Aims <p>Alpine meadows on the Qinghai–Tibet Plateau (QTP) are critical to global soil carbon storage, yet how seasonal freeze–thaw cycles (FTCs) regulate the allocation of photosynthetic carbon into soil particulate (POC) and mineral-associated organic matter (MAOC) remains poorly understood.</p> Methods <p>Using ¹³CO₂ pulse labelling, we investigated the dynamics of photosynthetic carbon in leaves, roots, and rhizosphere soils, along with its distribution in POC and MAOC (0–10&#xa0;cm) during FTCs.</p> Results <p>The results showed that total ¹³C assimilation during the early freezing period (EFP) was 0.572&#xa0;g&#xa0;m⁻², representing only 30% of that observed in the thawing period (TP, 1.955&#xa0;g&#xa0;m⁻2). In contrast, a larger proportion (86%) of the assimilated carbon was allocated belowground during the EFP. Photosynthetic carbon was primarily allocated into soil MAOC during the EFP, whereas it was mainly directed to POC during the TP, with a rapid peak but fast turnover. Photosynthetic carbon enrichment in rhizosphere soil was initially positively correlated with macropore abundance, and later with smaller pores. This pore structure facilitated efficient conversion of photosynthetic carbon into MAOC during the EFP, thereby enhancing soil carbon stability. Additionally, a higher abundance of fungi promoted greater carbon input into rhizosphere soils during the EFP.</p> Conclusions <p>Given that climate warming is expected to extend the thawing period and shorten the freezing period, the contrasting carbon dynamics observed in this study imply that future changes in freeze–thaw processes could increase topsoil derived-carbon input but reduce its long‑term stability.</p>