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.