Increased phosphorus availability mitigates the inhibition of nitrogen deposition on CH₄ uptake in an old-growth tropical forest, southern China

Abstract. 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 (CH₄) uptake, and how P interacts with N deposition. We examine the effects of N and P additions on soil CH₄ uptake in an N saturated old-growth tropical forest in southern China to test the following hypotheses: (1) P addition would increase CH₄ uptake; (2) N addition would decrease CH₄ uptake; and (3) P addition would mitigate the inhibitive effect of N addition on soil CH₄ uptake. Four treatments were conducted at the following levels from February 2007 to October 2009: control, N-addition (150 kg N ha⁻¹ yr⁻¹), P-addition (150 kg P ha⁻¹ yr⁻¹), and NP-addition (150 kg N ha⁻¹ yr⁻¹ plus 150 kg P ha⁻¹ yr⁻¹). Static chamber and gas chromatography techniques were used to quantify soil CH₄ uptake every month throughout the study period. Average CH₄ uptake rate was 31.2 ± 1.1 μg CH₄-C m⁻² h⁻¹ in the control plots. The mean CH₄ uptake rate in the N-addition plots was 23.6 ± 0.9 μg CH₄-C m⁻² h⁻¹, significantly lower than that in the controls. P-addition however, significantly increased CH₄ uptake by 24% (38.8 ± 1.3 μg CH₄-C m⁻² h⁻¹), whereas NP-addition (33.6 ± 1.0 μg CH₄-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 CH₄ uptake in tropical forests.