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.