Response of grassland soil respiration to experimental warming: The long-term effects may be greater than we thought

Soil respiration (Rs) profoundly affects the global carbon cycle, and its response to climate warming in grasslands shows significant heterogeneity for poorly understood reasons. A meta-analysis was undertaken to evaluate the effects of warming (including the magnitude, methods, and duration of warming) and environmental factors (including climate, soils, and plants) on the response of Rs to warming in grasslands, using a global dataset of 168 independent measurements. Multi-model inference, an information-theoretical method for synthesizing results of multiple alternative models, was used to quantify the relative importance of the environmental factors. We found that the response of Rs to warming followed a three-phase pattern: Rs increased initially, then remained unchanged, and then increased again as the duration of warming increased. The long-term response was greater than the short-term response. The Rs response was also affected by the method of warming, with open-top chambers promoting Rs more than infrared heaters. The response of Rs to warming was also positively related to the magnitude of warming. Rs was greatly increased with warming in temperate grasslands compared with that in cold grasslands but did not change significantly in arid grasslands. According to multi-model inference, this can be attributable to differences in soil nutrients and the mean annual precipitation. Our findings suggest a greater long-term stimulation of Rs in a warming world than we previously thought, so short-term studies cannot provide a reliable scientific basis for carbon projections under long-term climate warming. Our results also demonstrate that the carbon released in cold grasslands could be overestimated if the water deficiency was not considered.