Agricultural practices reshape molecular fingerprints of dissolved organic matter in aquatic ecosystems: A case study in China's Dai Village.

Agricultural management practices significantly influence the biogeochemical cycling of dissolved organic matter (DOM) in freshwater ecosystems, yet the molecular-level signatures of DOM in conventional farming regions (CFR) versus organic farming regions (OFR) remain poorly understood. Here, an OFR with a 20-year history of organic farming and a corresponding CFR were selected to explore differences in DOM molecular characteristics and functional diversity under contrasting agricultural management regimes. We found that both CFR and OFR were dominated by CHO molecules, while CFR showed higher relative abundance of low molecular weight (<300 Da) and nitrogen-containing molecules than OFR. Unique molecules in CFR were characterized by higher proportion of lipid-like compounds, and significantly lower molecular weight, degrees of unsaturation and oxidation, while unique molecules in OFR were dominated by lignin-like compounds with higher degree of oxidation. The functional divergence and dispersion of unique molecules was significantly higher in CFR than in OFR, illustrating more convergent DOM functional traits under organic farming. Furthermore, physicochemical properties explained higher variance in DOM molecular composition in CFR (R2 = 0.92) than in OFR (R2 = 0.49), with total nitrogen and total phosphorus as dominant drivers in CFR, whereas nitrite and total organic carbon showed the highest contribution in OFR. These findings demonstrate that conventional farming promotes the accumulation of nitrogen-rich, functionally diverse DOM molecules influenced by both terrestrial inputs and autochthonous, while organic farming leads to more oxidized and recalcitrant DOM in ponds, driven primarily by terrestrial inputs.