[Insert Figure 1 here]

A Meta-analysis of N and P Impacts on CH4 Uptake in Grasslands

To investigate the universality of these N and P effects on CH4 uptake in grassland soils, we carried out a global meta-analysis to quantify the responses of CH4 uptake rates in grassland ecosystems to nutrient addition treatments (N-alone, P-alone, and concurrent N + P additions) (Fig. 2 ). This global meta-analysis showed that, relative to the control treatments, N addition and concurrent N + P additions cause significant declines in annual CH4 uptake: -27% (p < 0.01) and -14% (p < 0.01), respectively (Fig. 2 ). The smaller inhibition effect of concurrent N + P additions than the N-only addition on CH4 uptake (p = 0.02) (Fig. 2 ) is consistent with the results of our field experiment (Figs. 1a and2 ).