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−2, 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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亚高山森林演替序列中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，在全球碳循环中起着至关重要的作用。然而，随着森林演替，土壤有机碳储量和质量的变化仍然存在很大的不确定性。本研究对位于青藏高原东南部的灌木、落叶阔叶林、阔叶针叶混交林、中龄针叶林和成熟针叶林等亚高寒林系列土壤碳储量和质量进行了调查。结果表明，土壤有机碳储量范围为 9.8 至 29.9 kg⋅m−2，并在整个森林演替序列中表现出驼峰状响应模式。在混交林和灌木林中分别观察到最高和最低的 SOC 存量。土壤有机碳蓄积量与土壤温度和凋落物蓄积量无显著关系，但与木材碎屑蓄积量呈正相关。同时，基于 FTIR 光谱的多糖、木质素、芳香族和脂肪族的平均百分比分别为 79.89%、0.94%、18.87% 和 0.29%。此外，除混交林中突然减少外，多糖的百分比在整个森林演替序列中表现出增加的趋势，而木质素、芳香族和脂肪族的比例在整个森林演替序列中表现出下降趋势，除了混交林中突然增加。因此，混交林的腐殖化指数 （HIs） 高于其他 4 个演替阶段，这意味着混交林的 SOC 质量差于其他演替阶段。 此外，土壤有机碳储量、顽固组分和 HIs 随着土壤深度的增加而减少，而多糖呈增加趋势。这些结果表明，与其他演替阶段相比，混交林的土壤有机碳储量更高，土壤有机碳质量更差。高比例的土壤有机碳储量（20-100 厘米深度为 66%）和较好的土壤有机碳质量（较低的 HIs）表明深层土壤具有储存土壤有机碳的巨大潜力，在全球变化下需要更多关注。