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Soil Carbon Release and Microbial Biomass Characteristics in Different Pinus massoniana Forests in Three Gorges Reservoir area
LI Zheng, ZHU Hai-qin, XIE Ping, JIA Bao-jie, HE Shu-fang
Journal of Changjiang River Scientific Research Institute ›› 2026, Vol. 43 ›› Issue (2) : 105-110.
PDF(4251 KB)
PDF(4251 KB)
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<0.05), but is not correlated with microbial biomass nitrogen. The positive correlation and similar patterns between microbial biomass carbon and soil respiration can directly reflect soil carbon release. Microbial biomass in the mixed forest is higher than that in the pure forest both before and after root exclusion, resulting in greater soil carbon emissions. In the absence of roots and their exudates, the C to N ratio in the mixed forest increases to facilitate the decomposition of more recalcitrant substrates. The findings of this study provide an important reference for research on carbon cycling and ecological construction in the Three Gorges Reservoir area.
Three Gorges Reservoir Area / soil respiration / microbial biomass / Pinus massoniana / seasonal variation
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目前有关森林根系分泌物及其诱导的土壤生态学效应研究主要关注根系碳(C)源输入, 而极少关注根系分泌物氮(N)源输入及其伴随的C:N化学计量特征对土壤过程和功能的影响, 极大地限制了我们对森林根系-土壤-微生物互作机制的深入认识。该研究以川西亚高山天然林和云杉(Picea asperata)人工林土壤为对象, 模拟配制不同C:N化学计量特征(只有N、C:N = 10、C:N = 50、C:N = 100和只有C处理)的根系分泌物溶液进行人工添加试验, 以探究根系分泌物化学计量特征对两种林分土壤碳动态及其微生物群落结构的影响差异。结果表明: 模拟根系分泌物C添加总体促进了两种林分土壤有机质分解激发效应而降低了土壤总碳(TC)含量, 而N添加在一定程度上缓和了两种林分土壤TC含量的降低幅度, 且C添加导致天然林土壤TC含量的降低幅度明显低于土壤N有效性更低的人工林。几种根系分泌物添加处理对两种林分土壤活性和惰性碳库的影响无明显规律。另外, 根系分泌物C添加总体降低了天然林土壤微生物总磷脂脂肪酸(PLFA)含量和细菌、放线菌、真菌PLFA含量, 而总体增加人工林土壤微生物PLFA总量和细菌、放线菌、真菌PLFA含量, 并诱导两种林分土壤微生物群落结构(细菌:真菌相对丰度)也发生了各自不同的变化。上述结果表明森林根系分泌物N源输入和土壤N有效性共同调控根系C源输入对土壤有机质分解激发效应的方向和幅度。研究结果为深入揭示典型森林根系分泌物化学计量特征对土壤生物化学循环过程的调控机制提供了一定的理论依据。
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<sec><title>Aims </title><p>Exudation measurements focus exclusively on total exudate carbon (C) fluxes without considering how root-derived nitrogen (N) inputs and variable exudate stoichiometries may influence microbial activity and biogeochemical cycles. As a result, the biogeochemical consequences of exudate stoichiometry for soil C-nutrient couplings and feedbacks to environmental changes remain largely unknown. Our objective is to explore to what extent N availability modifies soil microbial processes and the dynamics of soil carbon pool induced by labile C. </p></sec><sec><title>Methods </title><p>We conducted a 50-day laboratory incubation experiment by addition of simulated exudates varying in C:N to two coniferous forest soils: a natural forest and Picea asperata plantation. The five exudate addition treatments are C alone, N alone, and combinations of C and three N levels (C:N ratio of 10, 50 and 100). </p></sec><sec><title>Important findings </title><p> The addition of labile C exudates decreased soil total C for both natural forest and the plantation by stimulating soil organic matter (SOM) mineralization (i.e. greater priming effect), while the addition of N decreased total C. The decreased soil total C induced by exogenous labile C addition was greater in the plantation than that in the natural forest. The influence of exudate additions produced no significant influence on labile and recalcitrant carbon pools at either soil. The addition of labile C exudate decreased the total phospholipid fatty acid (PLFA), actinomycetic, bacterial and fungal PLFA for the natural forest, but increased them in the plantation. Moreover, the microbial community composition (i.e. the value of bacterial PLFA:fungi PLFA) varied greatly among the treatments. These results indicate that both root-derived N inputs and soil N availability co-regulate the direction and magnitude of priming effects on SOM decomposition by controlling the activity and the relative abundance of bacterial and fungal. Our results provide additional evidences toward a robust theoretical foundation for better understanding the ecological consequences of exudate stoichiometry on soil C cycling in forests.</p></sec>
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Declining biodiversity is a critical component of global change owing to its influence on ecosystem functioning. Decomposition rate frequently increases with fungal species number, but the responses of extracellular enzymes to fungal species number have not been tested. To test the effect of biodiversity on decomposition and enzyme activities, quaking aspen ( Populus tremuloides Michx.) litter was inoculated with mixtures of one, two, four, or eight fungi from a pool of 16 fungi that had been isolated from a boreal forest in Alaska. Total CO2 release and the activities of β-glucosidase, which targets cellulose, and polyphenol oxidase, which targets lignin and other recalcitrant phenolic compounds, were observed across the range of species numbers in the mixtures. Total CO2 release and β-glucosidase activity increased with number of species but were only weakly correlated with each other; polyphenol oxidase activity had no correlation with number of species or CO2 release. The results indicate that, over 4 months, decomposition of labile carbon is positively correlated with number of species.
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