Responses of soil nitrogen cycling to the interactive effects of elevated CO2 and inorganic N supply
Despite increasing interest in the effects of climate change on soil processes, the response of nitrification to elevated CO2remains unclear. Responses may depend on soil nitrogen (N) status, and inferences may vary depending on the methodological approach used. We investigated the interactive effects of elevated CO2 and inorganic N supply on gross nitrification (using 15N pool dilution) and potential nitrification (using nitrifying enzyme activity assays) in Dactylis glomerata mesocosms. We measured the responses of putative drivers of nitrification (NH4+ production, NH4+ consumption, and soil environmental conditions) and of potential denitrification, a process functionally linked to nitrification. Gross nitrification was insensitive to all treatments, whereas potential nitrification was higher in the high N treatment and was further stimulated by elevated CO2 in the high N treatment. Gross mineralization and NH4+ consumption rates were also significantly increased in response to elevated CO2 in the high N treatment, while potential denitrification showed a significant increase in response to N addition. The discrepancy between the responses of gross and potential nitrification to elevated CO2 and inorganic N supply suggest that these measurements provide different information, and should be used as complementary approaches to understand nitrification response to global change.