Effects of Salt Stress on Earthworm Function and Compost Quality During Vermicomposting of Kitchen Wastes

The high salt concentration in kitchen waste (KW) can impede the performance of subsequent biological treatment. However, the impact of salt stress on the quality of vermicomposting products generated from KW remains unclear. In this study, the effects of high salt concentration in KW on earthworm function and vermicompost quality were investigated by comparing two groups: a 1.5% salt (ST) group and a control (CK) group without salt. Results showed a significant decrease in the number and weight of earthworms in the ST (p < 0.01), with a mortality rate of 24.33% (p < 0.05) after vermicomposting. Compared to the CK, ST treatment resulted in a significant increase in catalase activity and a significant decrease in superoxide dismutase activity (p < 0.01). In addition, mucus secretion by earthworms decreased by 82.6% in ST (p < 0.01). Moreover, salt stress reduced KW humification during vermicomposting, lowering the humification index and β:α index by 23.7% and 41.2%, respectively. Microbial composition shifted under spatially heterogeneous selection pressures, leading to a 37.5% decrease in Ascomycota abundance, a 58.3% increase in Bacteroidetes abundance, and a 72.3% reduction in Proteobacteria abundance. Furthermore, the vertical stratification of physicochemical conditions significantly affected both microbial abundance and earthworm biomass in the ST treatment (p < 0.01), suggesting a salt-microbe-earthworm interaction mechanism. This study reveals that salt stress disrupts humification by impairing key microbial functions and ecological roles of earthworms during vermicomposting of KW.