Tropical forest soils serve as substantial and persistent methane sinks

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.

Figure 1

The geographic location of the research site.

Figure 2

The variation on soil temperature (Ts) and soil water content (SWC) during the observation period.

Figure 3

Temporal pattern on methane flux expressed as box plot. The red solid line connected the mean values.

Figure 4

The dependence of methane flux on soil moisture (a) and temperature (b).

Figure 5

The co-control of soil moisture and temperature on methane flux.

Figure 6

The relationship between soil respiration (${\mathit{F}}_{\mathbf{C}{\mathbf{O}}_2}$) and soil methane flux (${\mathit{F}}_{\mathbf{C}{\mathbf{H}}_4}$). Data points in different measurements were indicated with different colors. Lines represent linear regressions. The black bold line shows the linear regression with all data points.

Figure 7

The variation of methane oxidation to soil respiration ratio (a) and the dependence of the ratio on soil moisture (b).

Figure 8

The literature review based annual methane flux estimates globally.