Soil quality degradation induces synergistic evolution of carbon components: Microbial extracellular polymeric...

The complex climatic background of cold regions induces and aggravates soil erosion, which accelerates soil quality degradation and weakens the stability of farmland soil carbon pools. As a biological cementing agent sustaining soil aggregate structure, extracellular polymeric substances (EPS) serve as a core mediator regulating soil structural stability and carbon pool loss. However, the cascade response mechanism by which EPS drives the transformation of labile organic carbon under environmental stress remains unclear. To effectively reveal the regulatory rule of EPS on the evolution of soil carbon fractions, four typical farmland soils in cold regions (Chernozem, Albic soil, Meadow soil, Dark brown soil) were selected for simulated erosion experiments under rainfall, freeze-thaw and snowmelt conditions. This study systematically analyzed the coupling effects of erosion stress on soil aggregate structure, microbial community function and variations in labile organic carbon fractions, and identified the key driving factors responsible for soil carbon loss. The results indicated that EPS plays a core mediating role in the coupling process of soil structural stability and carbon cycling. Erosion disrupts soil aggregate structure and thereby increases soil porosity. Relative to the control, the porosity of dark brown soil rose by 14.56%, 15.75% and 15.91% under rainfall, freeze-thaw and snowmelt erosion, respectively. Meanwhile, soil erosion markedly reduced key EPS component contents. The degraded soil-water environment decreased microbial community diversity and suppressed the expression of EPS synthesis-related genes. Correspondingly, EPS-polysaccharide content declined by 42.63%, 23.67% and 41.71% under the three erosion treatments above. In addition, the impairment of EPS barrier function results in the reduction of labile organic carbon content in soil. The stability of the four soil types follows the order: Dark brown soil > Chernozem > Meadow soil > Albic soil. In conclusion, EPS is not only a cementing substance for maintaining soil structure, but also a biological barrier against carbon loss. This study is expected to provide a scientific basis for the remediation of degraded soil and the establishment of stable soil carbon pools.