Methane sink of subterranean space in an integrated atmosphere-soil-cave system

CH4 serves as an important greenhouse gas, yet limited knowledge is available in global and regional CH4 cycling, particularly in widely distributed karst terrain. In this study, we investigated an upland in Puding Karst Ecosystem Research Station, and explored CH4 concentration and/or flux in atmosphere, soil and cave using a closed static chamber method and an eddy covariance system. Meanwhile, we monitored atmospheric temperature, precipitation, temperature and wind velocity in the cave entrance. The results demonstrated that atmospheric CH4 and actual soil CH4 fluxes in the source area of eddy covariance system were -0.19±8.64 nmols-1m-2 and -0.16 nmols-1m-2 respectively. The CH4 concentrations in Shawan Cave exhibited 10∼100-fold lower than that of the external atmosphere. CH4 oxidation rate dominated by methane-oxidizing bacteria was 1.98 nmols-1m-2 in Shawan Cave when it combined with temperature difference between cave and external atmosphere. Therefore, CH4 sink in global karst subterranean spaces was estimated at 106.2 Tg CH4 yr-1. We supplemented an understanding of CH4 cycling paths and fluxes in karst terrain, as well as CH4 sinks in karst subterranean space. Further works require to establish a karst ecosystem observation network to conduct long-term integrated studies on CH4 fluxes regarding atmosphere, soils, plants and caves.