Cascading multiscale watershed effects on differential carbon isotopic characteristics and associated hydrological processes

Understanding land-use change accompanied by anthropogenic activities under alterations in watershed size regulations or differential carbon (C) isotope characteristics remain a challenge in C cycling research. In this study, we investigate changes in the export of C composition and its isotopic characteristics at multiple scales in a subtropical cascading watershed in China. Results show that C concentrations in rainfall and dissolved total carbon (DTC), dissolved organic carbon (DOC) and delta C-13 in runoff seasonally fluctuate at a temporal scale. On average, the delta C-13 from silicate rock weathering was 31-32%, contributing the largest amount of delta C-13 in the different watersheds. Moreover, the contribution of isotopic composition from atmospheric deposition to the delta C-13 fraction increased as watershed size increased, while the corresponding contribution from soil organic matter (SOM) decomposition decreased. On the other hand, anthropogenic activities play a dominant role in the isotopic composition of large watersheds. In addition, the correlation coefficient between C transport via runoff and the delta O-18 value in rainfall increased as watershed size increased. This indicated that as a source rainfall had an obvious influence on C transport in runoff according to proportional values measured in event and pre-event water.