Soil Carbon Release and Microbial Biomass Characteristics in Different Pinus massoniana Forests in Three Gorges Reservoir area

[Objective] As a hotspot region of soil respiration and carbon pool dynamics, the Three Gorges Reservoir area is most extensively covered by Pinus massoniana forests, whose soil organic carbon storage is the largest among all forest types. Therefore, investigating soil carbon release is particularly important. Soil carbon emissions are mainly regulated by microbial decomposition of aboveground litter, organic matter substrates, and root exudates; however, studies on the coupling relationship between soil respiration and microorganisms in Pinus massoniana forests in the Three Gorges Reservoir area are scarce, which affects the accurate quantification of the reservoir-area carbon pool. [Methods] This study used a Li-8100 automated soil CO2 flux system (Li-Cor Inc., Lincoln, NE, USA) and the chloroform fumigation-extraction method (FE) to monitor soil respiration rates and microbial biomass carbon and nitrogen in two Pinus massoniana forest types (a Pinus massoniana pure forest and a Pinus massoniana-Quercus acutissima mixed forest) across spring, summer, autumn, and winter. The trenching method was further used to determine the contributions of autotrophic and heterotrophic respiration components, and one-way analysis of variance was used to analyze differences. [Results] Both total respiration and heterotrophic respiration rates in the mixed forest were higher than those in the pure forest. Under both forest types, heterotrophic respiration accounted for an average of 88%, while autotrophic respiration accounted for an average of 12%, which was also a consequence of Pinus massoniana being the main constructive or dominant species in the Three Gorges Reservoir area. The proportions of autotrophic and heterotrophic respiration exhibited significant seasonal variation: the contribution of heterotrophic respiration first decreased and then increased with season, whereas autotrophic respiration first increased and then decreased. Autotrophic respiration contributed approximately 3%-25% to total soil carbon release. In the pure forest, heterotrophic respiration contributed 75%-97% of total soil carbon release, while in the mixed forest, heterotrophic respiration accounted for 82%-98% of total soil respiration. During the growing season (May-October), the average rates of all respiration components were higher and reached their maximum in summer. Microbial biomass carbon and nitrogen contents in the mixed forest were higher than those in the pure forest, reaching 259.54 and 20.25 mg/kg, respectively. After root exclusion treatment, microbial biomass carbon and nitrogen only decreased by 30.53% and 34.56%, respectively. The microbial biomass C to N ratio was 15.70 in the pure forest and 12.82 in the mixed forest. Both microbial biomass carbon and the C to N ratio drove soil carbon release. [Conclusion] In both Pinus massoniana pure and mixed forests, the contribution of heterotrophic respiration is much higher than that of autotrophic respiration, indicating that heterotrophic respiration is the main pathway of soil carbon release in the Three Gorges Reservoir area. After root exclusion treatment, the decreases in microbial biomass carbon and nitrogen are relatively small, suggesting that microorganisms mainly release carbon by decomposing litter and organic matter substrates rather than root exudates. Soil respiration, in both pure and mixed forests, is significantly positively correlated with microbial biomass carbon and the C to N ratio (P