Soybean nitrogen dynamics: Nitrogen fixation, yield, and residue decomposition

Nitrogen (N) a crucial element for plant growth, often limits net primary productivity of agroecosystems. In agricultural systems it is vital to have proper management of N to secure productivity and environmental sustainability. Soybean [Glycine max (L.) Merr.] is the second most important crop in the US Corn Belt, and it can fix part of its N through biological N fixation. The overarching goal of this research was to investigate soybean N fixation dynamics in response to environmental factors, and plant tissue decomposition to understand better N cycling and improve Midwestern US N budget estimations. A long-term incubation study was conducted by using aboveground plant litter of soybean along with three other plants (maize, oats, and alfalfa), to investigate the effect of different litter amendments on mineral associated organic matter (MAOM) accumulation and stabilization. Soils amended with soybean and maize litter had more stable MAOM-N, with greater efficiency of accumulation compared to oats and alfalfa litter amendments. In my second study, I used two soybean fields to estimate N-fixation dynamics by using two methodologies (15N isotope dilution method, and isolines), and measured crop growth, soil inorganic N and weather dynamics throughout the season. On average soybean grown in Iowa fixed from 23 to 65% of total aboveground N reaching a maximum rate of 3 kg ha-1 d-1 during early seed fill period. Finally, I conducted a field experiment to test the effect of supplemental N fertilization addition to soybean at three different rates and three application timings. The experiment showed inconsistent results among years and methodologies used to determine yields, but provided evidence that high yielding soybean crops will respond to N fertilizer. Therefore, it is suggested to improve the methodologies in the near future for advanced soybean research. Overall findings from this dissertation provide novel information about soybean physiological and biological processes as a result of soil-plant-atmosphere interactions. Our results indicate that in order to boost soybean yields, there is a need to use a systematic approach that focuses on plant-environment interactions to increase available N supply either from litter mineralization, N fixation, or additional N fertilizer