In order to predict and mitigate future climate change, it is essential to understand plant-mediated effects of elevated CO2 (eCO(2)) and O-3 (eO(3)) 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 eCO(2) and eO(3) 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 (delta N-15). This method relies on the concept that soil delta N-15 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 eCO(2) and eO(3) 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 eCO(2), 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 eO(3), leading to increased gaseous N loss, most likely in the form of N-2. Given that effects of eCO(2) and eO(3) 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 eCO(2) decelerates whereas eO(3) accelerates N-cycling in the longer-term, but feedback through changed N2O emissions is not occurring in this soybean system. (C) 2012 Elsevier Ltd. All rights reserved.
