Variation in the pattern of nitrogen accumulation and distribution in soybean

The contribution of early nodulation and dinitrogen (N₂) fixation to overall soybean [Glycine max (L.) Merr.] plant performance in the cooler northern states of the U.S. Midwest needs further investigation. In a previous growth chamber study, we identified traits associated with improved early nodulation and N₂ fixation in this crop. This study examined the nodulation, N accumulation, and N distribution patterns for the same 19 soybean lines in the field, and relates field performance to previous growth chamber results. The study was conducted in 1992, 1993, and 1994 at Becker, MN, in a soil with 7 to 15 kg NO₃/^- ha^-1. The following data was recorded: nodule fresh weight (NFW), plant dry weight (PDW), and total N accumulation (TNA) at 20 to 60 d after planting (DAP), PDW and TNA at the R3 and R6 growth stages, and grain and seed protein yield at maturity. Differences in NFW, PDW, and TNA were found at most sampling dates, and were indicative of significant genetic differences in N₂-fixing ability. Lines also differed in partitioning of N, with the cultivar Lambert distributing 88% of total N accumulated to reproductive tissue at the R6 growth stage, compared with only 31% for PI 437966. Two canonical variables, one created by analyzing together the NFW data collected at 20, 40, and 60 DAP in the field, the other derived from NFW, PDW, and nodule soluble protein data obtained in the previous growth chamber study each correlated with the seed protein yield of field-grown plants. This suggests that the measurement of early nodulation and N₂ fixation in either the growth chamber or field can serve as an indicator on which to base selection for enhanced N₂ fixation.