Seasonal hydrological change shaping the relationship between dissolved organic matter and land use in the middle reaches of the Yangtze river

The effect of hydrology and land use on the sources and molecular composition of dissolved organic matter (DOM) within large rivers is vital for carbon cycling. However, it is still unclear how hydrological change influences the response of DOM composition to land use. Here, we report longitudinal patterns in DOM content and composition based on absorbance and fluorescence measurements along the middle Reaches of the Yangtze river, during two contrasting hydrological seasons. We found that soil leachate, metropolitan effluent, and microbiological production mainly contribute to the composition of DOM in the middle Yangtze. In addition, the relationship between land use and the fluorescence fingerprint of riverine DOM was season dependent. During the wet season, a higher content of aromatic and humic DOM was mobilized due to enhanced forest soil erosion, and relatively shallow soil-to-mainstem flow paths (R-2 = 0.76, p < 0.01). While in the dry season, a higher content of aliphatic and protein-like DOM was likely derived from the deeper subsoil horizons or groundwater from the waters area (R-2 = 0.54, p < 0.01), due to declined water level of the mainstem. Changes in water residence time modulate the fate of DOM in the Three Gorges Reservoir. Flocculation, adsorption and photochemical degradation promoted the removal of terrestrial organic substances in the wet season, while microbial activity was beneficial to autochthonous DOM production in the dry season. Overall, this study advances our evaluation of the effect of seasonal hydrological scenarios on the relationship between DOM and land use in large monsoonal river systems.