Linkage between Leaf-Litter-Soil, Microbial Resource Limitation, and Carbon-Use Efficiency in Successive Chinese Fir (Cunninghamia lanceolata) Plantations

Chinese fir (Cunninghamia lanceolata) is a kind of evergreen coniferous tree species, the expansion of its pure forest area and multiple generations of continuous planting has led to a decline of stand quality and woodland fertility. To further investigate the relationship between leaf, litter, and soil stoichiometry, microbial community status, and microbial resource limitation of Chinese fir after continuous planting. We studied the C, N, and P stoichiometries of leaf, litter, and soil from successive rotations of Chinese fir plantations. In addition to this, soil microbial biomass C, N, and P, extracellular enzymes, as well as the soil microbial community composition, were determined. The continuous planting of Chinese fir significantly increased the leaf N and P contents and decreased the C content of litter, and the soil C:N and C:P ratios, thus leading to a soil stoichiometric imbalance. The continuous planting of Chinese fir plantations significantly increased the soil microbial biomass. Compared with the first-generation plantations, the N and P contents of the second- and third-generation plantations increased by 37.11%, 21.83% and 46.28%, 73.38%, respectively, thus alleviating the restriction of microbial N and P. Under continuous planting, the extracellular enzyme activities of N (NAG + LAP) and P (AP) were significantly decreased, and those of the second- and third-generation plantations were significantly decreased by 7.05%, 9.43% and 11.79%, 48.94%, respectively, compared with those of the first-generation plantations, resulting in an increase of 7.85 and 3.19% in carbon-use efficiency. The fungi:bacteria (F:B) ratio of the soil microbial community was elevated in successive plantations. The least squares pathway model (PLS-PM) indicated that the stoichiometric ratio of ecological enzymes had an indirect negative effect on CUE, and was the strongest predictor. This study showed that the successive plantation of Chinese fir resulted in a leaf, litter, and soil stoichiometric imbalance, further affecting community composition and resource limitation of soil microorganisms.