Effects of Bamboo-Sourced Organic Fertilizer on the Soil Microbial Necromass Carbon and Its Contribution to Soil Organic Carbon in Moso Bamboo (Phyllostachys edulis) Forest

Microbial necromass carbon (MNC) is crucial for soil carbon sequestration in bamboo (Phyllostachys edulis) forests. However, the response of MNC to bamboo-sourced organic fertilizers (BSOF) prepared by composting bamboo plant growth-promoting microorganisms and bamboo residues remains unclear. This study examined MNC and its contribution to soil organic carbon (SOC) in Moso bamboo plantations under four BSOF treatments: control (CK, 0 t·hm−2), low fertilizer application (LF, 7.5 t·hm−2), medium fertilizer application (MF, 15 t·hm−2), and high fertilizer application (HF, 30 t·hm−2) across 0–20 cm and 20–40 cm soil layers. In these two layers, HF and MF significantly (p < 0.05) increased the total MNC, fungal necromass carbon (FNC), and their contributions to SOC compared to CK, and HF led to higher (p < 0.05) bacterial necromass carbon (BNC) levels and SOC contributions than LF and CK. Soil depth and BSOF treatment were found to interact significantly. A random forest model showed that in the 0–20 cm layer, SOC was the best predictor of total MNC and FNC, whereas available potassium was optimal for BNC. Nitrate-nitrogen (NO3−-N) was the top predictor for total MNC, BNC, and FNC in the 20–40 cm layer. Partial least squares path modeling indicated that available soil nutrients directly influenced BNC and FNC, affecting SOC accumulation. These findings suggest a new method for enhancing soil carbon sequestration in bamboo forests.