Nitrogen Availability Regulates the Soil Organic Carbon Sequestration by Promoting Microbial Necromass and Plant Lignin Phenol Accumulation in Orchard Soil Amended with Organic Residues

Plant carbon (C) inputs and their subsequent microbial transformation affect the soil organic C (SOC) net sequestration. However, the characteristics of plant- and microbial-derived C and SOC sequestration under organic matter plus different nitrogen (N) levels in orchard soils remain unclear. Therefore, a pot experiment over 120 days was conducted to investigate the plant and microbial biomarkers in soils under 13C-labelled branches chip combined with N of 225 mg/kg (BRN1), 180 mg/kg (BRN2), 160 mg/kg (BRN3), 140 mg/kg (BRN4) and 0 mg/kg (BR). Branch residue and N addition increased the net SOC sequestration; the 13C recovered in SOC under branch residue plus N treatments was higher than the BR treatment. The highest newly formed C was found under BRN1, followed by BRN2 and BRN3; BRN4 had the lowest newly formed C. Branch residue and N increased lignin phenol content, which promoted syringyl-to-vanillyl and decreased acid-to-aldehyde ratios of vanillyl phenol, indicating branch-C retention in the soil. The microbial necromass C content under residue plus N treatments was higher than under the branch alone treatment, and the highest values were found under the BRN2 treatment. Additional N supply resulted in a greater contribution of microbial necromass C to SOC in soil under branch residue amendment, rather than plant C. Accordingly, BRN2 is considered optimal for net SOC sequestration by plant-derived and fungal necromass C.