Bacteria Affect the Distribution of Soil-Dissolved Organic Matter on the Slope: A Long-Term Experiment in Black Soil Erosion

Soil erosion results in dissolved organic matter (DOM) loss and is one of the main paths of soil carbon loss. Bacteria affect the generation and transformation of DOM. However, the effect of bacteria on the composition and slope distribution of DOM has rarely been investigated under field conditions. Based on a long-term experiment of three gradients (3 degrees, 5 degrees, 8 degrees) in a black soil erosion area of Northeast China, the content, composition, and source of DOM were studied. The results showed that the DOM of the 3 degrees and 5 degrees slope was enriched midslope, and the DOM of the 8 degrees slope was enriched downslope. Parallel factor (PARAFAC) analysis indicated that the main substances in DOM were fulvic-like acid, humic-like acid, tryptophan-like protein, and soluble microbial metabolites. The upslope and downslope soils of 3 degrees and 5 degrees slopes showed high DOM bioavailability, while the downslope soil of the 8 degrees slope showed high DOM bioavailability. The content of new DOM in downslope soil increased with the gradient. Bacteria played an important role in the synthesis and transformation of DOM and affected its composition and slope distribution. Verrucomicrobiota, Firmicutes, Planctomycetota, and Gemmatimonadota were the main factors affecting soil DOM. The results could be helpful in understanding the loss mechanism of DOM in eroded black soil and provide support for soil carbon sequestration.