Effect of biogel C addition on biochar degradation and microbial activities

Biochar contributes to long-term soil carbon stabilization (C) by acting both as a stable carbon pool and as a sorbent for labile compounds. Biogels from roots and microbes are known to form persistent surface coatings with soil sorbents, where they host microbial hotspots. Yet research has mainly focused on their interactions with minerals. The interactive effects of biogels and biochar on soil carbon dynamics remain largely unexplored. This study aimed to assess the effects of biogel coatings on fresh and aged biochar, particularly under drought, with a focus on biochar degradation and microbial responses. We conducted a three-factorial soil incubation study to examine the effects of fresh and aged biochar, with or without biogel amendment, under two moisture levels (30% and 70% water holding capacity, WHC). Using 14C labelled biochar, we quantified biochar degradation and biochar- induced priming effects by measuring 14CO2respiration, microbial biomass carbon (MBC) and its 14C incorporation, hydrolytic and oxidative enzyme activities, and changes in biochar surface area. Mucilage strongly enhanced microbial incorporation of biochar-derived C, specifically from aged biochar under drought conditions, while the opposite effect was observed in soils amended with fresh biochar. This demonstrates the greater microbial accessibility of aged biochar surfaces and limited use of fresh biochar due to inaccessibility of the hydrophobic surfaces, and in consequence the preference for more easily accessible C sources. Additionally, mucilage significantly reduced the Michaelis Menten constant (Km) of {beta}-glucosidase by up to 58% in soils amended with aged biochar under drought. This indicates that under these conditions, the interaction of the aged biochar with the biogel may have created a unique habitat requiring specific enzyme systems with higher affinity. Our findings highlight the role of mucilage in regulating microbial surface access and thus the decomposition of biochar, particularly under moisture- limited conditions. The synergy between aged biochar and biogels provides a promising new perspective to further enhance biochar-based drought mitigation, specifically for managing microbial activity in drought-prone soils.