Global change, nitrification, and denitrification: a review

[1] 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.