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 δ13C in runoff seasonally fluctuate at a temporal scale. On average, the δ13C from silicate rock weathering was 31–32%, contributing the largest amount of δ13C in the different watersheds. Moreover, the contribution of isotopic composition from atmospheric deposition to the δ13C 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 δ18O 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.