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Climate and regional air quality models predict that Southern California will experience longer and more severe droughts, and possibly wetter, more intense storms and changing nitrogen (N) deposition. We investigated how the three major soil greenhouse gas (GHG) fluxes respond to 4-6 years of exposure to a full-factorial experiment of reduced and augmented precipitation crossed with increased N in a semi-arid grassland in Irvine, CA, USA. The mean emission fluxes across all treatments were 249.8 mg CO2 m(-2) h(-1), -16.41 mu g CH4 m(-2) h(-1), and 2.24 mu g N2O m(-2) h(-1). Added N plots released 3.5 times more N2O than ambient N plots, and N treatment and soil moisture interacted, such that volumetric soil moisture in added N plots correlated positively with N2O release. Soil moisture, which was higher in the added water plots, correlated positively with respiration. CH4 consumption increased with soil moisture in the drought treatment, an opposite trend to that observed in most other studies.

Our data suggest that CH4 consumption, N2O production, and soil respiration will decline if Southern California grasslands experience more frequent and extreme droughts. However, when drought is followed by high rainfall, the additional moisture will likely increase CH4 consumption and N2O release in periodic pulses. Overall, climatic shifts in this ecosystem may lead to a decrease in overall soil GHG emissions to the atmosphere. However, increased N deposition to Southern California will likely lead to increased N2O release and a shift in the dominant N loss pathway toward gaseous release of N. If N deposition continues to increase along with severity and duration of drought, our data predict a decrease in global warming potential (GWP) of 17.2% from this ecosystem.