Elevated CO2 and O3 modify N turnover rates, but not N2O emissions in a soybean agroecosystem

In order to predict and mitigate future climate change, it is essential to understand plant-mediated effects of elevated CO2 (eCO2) and O3 (eO3) on N-cycling, including N2O emissions. This is of particular interest for agroecosystems, since N-cycling and N2O emissions are responsive to adaptive management. We investigated the interaction of soil moisture content with eCO2 and eO3 on potential N2O emissions from SoyFACE during a 28-day laboratory incubation experiment. We also assessed field N2O fluxes during 2 soybean-growing seasons. In addition, we sought to link previously observed changes in soybean growth and production to belowground processes over a longer time scale by analyzing changes in natural abundance stable isotope ratios of soil N (δ15N). This method relies on the concept that soil δ15N can only change when inputs or outputs with an isotope signature different from that of soil N are altered. We found no major effects of eCO2 and eO3 on laboratory and field measured N2O emissions. Natural abundance isotope analyses suggested, however, a decrease in belowground allocation of biologically fixed N in combination with decreased total gaseous N loss by eCO2, resulting in a tighter N cycle in the longer-term. In contrast, the isotope data suggested an increase in belowground allocation of biologically fixed N under eO3, leading to increased gaseous N loss, most likely in the form of N2. Given that effects of eCO2 and eO3 on N pools and instantaneous transformation rates in surface soil layers of this agroecosystem have been minimal, our results illustrate the importance of evaluating longer-term changes in N turnover rates. We conclude that eCO2 decelerates whereas eO3 accelerates N-cycling in the longer-term, but feedback through changed N2O emissions is not occurring in this soybean system.