Local climate conditions explain the divergent climate change effects on (de)nitrification across the grassland biome: A meta-analysis

Ecosystem functions, such as soil nitrogen cycling, are being altered by climate change. The responses of soil (de) nitrification to climate change are different and sometimes opposite across global grasslands. However, it remains unclear how the local environmental conditions and the duration and magnitude of climate change experiments influence these responses. We synthesized the results of 49 studies corresponding to 518 observations on the effects of warming, elevated CO2, and elevated precipitation, and unfolded the reasons for the divergent responses in (de)nitrification to these climate change factors. We found that the responses of (de)nitrification are mainly related to annual precipitation and temperature, and less to the duration and magnitude of experimental treatment. In contrast, soil variables such as pH and organic carbon had no significant effects on these responses. Specifically, the effect of warming on the abundance of ammonia-oxidizing archaea (AOA) shifted from negative for mean annual precipitation (MAP) < 600 mm to positive for MAP>700 mm, whereas the effects on denitrification decreased with mean annual temperature (MAT). The effects of elevated CO2 on nitrification, denitrification and soil nitrate all decreased with MAP, with negative effects observed for MAP>700 mm. The effects of elevated P on nitrification and soil ammonium decreased with MAP, with negative effects observed for MAP>300 mm. The results suggested that in dry regions, altered water availability governs climate change effects, while changed nitrogen availability is likely the main determinant in wet regions. This shows how biochemical models should include local climatic conditions when predicting nitrogen dynamics across the grassland biome under future climate change scenarios.