Roxarsone reduces earthworm-mediated nutrient cycling by suppressing aggregate formation and enzymic activity in soil with manure application.

The application of manure and earthworms are frequently used in fertilization practices to improve C, N, and P cycling in soil, which may be adversely affected by roxarsone (ROX), as an organoarsenical pollutant. To effectively address this issue, in this work, the interactive impacts of ROX and earthworm Eisenia foetida on the aggregate formation, input of organic carbon (OC), and changes in the available N and P following 56-day cultivation were systematically investigated. Compared to the control, earthworms increased the mean weight diameter (MWD) of the soil aggregates from 0.6 to 1.1 mm. Thereby, they activated soil enzymes including catalase (CAT), sucrase (SC), urease (UE), and neutral phosphatase (NP), with the soil's pH decreased to 7.1. Consequently, the values of OC, soluble nitrite (NO-N), and Olsen-P content were respectively increased by 0.78-, 1.69-, and 0.87- folds in the E treatment (14.3 vs. 25.5 g/kg, 12.8 vs. 33.3 mg/kg, and 7.8 vs. 14.6 mg/kg). Although the changes in the R treatment were slight, ROX reduced the earthworm-mediated improvements of soil fertility during the application of the RE treatment compared to the E treatment, i.e., the values of MWD, OC, NO-N, and Olsen-P were reduced to 0.9 mm, 20.4 g/kg, 25.4 mg/kg, and 11.6 mg/kg, respectively. From the well-fitted structural equation models, it was demonstrated that earthworms enhanced the aggregate formation and nutrient cycling of OC, NO-N, and Olsen-P, which were inhibited by ROX. Overall, these adverse effects can be offset by earthworm addition, which can play the dual role of monitor and driver for the soil properties. Our work provides insightful strategies for ROX-bearing manure management.