Isotopically-tracked hydrological changes in carbon cycling and its sources in a Chinese subtropical forested watershed

Abstract Forests are important components of the terrestrial ecosystem that play major roles in carbon cycling and associated environmental changes, but more knowledge of the processes involved is required. Therefore, carbon wet deposition and rainfall-runoff processes were examined, as well as carbon fluxes and sources in water in the Xiangxi River watershed, a typical subtropical forested watershed in China. Wet deposition of carbon amounted to 1.31 × 104 kg km−2 between June 2015 and May 2016 (inclusive). The concentration of measured forms of carbon in rainwater of single rainfall event peaked in summer, when the concentration of dissolved total carbon (DTC) was 16.5 mg L−1. In 2016, rainfall-runoff amounted to 7440 kg km−2, accounting for about 57% of the carbon wet deposition in the watershed. The forest ecosystem in the watershed was found to be a carbon sink every month of the year, with net ecosystem CO2 exchange (NEE) in 2016 of −5.11 × 105 kg C km−2, and monthly NEE was lowest in July (−8.93 × 104 kg C km−2). The 13C/12C ratio in dissolved inorganic carbon in the river water, relative to Pee Dee Belemnite (δ13CDIC), was higher in the dry season (range, −12.00 to −9.70‰; mean, −11.0‰) than in the rainy season (range, −14.00 to −12.20‰; mean, −12.90‰), and higher in downstream reaches (mean values in winter and spring: −10.45 and −12.35‰, respectively) than in upstream and middle reaches (−10.63 and −13.30‰, −11.63 and −12.87‰). In addition, atmospheric precipitation made the highest contributions to the C concentration in the river water in spring and winter (48.6 and 43.2%, respectively).