Biochar addition regulates soil and earthworm gut microbiome and multifunctionality

The earthworm gut bacterial community and enzyme activities play a key role in many soil ecosystem functions, but how the addition of different types of biochar (used to improve soil quality) regulates soil and earthworm gut bacterial communities and associated enzyme activities is scarcely understood. An experiment was conducted using soil microcosms and seven treatments: no biochar application as control, and two types of biochar (maize straw and cow dung biochar, hereafter SB and CB treatments, respectively) with three biochar application rates (20, 50, and 100 g/kg soil). Eight enzymatic activities related to C-, N- and P-cycling (used to compute a “multifunctionality” index) and the abundance and composition of bacterial communities were assessed in both the soil and earthworm guts. Our results show that earthworm gut content, characterized by lower bacterial abundance but higher enzymatic activities than soil, was strongly influenced by biochar addition. Increasing biochar application rates significantly increased the nutrient content, bacterial abundance, and multifunctionality in the earthworm guts. Owing to the difference in physicochemical properties between SB and CB, the increased gut multifunctionality was linked to the significantly modified gut bacterial community composition in CB treatments but not in SB treatments. Structural equation modeling further revealed that the changed gut multifunctionalities in both biochar treatments were mostly related to the soil and gut physicochemical properties, especially in CB treatments, while the impacts of gut microbial variables on soil multifunctionality were higher in the SB than CB treatments. These results demonstrate that the influence of biochar amendment on earthworm gut microbiome and multifunctionality depends on the type of biochar, which also regulates the contributions of earthworm gut biochemical processes to soil ecosystem functions.