Molecular composition of dissolved organic matter in soil leachate following application of surface agricultural practices and its implications for groundwater

The application of surface agricultural practices (SAPs) to agricultural soils is gaining attention as a potential valuable method for sequestering carbon and improving soil fertility. However, the impacts of SAPs on the molecular properties of dissolved organic matter (DOM) in soil leachates are poorly understood. In this study, the molecular characteristics of DOM successively leached from agricultural soils applied with control, manure fertilization, lucerne planting, and straw return were unraveled by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results indicated that the greater proportion of low molecular weight labile DOM (lipids-like, proteins-like and carbohydrates-like) in initial soil leachates gradually changed to higher fractions of larger recalcitrant DOM (condensed aromatics-like and tannins-like) in later soil leachates. Compared to the control, the soil leachates treated with SAPs had greater percentage of labile DOM and lower percentage of recalcitrant DOM, along with higher abundance of CHNO and CHOS compounds. Furthermore, DOM in the manure, lucerne, and straw treatments showed smaller mass weights, higher H/C ratios and fewer double bonds, rings, and aromatic structures. DOM with different physicochemical properties play different roles in the processes of nitrogen cycling and arsenic migration. The implementation of SAPs may alleviate groundwater nitrogen pollution, but it may also enhance the potential risk of arsenic mobility in groundwater. This study deepens our understanding of the molecular characterization of DOM leached from agricultural soils applied with different SAPs, which holds significant implications for evaluating the environmental impacts of soil DOM leaching.