Unraveling the links between molecular weight and multidimensional dynamics of dissolved organic matter in the runoff from gentle sloping land under rainfall.

Molecular weight is a fundamental intrinsic property of dissolved organic matter (DOM), playing a crucial role in its bio-stability, microbial transformation, and contributions to regional carbon cycling. However, the complex relationships between the molecular weight and multidimensional dynamics of DOM in the runoff remain insufficiently understood. In this study, fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was employed to elucidate the associations between molecular weight and multidimensional dynamics of DOM in surface runoff and subsurface runoff from gentle sloping land (i.e. bare land and farmland). Overall, DOM-bacteria bipartite network analysis revealed a more complex co-occurrence network in subsurface runoff from farmland, with interaction strength rising alongside molecular weight. Furthermore, null modeling from different perspectives, including molecular characteristics, biochemical transformations, and a comprehensive perspective, revealed that the DOM compositional assembly processes in runoff was dominated by deterministic processes, particularly variable selection, with variation also linked to molecular weight. Notably, variable selection always dominated the assembly process of DOM potential transformation despite the scale-dependent effects affecting the assembly process of DOM molecular characteristics. Therefore, the relationship between DOM molecular weight and its dynamics may vary across microbial communities and geographic scales. Overall, these findings not only help to deepen the understanding of the role of DOM in natural ecosystems, but also provide an important reference for the study of soil carbon cycling, biogeochemical cycling and microbial ecological processes.