Soil organic carbon (SOC) affects the function of terrestrial ecosystem and plays a vital role in global carbon cycle. Yet, large uncertainty still existed regarding the changes in SOC stock and quality with forest succession. Here, the stock and quality of SOC at 1-m soil profile were investigated across a subalpine forest series, including shrub, deciduous broad-leaved forest, broadleaf-conifer mixed forest, middle-age coniferous forest and mature coniferous forest, which located at southeast of Tibetan Plateau. The results showed that SOC stock ranged from 9.8 to 29.9 ​kg⋅m, and exhibited a hump-shaped response pattern across the forest successional series. The highest and lowest SOC stock was observed in the mixed forest and shrub forest, respectively. The SOC stock had no significant relationships with soil temperature and litter stock, but was positively correlated with wood debris stock. Meanwhile, the average percentages of polysaccharides, lignins, aromatics and aliphatics based on FTIR spectroscopy were 79.89%, 0.94%, 18.87% and 0.29%, respectively. Furthermore, the percentage of polysaccharides exhibited an increasing pattern across the forest successional series except for the sudden decreasing in the mixed forest, while the proportions of lignins, aromatics and aliphatics exhibited a decreasing pattern across the forest successional series except for the sudden increasing in the mixed forest. Consequently, the humification indices (HIs) were highest in the mixed forest compared to the other four successional stages, which means that the SOC quality in mixed forest was worse than other successional stages. In addition, the SOC stock, recalcitrant fractions and HIs decreased with increasing soil depth, while the polysaccharides exhibited an increasing pattern. These findings demonstrate that the mixed forest had higher SOC stock and worse SOC quality than other successional stages. The high proportion of SOC stock (66% at depth of 20–100 ​cm) and better SOC quality (lower HIs) indicate that deep soil have tremendous potential to store SOC and needs more attention under global change.

中文翻译：

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亚高山森林演替系列土壤有机碳储量和质量的变化


土壤有机碳（SOC）影响陆地生态系统的功能，在全球碳循环中发挥着至关重要的作用。然而，随着森林演替，SOC 储量和质量的变化仍然存在很大的不确定性。这里，对亚高山森林系列（包括灌木、落叶阔叶林、阔叶针叶混交林、中龄针叶林和成熟针叶林）1米土壤剖面的SOC储量和质量进行了调查，这些森林位于青藏高原东南部。结果表明，SOC 储量范围为 9.8 至 29.9 kg·m，并在整个森林演替系列中表现出驼峰状响应模式。最高和最低的 SOC 储量分别在混交林和灌木林中观察到。 SOC储量与土壤温度和凋落物储量没有显着关系，但与木屑储量呈正相关。同时，基于 FTIR 光谱的多糖、木质素、芳香族化合物和脂肪族化合物的平均百分比分别为 79.89%、0.94%、18.87% 和 0.29%。此外，多糖的比例在整个森林演替系列中表现出增加的趋势，但在混交林中突然下降；而木质素、芳香族化合物和脂肪族化合物的比例在整个森林演替系列中表现出下降的趋势，除了混交林中的突然增加。混交林。因此，与其他四个演替阶段相比，混交林的腐殖化指数（HI）最高，这意味着混交林的SOC质量比其他演替阶段差。此外，随着土壤深度的增加，SOC 储量、顽固组分和 HI 减少，而多糖则呈现增加模式。 这些发现表明，混交林比其他演替阶段具有更高的 SOC 储量和更差的 SOC 质量。高比例的 SOC 储量（20-100 cm 深度为 66%）和较好的 SOC 质量（较低的 HI）表明深层土壤具有储存 SOC 的巨大潜力，在全球变化下需要更多关注。