Tight coupling of fungal community composition with soil quality in a Chinese fir plantation chronosequence

Predicting changes in carbon and nutrient cycles in plantations requires a mechanistic understanding of the effects of stand age on soil quality and microbial communities. Here, we evaluated soil quality by using an integrated soil quality index (SQI) and traced the parallel shifts in fungal community composition using high-throughput sequencing in a chronosequence of Chinese fir (Cunninghamia lanceolata) plantations (stand age of 3, 16, 25, 32, >80 years). Soil properties showed pronounced changes with stand age in the topsoil. Soil organic carbon (SOC), total nitrogen (TN) and available phosphorus (AP) were 2.1, 1.9 and 2.2 times higher, respectively, in the oldest stands than in the youngest stands. SQI of the top 5 cm increased logarithmically with stand age. Mycorrhizal fungi initially increased in younger stands, but then they were gradually replaced by saprotrophs in older stands due to larger litterfall. Strong positive correlations between saprotrophic fungi and key soil quality indicators, such as TN, AP and NH4+, confirmed that abundance of decomposers is tightly linked with higher soil quality. Mycorrhizal ordersThelephorales,SebacinalesandRussulalesincreased in abundance and raised the activity of acid phosphatase to mobilise limiting phosphorus from organic matter. Consequently, mycorrhizal fungi are especially relevant in younger stands to acquire nutrients to sustain tree productivity. In developed stands, however, saprotrophic fungi are crucial in recycling nutrients from the litter. Collectively, the increase of topsoil quality during the life cycle of Chinese fir plantations is closely linked with the observed transition of fungal communities from mycorrhizae to saprotrophs.