The mechanism of earthworms’ impact on soil aggregates under different moisture conditions

As “soil ecosystem engineers”, earthworms influence soil aggregate formation and stability. Soil moisture significantly influences the activity of earthworms and the composition of aggregates. However, there is still a lack of systematic explanation for the differences in the specific mechanism of earthworms on the formation and stability of aggregates under different soil moisture conditions, which limits the accurate assessment of the ecological function of earthworms and their effective use in agricultural management. We used soil column experiments with four moisture treatments (field water capacity (FC), 80% FC, 60% FC, and 30% FC) to explore the effect mechanisms of Metaphire guillelmi (anecic) on soil aggregates. The results showed: At 100% and 80% FC, earthworms significantly increased the proportion of >5 mm aggregates (4% and 6%, respectively) and decreased that of <0.25 mm aggregates (3% and 2%, respectively). Earthworms increased mean weight diameter, geometric mean diameter, and the content of mechanically stable aggregates (DR > 0.25 mm); these metrics decreased as moisture content declined. Earthworms significantly reduced soil organic carbon content of >2 mm aggregates, and the decrease decreased with the reduction of moisture content (FC to 30% FC decreased by 0.54, 0.40, 0.37, and 0.29 g kg −1 , respectively). They significantly increased total nitrogen (TN) content across aggregate sizes (0.24–0.47 g kg −1 ), with higher TN under high moisture conditions. The results of this study can provide a scientific reference for further understanding the role of earthworms in farmland ecosystems and exploring biological measures to improve soil quality.