Responses of molecular composition and biodegradation of dissolved organic matter to erosion in topsoil versus subsoil in a Mollisol agricultural ecosystem

Abstract

Although dissolved organic matter (DOM) has a substantial influence on biogeochemical processes relevant to erosion in agricultural ecosystems, the mechanisms underlying the dynamics and biodegradation of DOM in eroding landscapes are yet to clarified. In this study, we examined the changes in dissolved organic carbon (DOC), profile distribution, biodegradation, and chemical characteristics associated with erosion in a Mollisol agricultural landscape. The results showed that DOC content in the mid-slope was significantly higher than that in the down-slope, particularly at depths of 40–100 cm. The higher humification index and low biological index indicated that the DOM in the depositional position was characterized by a higher humic content compared to the up-slope positions. The up-slope samples were found to contain larger amounts of protein-like substances, whereas the mid- and down-slope soils comprised a larger proportion of anthropogenic humic-like materials, indicating that DOM in the up-slope soil was primarily microbial-derived. For the topsoils, the activities of β-glucosidase and cellulase were highest in the down-slope both before and after incubation, thereby suggesting that the deposition was conducive to the secretion of these enzymes by soil microbes. Overall, our observations indicated that erosion and deposition had a significant effect on the chemical composition and biodegradation pathways of DOM in soil profiles. These findings provided detailed information on the heterogeneous DOM characteristics at the molecular level, as well as DOC effects on environmental behavior in agricultural eroding environments.