VARIABILITY IN NITROGEN-FIXATION ACTIVITY AMONG SOYBEAN CULTIVARS GROWN UNDER FIELD CONDITIONS

Variability in nitrogen fixation activity among soybean cultivars was examined under well-managed field conditions. Four and 15 soybean cultivars were growth in 1989 and 1991, respectively, along with the non-nodulating cultivar T201. Top dry weight, total top nitrogen content and nodule dry weight (1991 only) were determined on three or four dates. Nitrogen fixed by each cultivar was estimated with the difference method. In both years, daily nitrogen fixation rate (DNF) increased until roughly stage R5, showing a close linear relationship with top dry weight. dDNF/dTOPDW calculated combining all cultivars was 1.62 mg day-1g-1 (r2 = 0.91) in 1989 and 1.69 mg day-1g-1 (r2 = 0.92) in 1991. At the same time, calculation of dDNF/dTOPDW for individual cultivars before R5 showed a substantial difference among cultivars ranging from 1.29 mg day-1g-1 (r2 = 0.47) of Harosoy in 1991 to 2.23 mg day-1g-1 (r2 = 0.93) of Fusanari in 1991. Calculated nodule specific activity (DNF/NDW, mg day-1g-1) in 1991 increased for all cultivars during vegetative growth, while the ratio of module dry weight to top dry weight (NDW/TOPDW, g g-1) decreased. The highest DNF of cultivars which occurred near stage R5 ranged approximately from 0.3 g m-2day-1 to 0.5 g m-2day-1. During pod filling, DNF was observed generally to decrease, and old cultivars showed rapid declines in DNF just after R5 compared to more modern cultivars.