Abstract
Although tropical forest soils contributed substantially global soil methane uptake, observations on soil methane fluxes in tropical forests are still sparse, especially in Southeast Asia, leading to large uncertainty in the estimation of global soil methane uptake. Here, we conducted two-year (from Sep, 2016 to Sep, 2018) measurements of soil methane fluxes in a lowland tropical forest site in Hainan island, China. At this tropical forest site, soils were substantial methane sink, and average annual soil methane uptake was estimated at 2.00 kg CH4-C ha−1 yr−1. The seasonality of soil methane uptake showed strong methane uptake in the dry season (−1.00 nmol m−2 s−1) and almost neutral or weak soil methane uptake in the wet season (−0.24 nmol m−2 s−1). The peak soil methane uptake rate was observed as −1.43 nmol m−2 s−1 in February, 2018, the driest and coolest month during the past 24 months. Soil moisture was the dominant controller of methane fluxes, and could explain 94% seasonal variation of soil methane fluxes. Soil temperature could not enhance the explanation of seasonal variation of soil methane fluxes on the top of soil moisture. A positive relationship between soil methane uptake and soil respiration was also detected, which might indicate co-variation in activities of methanotroph and roots and/or microbes for soil heterotrophic respiration. Our study highlights that tropical forests in this region acted as a methane sink.
Highlights
Tropical forest soils contributed substantially global soil methane uptake, observations on soil methane fluxes in tropical forests are still sparse, especially in Southeast Asia, leading to large uncertainty in the estimation of global soil methane uptake
More than 90% of these methane sources are destructed by atmospheric chemical loss, and 5%-7% of these methane sources are consumed by methanotrophic bacteria in unsaturated oxic soil
Soil methane uptake acts as an important methane sink, the global bottom-up estimation of soil methane uptake still has a large uncertainty of 9–47 Tg CH4 yr−1 2–4
Summary
Tropical forest soils contributed substantially global soil methane uptake, observations on soil methane fluxes in tropical forests are still sparse, especially in Southeast Asia, leading to large uncertainty in the estimation of global soil methane uptake. We conducted two-year (from Sep, 2016 to Sep, 2018) measurements of soil methane fluxes in a lowland tropical forest site in Hainan island, China At this tropical forest site, soils were substantial methane sink, and average annual soil methane uptake was estimated at 2.00 kg CH4-C ha−1 yr−1. In the comprehensive meta-analysis of global field measurements of methane uptake[6], tropical forest soils methane uptake (6.2 Tg CH4 yr−1) contributed to 28% of global soil methane uptake This significant methane sink in tropical forest soils has the potential to change the atmospheric methane growth rate. To address the gap of methane fluxes in tropical forest soils in Southeast Asia, we started to measure soil methane fluxes since 2016 at a 1 ha plot established in a lowland tropical rainforest within Diaoluoshan National Nature Reserve in Hainan island (18°40′N, 109°54′E, elevation 255 m, Fig. 1). With two consecutive years of soil methane fluxes measurements, our objectives were (1) to assess whether tropical forest soils at this study site were methane sources or sinks; (2) to explore the monthly and annual magnitude, seasonal variation in soil methane fluxes at this study site; and (3) to investigate the main drivers of soil methane fluxes at this study site
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