Do Soybeans in Iowa Use More Nitrogen Than They Fix?.

Soybean (Glycine max L.) production in the United States has increased exponentially in the last few decades. Farmers use soybean in crop rotations schemes, particularly with corn in the Midwest US. As a legume, soybean interacts with rhizobia bacteria which convert N₂ into NH₃ through biological N fixation (BNF). For this reason, many people believe there is no need for N fertilizer application, but instead believe a ‘N credit’ suggesting that soybean is a net contributor of N such that BNF exceeds grain N harvested. We tested this hypothesis using two methods:  ¹⁵N isotopic dilution and ¹⁵N isotopic natural abundance using two isolines (nodulating vs. non-nodulating).

A field experiment was conducted in two sites located in Central and North West Iowa at two planting dates (May 1^st and May 25^th). We used ¹⁵N tracer to track the fate of soil organic N (SON) into different pools throughout the growing season of 2015 within different plant parts including above and below-ground tissues such as leaves, stems, pods, seeds, and roots. The labeled N was applied as ¹⁵N-NH₄NO₃15 days before soybean planting. Then crop and soil samples were taken at different soybean growth stages (V3.5, R1, R2, R3, R4, R5.5, and R7). Soybean N uptake (Nup) quantification lead us to understand the N dynamics in soybean and its contribution to residual nitrogen stored/remained at residue (leaf + stems). The results showed in every case soybean is a net user of N rather than a contributor. Moreover, soybean stover had lower concentrations of reactive N (Nr) after the grain was harvested, leaving poor quality residue in the soil. We believe that if soybean is a net user of SON, it is most likely depleting SOM in the long term.