Nitrification, denitrification, and related functional genes under elevated CO 2 : A meta-analysis in terrestrial ecosystems.

Global change may have profound effects on soil nitrogen (N) cycling that can induce positive feedback to climate change through increased nitrous oxide (N 2 O) emissions mediated by nitrification and denitrification. We conducted a meta-analysis of the effects of elevated CO 2 on nitrification and denitrification based on 879 observations from 58 publications and 46 independent elevated CO 2 experiments in terrestrial ecosystems. We investigated the effects of elevated CO 2 alone or combined with elevated temperature, increased precipitation, drought, and N addition. We assessed the response to elevated CO 2 of gross and potential nitrification, potential denitrification, and abundances of related functional genes (archaeal amoA, bacterial amoA, nirK, nirS, and nosZ). Elevated CO 2 increased potential nitrification (+28%) and the abundance of bacterial amoA functional gene (+62%) in cropland ecosystems. Elevated CO 2 increased potential denitrification when combined with N addition and higher precipitation (+116%). Elevated CO 2 also increased the abundance of nirK (+25%) and nirS (+27%) functional genes in terrestrial ecosystems and of nosZ (+32%) functional gene in cropland ecosystems. The increase in the abundance of nosZ under elevated CO 2 was larger at elevated temperature and high N (+62%). Four out of 14 two-way interactions tested between elevated CO 2 and elevated temperature, elevated CO 2 and increased precipitation, and elevated CO 2 and N addition were marginally significant and mostly synergistic. The effects of elevated CO 2 on potential nitrification and abundances of bacterial amoA and nirS functional genes increased with mean annual temperature and mean annual precipitation. Our meta-analysis thus suggests that warming and increased precipitation in large areas of the world could reinforce positive responses of nitrification and denitrification to elevated CO 2 and urges the need for more investigations in the tropical zone and on interactive effects among multiple global change factors, as we may largely underestimate the effects of global change on soil N 2 O emissions.