Molecular insight into the vertical migration and degradation of dissolved organic matter in riparian soil profiles

Abstract

Due to the molecular complexity of dissolving organic matter (DOM), the vertical molecular distribution of riparian soil DOM (especially dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP)) in different land use types and their relationship with the bacterial community is still unclear. This study analyzed the spectral characteristics of riparian soil DOM from 0 to 100 cm in wild grassland, agricultural land, and bare land. The molecular distribution of DOM was revealed through Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and the specific relationship between DOM and bacterial community composition (BCC) was evaluated. The results showed that the DOM in the upper soil layer (0–40 cm) was mainly composed of recalcitrant macromolecular organics, while that in the lower layer (40–100 cm) was labile small molecular organics. In agricultural land, the total storage of DOM was lower than that in wild grassland, but with a higher abundance of recalcitrant organic carbon (lignin, etc.). At the same time, the bacterial community in agricultural land is shifting towards copiotrophs. In addition, the abundance of labile C degrading genes increases with nitrate as the main electron acceptor. However, sulfates are mainly used as electron acceptors in wild grasslands. Both DOP and DON were dominated by lignin and displayed higher chemical diversity in the upper soil. The bioavailability of DOP in three types of soil is higher than that of DON. DOM-BCC network analysis shows that the recalcitrant DON and DOP molecules in soil are positively correlated with phylum Actinobacteriota in agricultural land. These results emphasize that the DOM molecular characteristics were closely related to the function of the soil bacterial community.