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Mechanisms of N and P impacts on CH4uptake

Based on various statistical analysis, we hypothesize that the following mechanisms underpin the N and P impacts on CH4 uptake. Whereas experimental N addition leads to an accumulation of mineral N (NH4+ and NO3-) in the soil (N-only compared with control), P addition enhances plant N uptake and N accumulation in vegetation biomass (N+P treatment compared with N-only addition) (Figs. 1b, 1c, S1, 3a, 3b, 3c, and 3d ). In our field experiment, P-only addition significantly decreased soil NH4+ content (p < 0.01,Fig. 1b ), but did not significantly affect plant N content (P-only addition compared with control) (p = 0.35, Fig. 1c ). The concurrent N + P additions, in contrast, did stimulate N accumulation in vegetation to a much higher degree than N addition alone (p < 0.01, Fig. 1c ). This increase in vegetation N accumulation significantly reduced soil NH4+ content under the N + P additions treatment compared to the N-only treatment (p < 0.01,Fig 1b ). Therefore, the P addition substantially stimulates plant growth and plant uptake of NH4+(Dijkstra et al., 2012), which reduces the NH4+ concentration in soils and thereby alleviates the N suppression of CH4 uptake (Fig. S1 ).