Results

A Field Experiment for N and P Impacts on CH4Uptake

The field experiment showed that the N-only and concurrent N + P additions treatments significantly suppressed CH4 uptake over the entire duration of the experiment (2013-2016, Fig. 1a ). Suppression of CH4 oxidation by N addition and N + P additions amounted to -16.3% (p < 0.01) and -7.9% (p = 0.04) relative to the control plots in 2013, -7.8% (p<0.01) and -1.8% (p=0.10) in 2014, -12.3% (p<0.01) and -6.6% (p<0.01) in 2015, and -18.3% (p<0.01) and -12.5% (p<0.01) in 2016, respectively (Fig. 1d ). Over the study period, the average annual CH4 uptake rates were 2.94 ± 0.07 kg C-CH4 ha-1 in control plots, 2.54 ± 0.04 kg C-CH4 ha-1 in N addition plots, and 2.73 ± 0.06 kg C-CH4ha-1 in N + P addition plots (Fig. 1a ). In comparison with the ambient treatment, the P-only treatment slightly and not statistically significantly stimulated CH4 uptake throughout the experiment: +7.4% (p = 0.12) in 2013, +4.0% (p = 0.09) in 2014, +1.2% (p = 0.2) in 2015, and +2.3% (p = 0.16) in 2016, respectively (Fig. 1d ). Overall, P addition alone did thus not affect CH4 uptake, N addition alone suppressed CH4 uptake, and concurrent N + P additions suppressed CH4 uptake to a lower degree, suggesting that P addition alleviates the N suppression of CH4 uptake in semiarid grasslands (Figs. 1a and 1d ).