Urbanisation accelerates carbon cycling and greenhouse gas emissions along a river-ocean continuum in the humid subtropics.

Carbon is constantly transported from the land into the ocean via rivers, and urbanisation accelerates this process. Meanwhile, greenhouse gas emissions from rivers are expected to be significant, but precise drivers remain poorly understood. In this year-long study, we focused on the Lam Tsuen River and its estuary in Hong Kong to investigate how carbon was cycled in the urbanised river-ocean continuum. Compared to the natural upland sites, the urbanised lowland sites were found to have significantly higher concentrations of both dissolved organic carbon (DOC; mean: 2.9 mg L-1 and 0.9 mg L-1 at lowland and upland sites, respectively) and particulate organic carbon (POC; mean: 1.0 mg L-1 and 0.4 mg L-1 at lowland and upland sites, respectively). Soils in the steep upland sites may not have fully developed since the latest disturbance, whereas the lowland sites are influenced by unregulated sewage discharge through sewers. POC δ13C values at the lowland sites (mean: -25.5‰) were consistently less negative than those at the upland sites (mean: -27.8‰), indicating that allochthonous, anthropogenic inputs were more important than autochthonous production for the former. The lowland sites also had >10 times higher carbon dioxide (CO2) and >100 times higher methane (CH4) downstream transport rates than the upland sites, owing to the decomposition of organic matter contributed by sewage. Moreover, high flow velocities induced by channelisation at the most downstream site translated into high CO2 emission rates, comparable to those observed at upland sites. Our findings revealed significant spatial variation in carbon cycling processes and underscored the combined contributions of both pollution and channelisation to driving carbon emissions.