Biogeochemical cycling of nutrients in karstic catchments, southwestern China: Linkages to changes of eco-environments

Abstract

In southwestern China, karst rocky desertification, a process of land degradation, takes place widely due to human impacts on the fragile karstic ecological systems, which includes soil erosion, extensive exposure of basement rocks, and drastic decrease in soil productivity. A study on biogeochemical cycling of nutrients in the karstic catchments has been carried out to understand the relationships between degradation of ecological system and nutrient cycling. Our studies on chemical and isotopic compositions ( 6 13C, 6 34S, 5 15N, 87Sr/S6Sr) of river, lake, rain, and ground water in the karstic catchments of southwestern China provided important information on the sources of major ions to them, chemical weathering rates of river basins, fluxes of dissolved materials from the drainage basins, and on their linkages to changes of ecological systems and environments. The weathering rates of carbonate rocks and the fluxes of nutrients from the river basins are high as compared with karstic terrains in the world. Comparative studies on the carbon isotopic composition of dissolved inorganic carbon in surface water of karstic and none-karstic catchments indicate that more dissolved inorganic carbon in the karstic rivers originated from respiration of soil organic carbon. The interrelationships between isotopic composition and chemical composition of the surface water suggest that the high weathering rates of carbonate rocks and large fluxes of carbon, sulfur, and nitrogen from the karstic river basins may be ascribed to the roles of sulfur and nitrogen cycling in the catchments. Weathering of carbonate rocks by sulfuric and nitric acids can give rise to a net source of CO2 to atmosphere through precipitation of carbonate minerals, which will change our view that weathering of carbonate rocks can neither subtract nor produce greenhouse gas in atmosphere from a long-term point of view. The sulfur and nitrogen isotopic compositions respectively of sulfate and nitrate ions in rain and ground and surface waters indicate important anthropogenic sources of these ions, in addition to natural origin. It is concluded that human activities can disturb the natural cycling of nutrients, and result in changes in ecological environments.