Dissolved organic carbon in rainwater from a karst agricultural area of Southwest China: Variations, sources, and wet deposition fluxes

Dissolved organic carbon (DOC) plays a key role among the earth-surficial ecosystem, particularly in the climate change, but very few researches focused on the rainwater DOC in karst agricultural region (vulnerable agroecosystem). To identify the concentrations, seasonal variations, controlling factors, deposition fluxes, and potential sources of DOC in rainwater, 85 rainwater samples were collected at the Houzhai catchment, a representative karst agriculture-intensive region in Southwest China, from June 2016 to May 2017. The concentrations and deposition flux of DOC were 0.63 mg C L-1 (volume-weighted mean) and 0.67 g C m(-2) yr(-1). These values were lower than the Asian average value, reflecting a low level of contaminated atmospheric organic carbon in the studied area. Seasonally, low DOC concentrations were often accompanied by high deposition flux during the rainy season (heavy rainfall), suggesting that the amount of rainfall is the critical factor controlling the rainwater DOC. Moreover, long-distance migration and anthropogenic emissions were also the non-negligible impact factors. The source identification showed that both agriculture-related volatilization and fossil fuel burning (mainly coal-combustion) were the primary sources of rainwater DOC according to the correlation analysis with the major ions (typical indicators), while the contribution of other origins (e.g., plant-released, biomass burning, and windblown dust) were limited. This study clearly explores the karst-agricultural wet deposition process of DOC that would strongly benefit the study of the global carbon biogeochemical cycle.