Microbial necromass underpins long-term soil carbon stability and ecosystem carbon persistence in pine reforestations

The long-term carbon (C) sequestration potential of plantations hinges on the dynamics and persistence of mature forest C sinks, yet how C storage and stability evolve with increasing forest age remains unclear. Here, we examined a chronosequence of mature Pinus massoniana reforestations (32-, 45-, and 60-year-old) to quantify ecosystem C storage across plant (tree, shrub, and herb), litter, and soil (0–100 cm) pools, and to assess soil organic carbon (SOC) stability via the ratio of mineral-associated organic carbon (MAOC) vs. particulate organic carbon (POC). Results showed that the total ecosystem C storage remained relatively constant across stand developmental stages, reflecting that plant C storage increased 53.4% from 32 to 45 years, then declined, while SOC storage decreased 53.9% from 32 to 45 years, then increased. In contrast, the 64.0% rise in the MAOC/POC ratio from 32 to 60 years may reflect a trend of enhanced SOC stability. Microbial necromass constituted 45.9%–64.8% of SOC, with fungal necromass dominating bacterial necromass, especially in the subsoils (20–100 cm). Additionally, SOC, POC, and MAOC showed strong positive correlations with microbial necromass but exhibited weak associations with plant and litter C pools. The MAOC/POC ratio correlated strongly with the ratio of fungal necromass carbon (FNC) vs. bacterial necromass carbon (BNC). These results reveal that microbial—especially fungal—necromass may underpin the soil C stability and ecosystem C persistence of mature pine reforestations. Therefore, accurately predicting the long-term C sequestration of mature reforestation requires a mechanistic understanding that integrates both SOC stability and microbial necromass dynamics. • Ecosystem C storage remains relatively constant over stand age. • The MAOC/POC ratio rises by 64.0% with stand age, indicating greater SOC stability. • Microbial necromass constitute over half of SOC content, especially in deep soils. • Fungal necromass contributes more to SOC than bacterial necromass.