NITROGEN FIXATION AND THE FATE AND TURNOVER OF CARBON FIXED THROUGH HYDROGEN-COUPLED CARBON DIOXIDE FIXATION IN SOYBEANS
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Global production of soybeans continues to increase, reflecting the value of soybeans in food products, soil amendments, and petroleum replacements. While the N2O aspect of the legume greenhouse gas (GHG) budget has been given substantial, and mostly negative, attention, the CO2 side needs to be explored. During nitrogen fixation, hydrogen is produced. As a result of hydrogen-coupled CO2 fixation, legume soils have the ability to increase the amount of atmospheric carbon fixed into the soil. However, the amount and fate of this fixed carbon has yet to be determined. Therefore, two experiments were conducted. The first experiment explored the methods of 13C labeling of four soil treatments over a period of 28 days. Results showed that most of this carbon uptake was through biotic processes, and that the pattern of carbon uptake was similar for all four soil treatments. The ideal labeling time for both ‘root’ and ‘nodule’ soil was determined to be between 7 and 14 days. The second experiment incubated four 13C labeled soil treatments in a continuous soybean field for 11 months. Throughout this time, bags of soil were removed and separated into three soil organic matter (SOM) fractions; the microbial biomass fraction (MBF), the light fraction (LF), and the acid stable fraction (ASF). These fractions were then measured for δ13C, providing a time series showing carbon movement through the soil which enabled an estimation of the MRT of fixed carbon in the soil. Results showed that the MBF was responsible for the initial flux of carbon uptake in all soil treatments, and that the ‘nodule + H2’ MBF continued to fix carbon at the end of the 11 months. While the LF was not ideal at representing the intermediate SOM pool, the ASF did increase in enrichment at the end of the experiment, showing the movement of newly fixed carbon into long-term carbon storage. Therefore, hydrogen-coupled CO2 fixation does contribute to long-term carbon storage in legume soils, however, longer study times with focus on different legume plants and soil types are required to determine the mean residence time (MRT) of fixed carbon in the ASF.