Application of leaves to induce earthworms to reduce phenolic compounds released by decomposing plants

• Earthworms can accelerate degradation of soil phenolic compounds.
• Microorganisms and the source of phenolic compounds affect the degradation rate.
• Earthworms show avoidance to high concentration of phenolic acids.

The negative impacts (allelopathic effects) of phenolic compounds (PCs) in soil on plant growth and microbial communities have recently received considerable attention. Similarly, there have been several recent studies on the effects of microbes on the degradation of PCs. Because of the profound effects of their feeding and burrowing activity, earthworms should have significant effects on PCs degradation; however, few studies have examined this potential effect, and particularly, factors that may affect the course of degradation. We tested different earthworm species and density, different conditions (sterilization or not) of mixture of soil and plant residual, and different source of PCs to evaluate the capacity of earthworms to accelerate PCs degradation. In addition, earthworm behavior experiments were set up to test whether adding plant leaves can stimulate earthworm feeding activity. The results showed that native earthworms exhibited a higher capacity than compost earthworms for degrading PCs; furthermore, when the number of Metaphire guillemi reached 300 individuals m−2 in our experimental units, the PCs decreased most quickly, and the residual PCs concentration was 105 μg (p-coumaric acid)g−1 (dry soil) less than that of control group. The source of PCs also affected their degradation rate, as PCs derived from leaves seemed to degrade more quickly. The results of our experiment suggested that earthworms avoid feeding on phenolic acids, but can be induced to do so by adding leaves to the substrate. These results indicate that earthworm activity can accelerate the degradation of total PCs, and that this may be further facilitated by incorporation of organic matter, which may be used to alleviate allelopathic effects of PCs in soil.