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 cult ivars were growth in 1989 and 1991, respectively, along with the non-n odulating cultivar T201. Top dry weight, total top nitrogen content an d nodule dry weight (1991 only) were determined on three or four dates . Nitrogen fixed by each cultivar was estimated with the difference me thod. In both years, daily nitrogen fixation rate (DNF) increased unti l roughly stage R5, showing a close linear relationship with top dry w eight. 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 bef ore 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 vegetativ e 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 n ear 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.