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Title:   Author  item
                                            DECOCK, CHARLOTTE - University Of Californiaitem
                                            CHUNG, HAEGEUN - Korea Universityitem
                                            Venterea, Rodney

                                                         - *Rod*item
                                            LEAKEY, ANDREW D - University Of Illinoisitem
                                            SIX, JOHAN - University Of California
  Submitted to: Journal of Soil Biology and Biochemistry
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 4/7/2012
                                    Publication  Date: 5/1/2012
                                    Citation: Decock, C., Chung, H., Venterea, R.T., Leakey, A.B., Six, J. 2012. Elevated CO2 and O3 modify N turnover rates, but not N2O emissions. Journal of Soil Biology and Biochemistry. 51(1):104-114.Interpretive Summary: In order to predict and possibly to mitigate future climate change, it is essential to understand effects of elevated concentrations of carbon dioxide (CO2) and ozone (O3) in the atmosphere on soil nitrogen (N) cycling in agricultural cropping systems. These impacts include potential effects on the potent greenhouse gas nitrous oxide (N2O). These changes in soil N cycling may occur due to changes in plant processes caused by elevated CO2 (eCO2) and elevated O3 (eO3). In this study, we investigated the effects of soil moisture content together with eCO2 and eO3 on N2O emissions from the SoyFACE field experiment during a 28-day laboratory incubation experiment. We also measured field N2O fluxes during two soybean growing seasons (2005 and 2006). We also used stable isotope techniques which can reveal changes in  belowground N cycling processes occurring over longer time scales than examined in the lab and field experiments. This technique assesses changes in natural abundance ratios of 15N content in soil (also known as delta 15N), and 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. This resulted in a tighter N cycle in the longer-term with eCO2. In contrast, 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. These results will be useful to scientists and policy-makers with regard to development of plans to mitigate and/or adapt to changes in atmospheric concentrations of CO2 and O3.
                                Technical Abstract: 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.


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