Optical and molecular techniques are complementary to understand the characteristics of dissolved organic matter in the runoff from sloping croplands with various micro-topographies during rainfall

Dissolved organic matter (DOM) is a vital component of biogeochemical cycles in soil and aquatic ecosystems. The distribution of surface and sub-surface runoff was affected by surface micro-topographic conditions during rainfall, which results in the differences in DOM content and composition. Whereas, the connections between the optical and molecular characteristics of DOM in the runoff from different micro-topographies caused by tillage managements remains unclear. Therefore, optical spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were combined to explore the DOM chemistry in the runoff from different runoff plots with various micro-topographies caused by four typical tillage managements (i.e. flat tillage, longitudinal tillage, contour tillage, and artificial digging tillage) in this study. The results observed a significant difference in optical and molecular parameters between surface and sub-surface runoff for the specified runoff plot, but little variations in DOM chemistry between different runoff plots with various micro-topographies for the given runoff type. These differences in the DOM content and composition of runoff were limited by the flow carrying capacity and source supplying capacity of DOM. Significant correlations between optical and molecular parameters in surface and sub-surface runoff were found by Spearman correlation analysis. Furthermore, the bipartite networks further indicated the optical and molecular datasets in sub-surface runoff showed greater consistency and correlation intensity. These correlations and some of the inconsistencies indicated that optical and molecular technologies are complementary to trace DOM chemistry. This research is of significance to further clarify the migration patterns of DOM in global soil and aquatic ecosystems, and reveal the influence of rainfall-runoff processes on the migration of biogenic elements in ecosystems.