Climatic and geomorphic controls on the erosion of terrestrial biomass from subtropical mountain forest

Erosion of particulate organic carbon (POC) occurs at very high rates in mountain river catchments, yet the proportion derived recently from atmospheric CO 2 in the terrestrial biosphere (POC non‐fossil ) remains poorly constrained. Here we examine the transport of POC non‐fossil in mountain rivers of Taiwan and its climatic and geomorphic controls. In 11 catchments we have combined previous geochemical quantification of POC source (accounting for fossil POC from bedrock), with measurements of water discharge (Q w ) and suspended sediment concentration over 2 years. In these catchments, POC non‐fossil concentration (mg L −1 ) was positively correlated with Q w , with enhanced loads at high flow attributed to rainfall driven supply of POC non‐fossil from forested hillslopes. This climatic control on POC non‐fossil transport was moderated by catchment geomorphology: the gradient of a linear relation of POC non‐fossil concentration and Q w increased as the proportion of steep hillslopes (>35°) in the catchment increased. The data suggest enhanced supply of POC non‐fossil by erosion processes which act most efficiently on the steepest sections of forest. Across Taiwan, POC non‐fossil yield was correlated with suspended sediment yield, with a mean of 21 ± 10 tC km −2 yr −1 . At this rate, export of POC non‐fossil imparts an upper bound on the time available for biospheric growth, of ∼800 yr. Over longer time periods, POC non‐fossil transferred with large amounts of clastic sediment can contribute to sequestration of atmospheric CO 2 if buried in marine sediments. Our results show that this carbon transfer should be enhanced in a wetter and stormier climate, and the rates moderated on geological timescales by the regional tectonic setting.