Figure Legends
Fig. 1 Differential effects of N, P, and concurrent N and P additions on surface CH4 flux, soil NH4+ and plant N content in semiarid grassland from 2013 to 2016. (a) Net nutrient effects on mean CH4 uptake; (b) Net nutrient effects on soil NH4+ content in top 50 cm; and (c) Net nutrient effects on plant N content. The error bars indicate the standard error of means (n = 3 × 2). Different letters represent significant differences among treatments (p<0.05) by the one-way ANOVA. (d) N and P effects on CH4 uptake as normalized to the control treatment in semiarid grassland from 2013 to 2016; error bars indicate the standard error (note that the N + P all have suppression impact but in a lesser degree)
Fig. 2 A meta-analysis of N and P effects on CH4 uptake in grassland ecosystems (percentage is used to report the N and P impacts)
Fig. 3 Structural equation modeling (SEM) for the relative controls of soil NH4+ content, soil P content, plant N and plant P on CH4 flux under various treatments; (a) ambient (n=96, χ2=1.389, DF=0, CFI=0.986, NFI=1), (b) N addition (n=96, χ2=5.671, DF=0, CFI=0.968, NFI=1), (c) P addition (n=96, χ2=1.644, DF=0, CFI=0.993, NFI=1) and (d) concomitant N + P additions (n=96, χ2=10.347, DF=0, CFI=0.943, NFI=1) (black lines represent positive paths; red lines represent negative paths; the solid lines represent the path is statistically significant [p<0.05]; the dotted lines represent non-significant effects [p>0.05]).
Fig. 4 The model simulated contemporary (2005-2014) CH4 oxidation and N and P impacts across global grasslands; (a) absolute CH4 flux (Kg C ha-1 y-1) under ambient condition (pre-industrial N + P deposition); (b) relative impact of N deposition (contemporary N deposition and pre-industrial P deposition) compared to the ambient condition; (c) relative impact of P deposition (pre-industrial N deposition and contemporary P deposition) compared to the ambient condition; (d) relative impact of N + P depositions (contemporary N + P depositions) compared to the ambient condition.
Fig. 5 Graphic diagram showing substrate competition between CH4 and NH4+ for methane monooxygenase (MMO); under ambient condition (left panel), N addition condition (middle panel), and concomitant N + P additions (right panel); the trivial soil NH4+is in weak competitive equilibrium with CH4 for MMO under ambient condition; and the added NH4+ shifts the competitive equilibrium and thus reduces the CH4 oxidation; the P addition pushes the shifted equilibrium back and alleviates the N suppression on CH4 oxidation by stimulating N cumulation in vegetation biomass.