362-14 Do Soybeans in Iowa Use More Nitrogen Than They Fix?.
A field experiment was conducted in two sites located in Central and North West Iowa at two planting dates (May 1st and May 25th). 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.