Global change, nitrification, and denitrification: A review

We reviewed responses of nitrification, denitrification, and soil N 2 O efflux to elevated CO 2 , N availability, and temperature, based on published experimental results. We used meta‐analysis to estimate the magnitude of response of soil N 2 O emissions, nitrifying enzyme activity (NEA), denitrifying enzyme activity (DEA), and net and gross nitrification across experiments. We found no significant overall effect of elevated CO 2 on N 2 O fluxes. DEA and NEA significantly decreased at elevated CO 2 ; however, gross nitrification was not modified by elevated CO 2 , and net nitrification increased. The negative overall response of DEA to elevated CO 2 was associated with decreased soil NO 3 − , suggesting that reduced availability of electron acceptors may dominate the responses of denitrification to elevated CO 2 . N addition significantly increased field and laboratory N 2 O emissions, together with gross and net nitrification, but the effect of N addition on field N 2 O 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 CO 2 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 CO 2 .