Freeze-thaw processes correspond to the protection-loss of soil organic carbon through regulating pore structure of aggregates in alpine ecosystems

Abstract. Seasonal freeze‒thaw (FT) processes alter soil formation and causes changes in soil structure in 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 FT processes on pore structure and its impact on SOC fractions have been overlooked. This study characterized the pore structure and SOC fractions of aggregates during the unstable freezing period (UFP), stable frozen period (SFP), unstable thawing period (UTP) and stable thawed period (STP) in typical alpine ecosystems via the dry sieving procedure, X-ray computed tomography (CT) scanning and elemental analysis. The results showed that pore characteristics of 0.25–2 mm aggregates were more vulnerable to seasonal FT processes than that of > 2 mm aggregates. The freezing process promoted the formation of > 80 μm pores of aggregates. The total organic carbon (TOC), particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) contents of macroaggregates were high in the stable frozen period and low in unstable thawing period, demonstrating that freezing process enhanced SOC accumulation while early stage of thawing led to SOC loss. The vertical distribution of SOC of aggregates was more uniform in 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 importance of pore structure in regulating SOC protection was more obvious and pore structure inhibited SOC loss by promoted the formation of >80 μm pores. In the thawing period, pores of 15–30 μm inhibited SOC protection. Our results are valuable for evaluating potential changes in alpine soil carbon sinks under global warming.