Yield, Soil Nitrogen Uptake, and Nitrogen Fixation by Soybean from Four Maturity Groups Grown at Three Elevations

The exploitation of the soybe an [Glycine max (L.) Merr.] -Bradyrhizobium japonicum symbiosis in varied environments requires an understanding of factors that may affect fixed and soil N assimilation. Temperature affects both soybean maturity and N requirement, and soil N availability. Five soybean varieties belonging to four maturity groups (00, IV, VI, and VIII) and their respective nonnodulating isolines were planted at three sites along an elevational transect in Hawaii to study the effect of elevation-associated changes in temperature on yield, soil N uptake, and N= fixation. The mean soil/air temperatures during the experiment were 24/23, 23/21, and 20/IS°C, with 7, 8, and 9°C differentials between mean maximum and mean minimum air temperatures at elevations of 320, 660, and 1050 m , respectively. The soils of the two lower sites were Humoxic Tropohumults and that of the highest site was Entic Dystrandept. The seeds were inoculated at planting with B. japonicum. Plants were harvested at physiological maturity. Average dry matter yield and N accumulation at the highest site was only 48 and 41%, respectively, of that at the lowest site. Early maturing varieties were relatively more affected by high elevation than were later varieties. Within each site, late -maturing varieties produced greater yield and accumulated more N than did the early varieties. Nitrogen fixation contributed 80, 66, and 97% to total plant N at the lowest, intermediate, and highest sites, respectively. Differences between sites in the proportion of N from fixation we re due to differences in the availability and uptake of soil N. Although total N assimilation between varieties differed in some cases by more than 400%, the proportions of N derived from fixation were similar within a site. On a per-day basis, N accumulation was similar among varieties at a site. Since the different maturity groups had similar average N assimilation rates per day within a site, and since soil N mineralization rate per day at a site can be assumed to be constant, the proportion of total N derived from N2 fixation is necessarily constant. Since the proportion of N derived from fixation is yield independent at a site, it may be possible to predict the relative contributions of soil N and symbiotic N2 fixation to soybean through characterization of the processes that affect N availability at a site. Data from in vitro soil N mineralization and a greenhouse pot test indicated that low temperature may have decreased root growth and uptake of soil N, or soil N availability more than it reduced N2 fixation. Additional Index Words: Glycine max, Temperature, Non-nodulating isoline, Symbiotic nitrogen fixation, Bradyrhizobium japonicum.