Composition and activity of nitrifier communities in soil are unresponsive to elevated temperature and CO2, but strongly affected by drought

Nitrification is a fundamental process in terrestrial nitrogen cycling. However, detailed information on how climate changeaffects the structure of nitrifier communities is lacking, specifically from experiments in which multiple climate changefactors are manipulated simultaneously. Consequently, our ability to predict how soil nitrogen (N) cycling will change in afuture climate is limited. We conducted afield experiment in a managed grassland and simultaneously tested the effects ofelevated atmospheric CO2, temperature, and drought on the abundance of active ammonia-oxidizing bacteria (AOB) andarchaea (AOA), comammox (CMX)Nitrospira, and nitrite-oxidizing bacteria (NOB), and on gross mineralization andnitrification rates. We found that N transformation processes, as well as gene and transcript abundances, and nitrifiercommunity composition were remarkably resistant to individual and interactive effects of elevated CO2and temperature.During drought however, process rates were increased or at least maintained. At the same time, the abundance of activeAOB increased probably due to higher NH4+availability. Both, AOA and comammoxNitrospiradecreased in response todrought and the active community composition of AOA and NOB was also significantly affected. In summary, ourfindingssuggest that warming and elevated CO2have only minor effects on nitrifier communities and soil biogeochemical variablesin managed grasslands, whereas drought favors AOB and increases nitrification rates. This highlights the overridingimportance of drought as a global change driver impacting on soil microbial community structure and its consequences for Ncycling.