Responses of soil N 2 O emissions and their abiotic and biotic drivers to altered rainfall regimes and co‐occurring wet N deposition in a semi‐arid grassland

Abstract

Global change factors such as changed rainfall regimes and nitrogen (N) deposition contribute to increases in the emission of the greenhouse gas nitrous oxide (N 2 O) from the soil. In previous research, N deposition has often been simulated by using a single or a series of N addition events over the course of a year, but wet N deposition actually co‐occurs with rainfall. How soil N 2 O emissions respond to altered rainfall amount and frequency, wet N deposition, and their interactions is still not fully understood. We designed a three‐factor, fully factorial experiment with factors of rainfall amounts (ambient, −30%) rainfall frequency (ambient, ±50%) and wet N deposition (with/without) co‐occurring with rainfall in semi‐arid grassland mesocosms, and measured N 2 O emissions and their possible biotic and abiotic drivers. Across all treatments, reduced rainfall amount and N deposition increased soil N 2 O emissions by 35% and 28%, respectively. A significant interactive effect was observed between rainfall amount and N deposition, and to a lesser extent between rainfall frequency and N deposition. Without N deposition, reduced rainfall amount and altered rainfall frequency indirectly affected soil N 2 O emissions by changing the abundance of nirK and soil net N mineralization, and the changes in nirK abundance were indirectly driven by soil N availability rather than directly by soil moisture. With N deposition, both the abundance of nirK and the level of soil water‐filled pore space contributed to changes in N 2 O emissions in response to altered rainfall regimes, and the changes in the abundance of nirK were indirectly driven by plant N uptake and nitrifier (ammonia‐oxidizing bacteria) abundance. Our results imply that unlike wetter grassland ecosystems, reduced precipitation may increase N 2 O emissions, and N deposition may only slightly increase N 2 O emissions in arid and semi‐arid N‐limited ecosystems that are dominated by grasses with high soil N uptake capacity.

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