Effects of elevated CO2 and O3 on N-cycling and N2O emissions: a short-term laboratory assessment

Elevated atmospheric CO2 (eCO(2)) and tropospheric O-3 (eO(3)) can alter soil microbial processes, including those underlying N2O emissions, as an indirect result of changes in plant inputs. In this study, effects of eCO(2) and eO(3) on sources of N2O in a soybean (Glycine max (L.) Merr.) agroecosystem in Illinois (SoyFACE) were investigated. We hypothesized that increases in available C and anaerobic microhabitat under eCO(2) would stimulate N2O emissions, with a proportionally larger increase in denitrification derived N2O (N2OD) compared to nitrification plus nitrifier denitrification derived N2O (N2ON+ND). We expected opposite effects under eO(3). Isotopically labeled (NH)-N-15 (4) (14) NO3 and (NH)-N-14 (4) (15) NO3 were used to evaluate mineral N transformations, N2OD, and N2ON+ND in a 12-day incubation experiment. We observed minimal effects of eCO(2) and eO(3) on N2O emissions, movement of (15) N through mineral N pools, soil moisture content and C availability. Possibly, altered C and N inputs by eCO(2) and eO(3) were small relative to the high soil organic C content and N-inputs via biological N-2-fixation, minimizing potential effects of eCO(2) and eO(3) on N-cycling. We conclude that eCO(2) and eO(3) did not affect N2O emissions in the short term. However, it remains to be tested whether N2O emissions in SoyFACE will be unaltered by eCO(2) and eO(3) on a larger temporal scale under field conditions.