Runoff-driven export of terrigenous particulate organic matter from a small mountainous river: sources, fluxes and comparisons among different rivers

The land-to-ocean export and subsequent preservation of terrestrial particulate organic carbon (POC) in oceans represents a major carbon sink in the global carbon cycle. Small mountainous rivers (SMRs) are hotspots of this process; however, how the sources and fluxes of POC change during rain events in low-gradient SMRs and how they compare with those in steep SMRs are unknown. Here, we combined the bulk properties and lignin phenols to trace changes in POC sources and fluxes in a low-gradient SMR (stream gradient: 0.6%) in southeastern China. Our results were further compared with other SMRs to understand the geomorphological influences on the POC export from SMRs. The results indicated that POC sources changed from aquatic organisms and deep soils during low-flow periods to surface soils and plant debris during high-flow periods. The lignin concentration showed a significant positive relationship with the runoff, suggesting that hydrology played an important role in exporting terrigenous POC in low-gradient SMRs. We further estimated that approximately 22%, 56% and 79% of annual water, POC and particulate lignin were exported during high-flow periods, respectively. Furthermore, we found that with increasing water discharge, the magnitude of the lignin increase was greater in SMRs with smaller stream gradients. Our results suggest that the export of terrigenous POC from low-gradient SMRs is driven by hydrology and is more sensitive to rain events than that in steep SMRs.