest rates of N 2 fixation and greatest contributions of fixed N to soil followed several years of cereal cropping, or occurred where soybean had been double-cropped with wheat. In these situations nitrate concentrations in surface soils were low at sowing. The lowest amounts of N 2 fixation and poorest N-balance arose when the 1990–1991 crop followed 2 or 3 consecutive years of soybean, at sites with a recent history of legume-based pastures, or where N-fertilizer had been applied. Only 4 of the 33 crops followed through to final harvest exhibited evidence of prolognged N 2 fixation during seed maturation.