Yuhui Qiao

Earthworms act as key ecosystem engineers influencing the distribution of soil microplastics (MPs), however, the residence, transport, and fate of these particles within the drilosphere, particularly within biogenic cast aggregates, remain poorly understood. Here, we combined a field survey of 43 paired soil-cast samples across three agricultural land-use scenarios with complementary laboratory soil column experiments to elucidate earthworm-driven MP dynamics. The field survey revealed ubiquitous in situ MP occurrence and upward fluxes from bulk soils to casts, with transport efficiency modulated by soil clay and organic carbon contents. Laboratory simulations using epigeic and anecic species validated that different ecotypes actively ingest MPs from source soils and deposit them in surface casts. Crucially, both field and laboratory data demonstrated a significant reduction in particle size in casts compared to soil (6.48% and 19.8%, respectively), supporting the potential earthworm effects on MP mechanical attrition. Polymer compositions in casts mirrored those in soils, exhibiting a nonselective and passive ingestion pathway. Beyond physical transport, the formation of field biogenic polymer aggregates facilitated chemical aging of MPs, as evidenced by elevated oxidation indices. This process was likely accelerated by the enrichment of plastic-degrading microbial taxa (e.g., Flavobacterium) within casts, which exhibited up to a 35.6-fold increase in relative abundance. Collectively, these findings highlight the dual role of soil-engineering invertebrates in driving the vertical redistribution and physicochemical degradation of MPs in agricultural systems.

Although the potential of vermiremediation for restoring metal-contaminated soils is promising, the effects of earthworms on the availability of soil metals are still debatable. Most previous studies considered the soil as a “whole black box.” Mobilization or immobilization of metals are affected by earthworm activities within drilosphere hotspots under different soil conditions, which has not been specifically studied. Therefore, an improved 2D terrarium was designed to study the impact of earthworm activities on cadmium (Cd) fate in the drilosphere hotspots (burrow wall soils, burrow casts, and surface casts) of different artificially spiked Cd treatments (CK: 0 mg kg^-1; LM: 1 mg kg^-1; and HM: 5 mg kg^-1) with different organic amendments (2% and 10%). The results revealed that Cd increased earthworm activities with the highest cast production in HM and the highest burrow length in LM. Earthworms exhibited a stronger tendency to reduce total Cd concentration by 4.48–13.58% in casts of LM soils, while 3.37–5.22% in burrow walls under HM treatments. Overall, earthworms could increase the availability of Cd in casts under all conditions (55.46–121.01%). The organic amendments decreased the total Cd concentration and increased the availability of Cd in the disturbed soil. A higher amount of organic amendment significantly decreased total Cd concentration of the drilosphere by 1.16–5.83% in LM and HM treatments, while increasing DTPA-Cd concentrations in all components by 23.13–55.20 %, 14.63–35.11%, and 3.30–11.41% in CK, LM, and HM treatments, respectively, except for earthworm non-disturbed soil and no-earthworm soil in HM treatments. Redundancy analysis (RDA) revealed that the moisture, pH, and total carbon contents in soil are the main factors affecting Cd bioavailability. In this study, we decoded the “black box” of soil by making it relatively simple to better understand the effects and mechanisms of earthworm activities on soil metal availability and consequently provided comprehensive insights for using earthworms in soil vermiremediation. Graphical Abstract