Global change, nitrification, and denitrification: A review

We reviewed responses of nitrification, denitrification, and soil N2O efflux to elevated CO2, N availability, and temperature, based on published experimental results. We used meta-analysis to estimate the magnitude of response of soil N2O emissions, nitrifying enzyme activity (NEA), denitrifying enzyme activity (DEA), and net and gross nitrification across experiments. We found no significant overall effect of elevated CO2 on N2O fluxes. DEA and NEA significantly decreased at elevated CO2; however, gross nitrification was not modified by elevated CO2, and net nitrification increased. The negative overall response of DEA to elevated CO2 was associated with decreased soil [NO3-], suggesting that reduced availability of electron acceptors may dominate the responses of denitrification to elevated CO2. N addition significantly increased field and laboratory N2O emissions, together with gross and net nitrification, but the effect of N addition on field N2O efflux was not correlated to the amount of N added. The effects of elevated temperature on DEA, NEA, and net nitrification were not significant: The small number of studies available stress the need for more warming experiments in the field. While N addition had large effects on measurements of nitrification and denitrification, the effects of elevated CO2 were less pronounced and more variable, suggesting that increased N deposition is likely to affect belowground N cycling with a magnitude of change that is much larger than that caused by elevated CO2.