Abstract. Seasonal freeze–thaw processes alter soil formation and lead to changes in soil structure of alpine ecosystems. Soil aggregates are basic soil structural units and play a crucial role in soil organic carbon (SOC) protection and microbial habitation. However, the impact of seasonal freeze–thaw processes on pore structure and their impact on SOC fractions have been overlooked. This study characterized the pore structure and SOC fractions of soil aggregates of the unstable freezing period, stable frozen period, unstable thawing period and stable thawed period in typical alpine ecosystems via a dry-sieving procedure, X-ray computed tomography scanning and elemental analysis. The results showed that pore networks of 0.25–2 mm aggregates were more vulnerable to seasonal freeze–thaw processes than those of >2 mm aggregates. The freezing process promoted the formation of >80 µm pores of aggregates. The total organic carbon, particulate organic carbon and mineral-associated organic carbon contents of aggregates were high in the stable frozen period and dropped dramatically in the unstable thawing period, demonstrating that the freezing process was positively associated with SOC accumulation, while SOC loss featured in the early stage of thawing. The vertical distribution of SOC of aggregates was more uniform in the stable frozen period than in other periods. Pore equivalent diameter was the most important structural characteristic influencing SOC contents of aggregates. In the freezing period, the SOC accumulation might be enhanced by the formation of >80 µm pores. In the thawing period, pores of <15 µm were positively correlated with SOC concentration. Our results revealed that changes in pore structure induced by freeze–thaw processes could contribute to SOC protection of aggregates.

中文翻译：

---

冻融过程对应于高寒生态系统中通过调节团聚体孔隙结构对土壤有机碳的保护-损失


摘要。季节性冻融过程会改变土壤形成，并导致高山生态系统的土壤结构发生变化。土壤团聚体是基本的土壤结构单元，在土壤有机碳 （SOC） 保护和微生物栖息地方面起着至关重要的作用。然而，季节性冻融过程对孔隙结构的影响及其对 SOC 馏分的影响一直被忽视。本研究通过干筛、X射线计算机断层扫描和元素分析，对典型高寒生态系统不稳定冻结期、稳定冻结期、不稳定解冻期和稳定解冻期土壤团聚体的孔隙结构和SOC组分进行了表征。结果表明，0.25-2 mm 聚集体的孔隙网络比 >2 mm 聚集体的孔隙网络更容易受到季节性冻融过程的影响。冷冻过程促进了 >80 μm 聚集体孔隙的形成。团聚体的总有机碳、颗粒有机碳和矿物伴生有机碳含量在稳定冻结期较高，在不稳定解冻期下降急剧，表明冻结过程与 SOC 积累呈正相关，而 SOC 损失在解冻初期具有显著特征。稳定冻结期团聚体 SOC 垂直分布比其他时期更均匀。孔当量直径是影响团聚体 SOC 含量的最重要结构特征。在冻结期间，形成 >80 μm 孔可能会增强 SOC 积累。在解冻期，<15 μm 孔隙与 SOC 浓度呈正相关。 我们的结果表明，冻融过程诱导的孔隙结构变化可能有助于聚集体的 SOC 保护。