Increased phosphorus availability mitigates the inhibition of nitrogen deposition on CH ₄ uptake in an old-growth tropical forest, southern China

It is well established that tropical forest ecosystems are often limited by phosphorus (P) availability, and elevated atmospheric nitrogen (N) deposition may further enhance such P limitation. However, it is uncertain whether P availability would affect soil fluxes of greenhouse gases, such as methane (CH ₄) uptake, and how P interacts with N deposition. We examine the effects of N and P additions on soil CH ₄ uptake in an N saturated old-growth tropical forest in southern China to test the following hypotheses: (1) P addition would increase CH ₄ uptake; (2) N addition would decrease CH ₄ uptake; and (3) P addition would mitigate the inhibitive effect of N addition on soil CH ₄ uptake. Four treatments were conducted at the following levels from February 2007 to October 2009: control, N-addition (150 kg N ^-1 yr ^-1), P-addition (150 kg P ^-1 yr ^-1), and NP-addition (150 kg N ^-1 yr ^-1 plus 150 kg N ^-1 yr ^-1 plus 150 kg P ^-1 yr ^-1). Static chamber and gas chromatography techniques were used to quantify soil CH ₄ uptake every month throughout the study period. Average CH ₄ uptake rate was 31.2 ± 1.1 μg CH ₄-C m ^-2 h ^-1 in the control plots. The mean CH ₄ uptake rate in the N-addition plots was 23.6 ± 0.9 μg CH ₄-C m ^-2 h ^-1, significantly lower than that in the controls. P-addition however, significantly increased CH ₄ uptake by 24% (38.8 ± 1.3 μg CH ₄-C m ^-2 h ^-1), whereas NP-addition (33.6 ± 1.0 μg CH ₄-C m ^-2 h ^-1) was not statistically different from the control. Our results suggest that increased P availability may enhance soil mathanotrophic activity and root growth, resulting in potentially mitigating the inhibitive effect of N deposition on CH ₄ uptake in tropical forests.