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Elevated CO2 and O3 modify N turnover rates, but not N2O emissions

Decock, Charlotte, Chung, Haegeun, Venterea, Rodney, Gray, Sharon B., Leakey, Andrew D.B., Six, Johan
Soil biology & biochemistry 2012 v.51 pp. 104
agroecosystems, carbon dioxide, climate change, crop production, emissions, nitrogen content, nitrous oxide, soil surface layers, soil water content, soybeans, stable isotopes
In order to predict and mitigate future climate change, it is essential to understand effects of elevated CO2 (eCO2) and O3 (eO3) on N-cycling, including N2O emissions, due to plant mediated changes. 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 (delta 15N). This method relies on the concept that soil delta 15N 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 N2O emissions. Natural abundance isotope analyses suggested 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. Under eO3, increased belowground allocation of biologically fixed N led 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 previously observed for this agroecosystem have been minimal, our results illustrate the importance of tools that can detect 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 soybean systems.