Warming Reduces the Positive Effect of Nitrogen Addition on Soil Organic Carbon in Grasslands

Nitrogen (N) addition can increase soil organic carbon (SOC) in some grasslands, however, it is unknown whether N addition may enhance SOC in a warmer climate at broad spatial scales. We conduct a field experiment to test how N addition and warming interactively influence SOC in a temperate semiarid grassland. N addition significantly increases SOC by 14%, while simultaneous N addition and warming significantly decreases SOC by 25% relative to N-only addition. This result is further supported by a global meta-analysis, which shows that N addition (1 g N m^-2 yr^-1) increases SOC content by 0.2% relative to ambient conditions in grasslands, but the positive effect of N on SOC significantly declines at a rate of 0.06% per °C of increased mean annual temperature. Our field experiment and meta-analysis suggest that warming may reduce or even eliminate the positive effect of N addition on SOC in grasslands.