Impact of land use on the DOM composition in different seasons in a subtropical river flowing through a region undergoing rapid urbanization

The dissolved organic matter (DOM) composition in river ecosystems can reflect human impacts on the river ecosystem, and plays an important role in the carbon cycling process. We collected water and phytoplankton samples at 107 sites in the Dongjiang River in two seasons to assess the impact of the sub-catchments land use composition. The results showed that (1) the forested sub-catchments had higher humic-like C1 (16.45%) and C2 (25.04%) and lower protein-like C3 (22.57%) and C4 (35.95%) than the urbanized and mixed forest-agriculture sub-catchments, while the urbanized sub-catchments showed an inverse trend (4.54%, 15.51%, 33.97% and 45.98%, respectively). (2) A significant variation in the proportion of C1 and C4 between the dry and rainy seasons was recorded in both the forested and the mixed forest-agriculture sub-catchments (p < 0.01), but only C4 showed an obvious seasonal variation in the urbanized sub-catchments (p < 0.01); while the DOM composition was mainly related to the proportion of urbanized and forested land year-round (p < 0.01). (3) No significant correlation was found in either season between the DOM composition and the agricultural land proportion (p > 0.05). Our findings indicated that the DOM composition was strongly dependent on the land use composition of the sub-catchments and varied seasonally, but the seasonal variation pattern could be disturbed by human activities in the extensively urbanized catchments. Notably, when considering bioavailability, the higher C4 proportion compared with those in temperate rivers implied a larger amount of CO2 released from subtropical rivers into the atmosphere.