Dissolved organic matter in surface runoff in the Loess Plateau of China: The role of rainfall events and land‐use

Abstract

Abstract Exploring the chemical characterization of dissolved organic matter (DOM) is important for understanding the fate of laterally transported organic matter in watersheds. We hypothesized that differences in water‐extractable organic matter (WEOM) in soils of varying land uses and rainfall events may significantly affect the quality and the quantity of stream DOM. To test our hypotheses, characteristics of rainfall‐runoff DOM and WEOM of source materials (topsoil from different land uses and gullies, as well as typical vegetation) were investigated at two adjacent catchments in the Loess Plateau of China, using ultraviolet–visible absorbance and excitation emission matrix fluorescence with parallel factor analysis (PARAFAC). Results indicated that land‐use types may significantly affect the chemical composition of soil WEOM, including its aromaticity, molecular weight, and degree of humification. The PARAFAC analysis demonstrated that the soils and stream water were dominated by terrestrial/allochthonous humic‐like substances and microbial transformable humic‐like fluorophores. Shifts in the fluorescence properties of stream DOM suggested a pronounced change in the relative proportion of allochthonous versus autochthonous material under different rainfall patterns and land uses. For example, high proportions of forestland could provide more allochthonous DOM input. This study highlights the relevance of soils and hydrological dynamics on the composition and fluxes of DOM issuing from watersheds. The composition of DOM in soils was influenced by land‐use type. Precipitation patterns influenced the proportion of terrestrial versus microbial origins of DOM in surface runoff. Contributions of allochthonous, terrestrially derived DOM inputs were highest from forested landscapes.