Mechanism of methane uptake in profiles of tropical soils converted from forest to rubber plantations

Abstract

Land-use change modulates the balance between methane (CH4) oxidation by soil methanotrophs, and production by methanogens through changes in soil physical, chemical and biological properties. The large-scale expansion of rubber plantations in Southeast Asia has decreased CH4 uptake by soil, but a mechanistic understanding of the associated processes within the soil profile is missing. To assess such land-use change impacts, we quantified the relative controls of CH4 diffusion and oxidation processes on soil profile methane fluxes using two datasets from Xishuangbanna, Southwest China. CH4 concentration, bulk density, soil temperature and moisture were measured at 5, 10, 30 and 70 cm depths during the dry and rainy season to determine the effective CH4 diffusion coefficient, diffusive flux, turnover rate and methanotrophic activity. Isotopic fractionation of carbon during CH4 oxidation was estimated using δ13CH4 profiles from in situ soil probes sampled in the dry season. This fractionation factor was used to model the seasonality of CH4 fluxes under the two land uses. CH4 consumption at 0–5 cm soil depth was significantly greater in natural forest than in rubber plantations, with a mean CH4 flux of −23.8 ± 1.0 and −14.4 ± 1.0 μg C m−2 h−1, respectively. Atmospheric CH4 oxidized in the top 10 cm accounted for 93% and 99% of total consumption in forest and rubber profiles, respectively. CH4 diffusivity at the four sampled depths was significantly lower in rubber plantations than in forest. Higher soil water content predominately explained the weakened CH4 sink in converted rubber plantations. During the transition from dry season to rainy season, methanotrophic activity at 5–10 cm depth decreased by 99.6% and 83.3% in forest and rubber plantations, respectively. The estimated isotope fractionation factor for carbon due to CH4 oxidation was 1.0292 ± 0.0015 (n = 12). A diffusion-oxidation model of the 13CH4 profile explained the oxidative behavior in the dry season, but suggested CH4 production in the subsoil during the rainy season. This production needs to be considered in future studies to explain changes in CH4 processes caused by land use transformations. The weakened CH4 sink function in soils under rubber monoculture exerts a negative effect on the global CH4 budget and climate change, given the large extent of rubber plantations worldwide. More sustainable cultivation approaches, such as rubber agroforestry that also improve soil structure and aeration are needed to mitigate this negative impact.