Dynamic Output of Dissolved Organic Matter from Riparian Soil Driven by Rainstorms: Insight from Molecular Perspective

Dissolved organic matter (DOM) is the most active and mobile part of soil organic carbon. Surface and subsurface runoffs during rainstorms are important carriers transporting riparian soil DOM to adjacent water bodies. This study explored the molecular heterogeneity and driving mechanisms of DOM migration via runoffs from wild grassland, agricultural land, and bare land incorporating Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The molecular composition and bioavailability of runoff DOM changed over the rainstorm, with agricultural surface runoff displayed the highest DOM biodegradability (36.10 %) and CO2 yield (3.69 umol/mg COD). The DOM in surface runoff exhibited more dealkylation of lignin-derived molecules (CHO, CHON) and oxygenation reactions due to higher O2 availability, while subsurface runoff was dominated by decarboxylation reactions attributing to anaerobic metabolism of aromatic and aliphatic structures. Partial least squares path modeling indicated that DOM molecular composition directly determined its biodegradability. Microbial nutrient cycling contributed most to surface runoff DOM molecular composition (b = 0.82), while Fe redox reactions controlled that in subsurface runoff (b = 0.54). These results unveiled the overlooked molecular output pattern of riparian DOM driven by rainstorm and its critical role in stimulating downstream CO2 emissions.