Earthworms increase the potential for enzymatic bio-activation of biochars made from co-pyrolyzing animal manures and plastic wastes

Highlights: • Plastichar (manure+plastic-derived biochar) applied at 2.5% w/w was not toxic to earthworms. • Application (2.5% w/w) of poultry-litter alone biochar was toxic (100% mortality) to earthworms. • Plastichars displayed improved features to retain active esterase enzymes. • Incubation of plastichars with earthworms facilitated the biochar enzymatic activation. • Co-application of plastichar and earthworms increased soil enzyme activity in subsoil. We assessed the enzymatic activation of four different biochars produced from pyrolyzing swine manure and poultry litter, and by co-pyrolyzing these livestock residues with agricultural spent mulch plastic film wastes (plastichars). Enzymatic activation consisted of incubating biochars in soil inoculated with earthworms (Lumbricus terrestris), which acted as biological vectors to facilitate retention of extracellular enzymes onto biochar surface. The activity of carboxylesterase ‒a pesticide-detoxifying enzyme‒ was measured in non-bioturbed soils (reference), linings of the burrows created by earthworms, casts (feces) and biochar particles recovered from the soil. Our results revealed that: 1) biochar increased soil carboxylesterase activity respect to biochar-free (control) soils, which was more prominent in the presence of earthworms. 2) The maximum enzyme activity was found in soils amended with plastichars. 3) The plastichars showed higher enzyme binding capacities than that of the biochars produced from animal manure alone, corroborating the pattern of enzyme distribution found in soil. 4) The presence of earthworms in soil significantly increased the potential of the plastichars for enzymatic activation. These findings suggest that the plastichars are suitable for increasing and stabilizing soil enzyme activities with no toxicity on earthworms.