Understanding the variability in soybean nitrogen fixation across agroecosystems

Abstract

Legume-based cropping systems have the potential to internally regulate N cycling due to the suppressive effect of soil N availability on biological nitrogen fixation. We used a gradient of endogenous soil N levels resulting from different management legacies and soil textures to investigate the effects of soil organic matter dynamics and N availability on soybean (Glycine max) N2 fixation. Soybean N2 fixation was estimated on 13 grain farm fields in central New York State by the 15N natural abundance method using a non-nodulating soybean reference. A range of soil N fractions were measured to span the continuum from labile to more recalcitrant N pools. Soybean reliance on N2 fixation ranged from 36% to 82% and total N2 fixed in aboveground biomass ranged from 40 to 224 kg N ha−1. Soil N pools were consistently inversely correlated with % N from fixation and the correlation was statistically significant for inorganic N and occluded particulate organic matter N. However, we also found that soil N uptake by N2-fixing soybeans relative to the non-nodulating isoline increased as soil N decreased, suggesting that N2 fixation increased soil N scavenging in low fertility fields. We found weak evidence for internal regulation of N2 fixation, because the inhibitory effects of soil N availability were secondary to the environmental and site characteristics, such as soil texture and corresponding soil characteristics that vary with texture, which affected soybean biomass, total N2 fixation, and net N balance.