Biochar addition affects soil carbon stock by altering keystone fungal species and necromass abundance and oxidase activities in forest and paddy soils

Fungi play a crucial role in the utilization and storage of soil organic carbon (SOC). Biochars can potentially influence soil carbon (C) turnover by mediating extracellular electron transfer, which can be facilitated by fungi. However, the effects of biochar and soil type on the community, abundance, enzyme secretion, and necromass of fungi mediating SOC storage remain unclear. A mesocosm incubation experiment was conducted using forest and paddy soils from southern China to study the impact of biochars pyrolyzed at low (300°C, BL) and high (700°C, BH) temperatures on fungal abundance, community composition, necromass abundance, and C-degrading enzyme activities. The SOC retention ratio was higher under BL (84.0%) than under BH (76.3%). Addition of BL increased fungal abundance in the forest soil by 230%. In contrast, addition of BH decreased fungal abundance in the paddy soil by 20.8%. Biochar addition affected fungal necromass accumulation and oxidase activity and regulated SOC turnover. The high available C content and moderate liming effect of BL significantly increased fungal abundance and necromass abundance in the forest soil compared to the paddy soil. Moreover, after 16 weeks of incubation, BL addition decreased peroxidase activity by 32.1% in the forest soil due to the higher C use efficiency of fungi (i.e., the enrichment of Talaromyces, Umbelopsis, and Trichoderma), decreasing C-degrading enzyme secretion and reducing SOC degradation compared to the paddy soil. However, BH addition increased the Fusarium abundance, which regulated the polyphenol oxidase activity and promoted SOC degradation in the paddy soil. We concluded that biochars could alter the soil environment and extracellular electron transfer to mediate fungal necromass content and C-degrading enzyme activities, thus affecting SOC storage in the forest and paddy soils.