RESUMEN
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.
RESUMEN
The data of valuation of forest ecosystem service function (FESF) in 101 primary case studies of China were collected and obtained based on Specifications for Assessment of Forest Ecosystem Services in China (LY/T 1721-2008). FESF was then analyzed synthetically in terms of value coefficient. The results showed that the average value per unit area (VPUA) of FESF in China was 6.11×104 yuan·hm-2, and the order of VPUA of each service function was: water conservation (2.44×104 yuan·hm-2)> soil conservation (1.15×104 yuan·hm-2)> biodiversity conservation (1.00×104 yuan·hm-2)> carbon fixation and oxygen release (0.98×104 yuan·hm-2)> atmosphere environmental purification (0.28×104 yuan·hm-2)> forest recreation (0.23×104 yuan·hm-2)> action of forest against natural calamities (0.19×104 yuan·hm-2)> nutrient accumulation(0.16×104 yuan·hm-2). Water conservation, soil conservation, biodiversity conservation, carbon fixation and oxygen release were the four dominant service functions of forest ecosystem in China. The VPUA of FESF of the reserve level was higher than that of county level. The establishment of reserves played positive roles in biodiversity conservation and enhancement of service function, but the service function of forest recreation still existed with some insufficiency, and it needed to be further improved. Dominant service functions of forest ecosystem varied in different physicographic regions, and each type of service function presented different differentiation characteristics in space. The VPUA of FESF in South China was the highest up to 11.36×104 yuan·hm-2. The power regression correlation coefficients (R2) of the total value of FESF with forest area and forest stock volume were 0.905 (P<0.01) and 0.860 (P<0.01), respectively, indicating that forest area and forest stock volume were the two key factors affecting FESF and its total value. Moreover, latitude and mean annual precipitation also had significant effect on the VPUA of FESF.