Contrasting change patterns of lignin and microbial necromass carbon and the determinants in a chronosequence of subtropical Pinus massoniana plantations

The continuous input and degradation of organic matter is crucial for soil organic carbon (SOC) sequestration during stand development. However, it remains unclear how the relative contribution of plant residues and microbial necromass to SOC changes with stand age and what are the determinants of the change pattern. The space-for-time substitution (chronosequence) approach was used to establish plots of Pinus massoniana plantations at five stand ages (6, 13, 29, 38, and 57 years). Soil samples were collected in each plot to determine the relative contribution of plant residues and microbial necromass to SOC storage by using SOC-normalized content of lignin and microbial necromass carbon (MNC). Lignin increased initially from young to mature but then decreased in over-mature plantation (R2 = 0.54, P < 0.01) and was dominated by vanillyl phenols (79 %–89 %). MNC decreased at first from young to mature but then increased in over-mature plantation (R2 = 0.50, P < 0.05) and it was dominated by fungal necromass carbon (54–63 %). Partial least squares path modeling revealed that stand age directly positively affected lignin and MNC, and indirectly maintained their change patterns through tree growth and microbial attributes. Stand age had a total positive effect (0.57) on lignin and a total negative effect (−0.54) on MNC. Regression analysis showed that the ratio of SOC to total nitrogen (SOC/TN) was positively correlated with lignin (R2 = 0.76, P < 0.01) yet negatively correlated with MNC (R2 = 0.58, P < 0.01). Additionally, the diameter at breast height, pH, and the ratio of fungal to bacterial biomass together with SOC/TN regulated the patterns of lignin and MNC with stand age. Overall, this study highlights the opposite pattern of lignin and MNC and emphasizes the importance of considering both tree growth and microbial attributes when developing SOC sequestration strategies with stand age.