Straw mulching alters the composition and loss of dissolved organic matter in farmland surface runoff by inhibiting the fragmentation of soil small macroaggregates

Straw mulching is a widespread practice for reducing the soil carbon loss caused by erosion. However, the effects of straw mulching on dissolved organic matter (DOM) runoff loss from black soil are not well studied. How straw mulching affects the composition and loss of runoff DOM by changing soil aggregates remains largely unclear. Here, a straw mulching treatment was compared to a no mulching treatment (as a control) on sloping farmland with black soil erosion in Northeast China. We divided the soil into large macroaggregates (>2 mm), small macroaggregates (0.25-2 mm), and microaggregates (<0.25 mm). After five rain events, the effects of straw mulching on the concentration (characterized by dissolved organic carbon) and composition (analyzed by fluorescence spectroscopy) of runoff and soil aggregate DOM were studied. The results showed that straw mulching reduced the runoff amount by 54.7%. Therefore, although straw mulching increased the average dissolved organic carbon (DOC) concentration in runoff, it reduced the total runoff DOC loss by 48.3%. The composition of runoff DOM is similar to that of soil, as both contain humic-like acid and protein-like components. With straw mulching treatment, the protein-like components in small macroaggregates accumulated and the protein-like components in runoff declined with rain events. Fluorescence spectroscopy technology may help in understanding the hydrological paths of rain events by capturing the dynamic changes of runoff and soil DOM characteristics. A variation partitioning analyses (VPA) analysis indicated that the DOM concentration and composition of microaggregates explained 68.2% of the change in runoff DOM from no mulching plots, while the change in runoff DOM from straw mulching plots was dominated by small macroaggregates at a rate of 55.1%. Taken together, our results demonstrated that straw mulching reduces the fragmentation of small macroaggregates and the loss of microaggregates, thus effecting DOM compositions in soil and reducing the DOM loss in runoff. These results provide a theoretical basis for reducing carbon loss in sloping farmland.