Rubber Cultivation Weakened the Soil Methane Sink Function Compared to Natural Forest

The impact of transforming natural forest into rubber plantations on soil function as CH4 sink has not been well understood, especially on the seasonal dynamic and changes in underlying soil processes. Aimed to understand how this land use conversion changes the soil CH4 sink, we conducted measurements in both natural forests and rubber plantations in Xishuangbanna, SW China. Temporal dynamic of soil surface CH4 flux, CH4 concentration and isotope signature of 13CH4 in the soil profile at 5, 10, 30 and 70 cm depth were measured at representative time in dry and rainy season. CH4 uptake by soils in rubber plantations was only 41.8 % of uptake by forest soils, with annual CH4 cumulative flux of -2.41±0.28 and -1.01±0.23 kgC ha1yr1 for forests and rubber plantations respectively. The CH4 oxidation was stronger in dry season than in rainy season, mainly explained by changes in soil moisture. From dry season to rainy season, CH4 flux shifted to weak consumption in forest and young rubber plantation, or even towards emission in older rubber plantations; CH4 concentration increased in all four depths with higher increment in older rubber plantations. The enrichment of soil CH4 by 13CH4 was higher in forest than in rubber plantations. The decrement of delta 13CH4 from dry to rainy season in both land uses indicated the increased CH4 production in rainy season, while the flux showed the net consumption. The CH4 turnover rate suggested that the surface 0–5 cm soil was the most active layer responsible for CH4 oxidation. Conversion the forest into rubber plantation weakened soil CH4 sink function. Seasonal change of CH4 flux and 13CH4 enrichment was larger under rainforest than rubber plantations, indicating modified soil water regime under rubber. Converting forest into rubber plantations and rubber cultivation may have profound impact on greenhouse gas emission from soil.

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