Atmospheric CO2 elevation has little effect on nitrifying and denitrifying enzyme activity in four European grasslands

The objective of this study was to determine what patterns, if any, existed in the response of nitrifying enzyme activity (NEA), denitrifying enzyme activity (DEA), soil microbial N and soil inorganic N to elevated CO2 across a broad range of grassland environments. We studied the response of these N pools and microbial activities in four CO2-enrichment sites of the MEGARICH project (Managing European Grasslands as a Sustainable Resource in a Changing Climate). CO2 treatment was studied in factorial combination with a cutting frequency treatment at two sites and with a temperature treatment at one site. Our study showed that microbial biomass N, NEA, DEA and extractable soil [NH4+] and [NO3-] were generally not affected by elevated CO2 in these grassland ecosystems after several years of treatment, nor by cutting frequency or temperature at the sites that included these treatments. Exceptions to this were that DEA and soil [NO3-] decreased by 22% and 45%, respectively, at the French site at elevated CO2. We discuss the possible explanations for this lack of response.