Abstract
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.
Highlights
Methane (CH4) is considered the second most important greenhouse gas after carbon dioxide and with a global warming potential 25 times compare to carbon dioxide (CO2) over a 100 year horizon (IPCC, 2007)
We hypothesized that: (1) N addition would inhibit soil CH4 uptake as we found in a previous study in the same forest (Zhang et al, 2008); (2) P addition would increase soil CH4 uptake due to that P addition world stimulate plant root growth and lead to higher water uptake, and lower WFPS which lead to higher gas diffusion and, higher CH4 uptake; (3) NP addition would have less effect on soil CH4 uptake comparing with N or P addition alone
P addition could reverse such trends by stimulating forest soil CH4 uptake. In this 33 months field experiment, we showed that increased P availability might mitigate the inhibitive effect of N addition on soil CH4 uptake in this N-saturated oldgrowth tropical forest
Summary
Methane (CH4) is considered the second most important greenhouse gas after carbon dioxide and with a global warming potential 25 times compare to carbon dioxide (CO2) over a 100 year horizon (IPCC, 2007). Upland oxic soil that are continuously emerged and exposed to atmospheric concentrations of CH4 is another major sink of atmospheric CH4, which consumes CH4 through the activity of methanotrophs under aerobic conditions (∼30 Tg CH4 yr−1) (Lelieveld et al, 1998). Atmospheric methane originates mainly from a biological process. In forest soils, it is produced in anoxic layer by methanogenic bacteria during the anaerobic digesting of organic matter. Methane is eliminated into forest soils by microbial oxidation (methanotrophy) in the aerobic zone and which oxidises atmospheric methane.Methanotrophs in forest soil use CH4 as only a carbon (C) and energy source and oxygen availability is the main factor limiting their activity (Le Mer and Roger, 2001). Methanotrophy in forest soils is primarily dependent on physical factors controlling soil diffusion, e.g., water content and soil texture
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