C:N:P stoichiometry of different soil components after the transition of temperate primary coniferous and broad-leaved mixed forests to secondary forests

The balance of C, N, and P in plants, soil, and microbes is remarkably important for maintaining forest ecosystem stability. However, during forest transitions, the potential effects of the stoichiometric changes and correlations of different soil components on forest restoration are not clear. The restoration characteristics and ecological services of secondary forests, the world’s major forest resources, have been a hot topic of concern. In this study, we used primary Korean pine broad-leaved forests (PF) as controls and investigated the characteristics of the stoichiometry of different soil components and the microbial resource limitation in young and middle-aged (60 years) secondary broad-leaved forests (SF), which were formed after clear-cutting of PF. We found that soil resources, microbial biomass, enzymes, and their stoichiometry varied considerably with the period of sampling within the growing season, and the influence of sampling period was usually greater than that of forest type. Among the stoichiometric ratios of soil resources, the C/N ratio was found to be an important stoichiometric indicator, explaining 84.41% of the variation in forest biomass between the two forest types, and the soil C/N ratio of the SF was remarkably lower. Compared with soil microbial biomass in PF, that in SF exhibited obvious competitive patterns with plants and sensitive homeostasis regulation, suggesting that, in SF, soil microbes competed more strongly with plants for nutrients, and microbial metabolism was more susceptible to the changes in soil resources. In addition, with the exception of β-1,4-glucosidase, the seasonal coefficients of variation for β-1,4-N-acetylglucosaminidase, leucine arylamidase, and acid phosphatase were all significantly larger in SF. The temporal heterogeneity of soil enzymes between the two forest types was mainly related to the dynamic changes of microbial biomass C. The results of eco-enzymatic stoichiometry indicated that soil microbes in the two forest types were restricted by P, but the P limitation was more obvious in SF. Soil microbes in SF were more limited by C. Finally, the correlation analysis revealed that the stoichiometric ratios of the different soil components formed a complex network.