[Insert Figure 2 here]
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 ).