Deciphering the origins, composition and microbial fate of dissolved organic matter in agro-urban headwater streams.

Abstract

Landscape urbanization and intensive agriculture dramatically alter stream ecosystems, but it is little known how urban and agriculture land use change the quantity, quality and ultimate fate of dissolved organic matter (DOM) in stream ecosystems via nonpoint source pathways. Size-exclusion chromatography with fluorescence excitation-emission matrices, as well as absorbance and lignin biomarkers were applied to investigate the characteristics and microbial fate of DOM in 15 first-order agricultural, urban and forest headwater streams in Taihu Lake Watershed, a subtropical region of SE China. Results showed that both urban and agricultural land cover increased the amount of dissolved organic matter (DOC) with a lower C/N ratio, and that the majority of DOC was bound to terrestrial humic-like substances. Compared to forest-impacted headwater streams, the aromaticity and molecularity of OM were greatly decreased as the amounts of anthropogenic/aquagenic fulvic acid-like and protein-like DOM enriched in urban streams, and that of microbially soil-derived humic-like DOM and low molecular-weight substances (e.g., neutrals and acids) increased in agricultural streams, respectively. Human-influenced land use also influenced the seasonal variability of stream DOM biogeochemistry. Natural watersheds produced high DOC specific loads and concentrations only in the rainy season (high-flow period). Agricultural streams contributed a higher amount of terrestrial humic-like and protein-like fractions and urbanized watersheds had high values of autochthonous protein-like fractions only. Redundancy analysis (RDA) revealed that DOM quality (explainable variables: molecular source, 43.29%; molecular-weight, 29.74%) were the most prominent factor impacting microbial carbon processing, followed by inorganic nutrients (17.29%). A higher proportion of DOM from urban streams was eventually mineralized to carbon dioxide mainly due to the inefficient utilization of humic and nonhumic (e.g., protein, polysaccharides and lignin) substances as well as higher levels of inorganic nitrogen and phosphorus, whereas a significant fraction of DOM from agricultural and forest streams tended to enter microbial production and the recalcitrant-DOM pool due to the presence of less labile substrates. Our findings indicate that differences in stream-DOM and environmental properties due to urbanization and farming practices may influence stream microbial carbon processing and cause bottom-up changes in the fate of organic carbon moving through freshwater ecosystems.