Soybean Stover Nitrogen Affected by Dinitrogen Fixation and Cultivar

Grain legumes often are credited with leaving residual N for crops that follow in the rotation. This study was conducted to determine if increasing N2 fixation in soybean [Glycine max (L.) Merr.] would result in higher amounts of stover N at harvest. Eleven early‐maturing cultivars (Maturity group 00 and 0) in 1988 and 16 in 1989 were grown with five inoculant strain treatments and alone as uninoculated controls. There were slight cultivar‐by‐inoculant interactions but main effects were much larger. The order of N2 fixation with inoculant treatments, measured by the difference method, was 532C or three‐strain mixture > USDA 110 > CB 1809 > HH303. Dinitrogen fixation ranged from 60 to 125 kg ha−1 in 1988 and from 31 to 84 in 1989. Higher N2 fixation caused related significant (P ≤ 0.05) increases in stover dry matter and stover N yields in both years, despite concomitant increases in the apparent harvest index (AHI) and apparent NHI. Stover N concentration was not affected by increased N2 fixation in either year. In 1988, cultivars differed only slightly in stover N content. In 1989, amounts of stover N in different cultivars depended on how completely N was translocated to seed. Over the 2 yr, amounts of N in harvestable stover of the early‐maturing cultivars ranged from 9 to 24 kg ha−1, averaged across inoculant treatments. Although stover N increased with N2 fixation in both years, the amount of stover N available for return to the soil was quite low.