Soil Microbial Biomass and Community Composition across a Chronosequence of Chinese Cedar Plantations

Afforestation is one of the most important forestry practices, but its impact on soil microbial communities remains poorly understood. In this study, we sampled the soil from 0-15 cm and 15-30 cm soil depths of 7-, 13-, 24-, 33-, and 53-year-old Chinese cedar (Cryptomeria japonica var. sinensis) plantations. To investigate the effect of stand age on soil microbial communities and their potential drivers, we measured phospholipid fatty acids (PLFAs) and soil physicochemical properties. At the 0-15 cm soil depth, the biomass of total PLFAs and functional microbial groups such as bacteria (B), fungi (F), Gram-negative bacteria (GN), Gram-positive bacteria (GP), actinomycetes (ACT), and arbuscular mycorrhizal fungi (AMF) increased sharply in 7- to 13-year-old stands, but then gradually leveled off in older stands. On the other hand, the biomass of total PLFAs and functional microbial groups at the 15-30 cm soil depth peaked in the 33-year-old stand. The biomass of total PLFAs and functional microbial groups was strongly influenced by stand age and soil depth, and was significantly lower at the 15-30 cm soil depth than at the 0-15 cm soil depth except for the 7-year-old stand. The F/B and fungi/total PLFAs ratios of both soil depths were markedly lower in the 13-year-old stand than in the remaining four stand ages, while the proportions of the bacterial group (GP and GN) showed contrasting trends. The biomass of all functional microbial groups and the GP/GN ratio were mainly mediated by soil organic carbon (SOC) concentration and the soil organic carbon to total phosphorus (C/P) ratio at the 0-15 cm soil depth, but primarily affected by ammonium nitrogen (NH4+-N) concentration at the 15-30 cm soil depth. The F/B ratio of the two soil depths was prominently affected by nitrate nitrogen (NO3--N) concentration. Our results highlighted that SOC concentration and mineral N (i.e., NH4+-N and NO3--N) concentration mainly drove changes in the soil microbial biomass and community composition with stand age in Chinese cedar plantations, and that the 13-year-old stand may be the key period for management.