Soil Open Pore Structure Regulates Soil Organic Carbon Fractions of soil Aggregates under Simulated Freeze‑Thaw Cycles as Determined by X‑ray Computed Tomography

Purpose: Freeze-thaw cycles (FTCs) in the alpine region driven by global warming is an important abiotic perturbation that affects soil pore structure and soil organic. However, the mechanisms of interaction between soil aggregate structure and carbon fractions during FTCs are unclear. Methods: In this study, soil samples were collected from two typical alpine ecosystems in the Qinghai-Tibet Plateau region, and soil aggregates were categorized into three sizes: >2 mm, 0.25–2 mm, and < 0.25 mm. The experiments consisted of 12 FTCs (0, 1, 3, 6 and 12 cycles) (freezing at -10 ℃ for 24 h and thawing at 15 ℃ for 24 h). Aggregate structure and carbon fractions were quantified using CT scanning and physical classification, respectively. Results: FTCs increased total porosity, open porosity, pore volume and the surface area density of soil aggregates. After freeze-thaw cycles, the pore volume of > 2 mm and 0.25–2 mm aggregates increased by 65.55% and 31.85%, respectively. FTCs greatly reduced the particulate organic carbon (POC), mineral-associated organic carbon (MAOC) and total organic carbon (TOC) contents of soil aggregates, while the dissolved organic carbon (DOC) content exhibited an initial increase followed by a decrease trend. During the FTCs, the structure of soil aggregates, including aggregate size and open pore structure, significantly affected carbon fraction content. In > 2 mm aggregates, the POC, MAOC, and TOC contents were negatively correlated with open pore porosity, surface area density, porosity (< 30 μm) and pore mean volume. In 0.25–2 mm aggregates, the POC, MAOC, and TOC contents were negatively correlated with the pore number density and pore length density of soil aggregates, and were positively correlated with the mean pore volume and porosity (> 200 μm) of soil aggregates. Conclusion: In typical alpine ecosystems, the pores within soil aggregates were mainly open pores. Freeze-thaw cycles substantially influenced the pore structure, especially open pores, and the carbon fractions content. There was a close interaction between the pore structure of soil aggregates and carbon content under repeated freeze-thaw cycles.