The photosynthetic carbon was mainly allocated to soil MAOC of typical meadows in the freezing period

PeiYing LiuXia Hu^*

• Beijing Normal University, Beijing, China

Alpine meadows on the Qinghai-Tibet Plateau play a crucial role in global soil carbon storage, and seasonal freeze-thaw cycles are important factors influencing the carbon cycle.However, few studies have quantified how the seasonal freeze-thaw process regulates the allocation and stabilization of photosynthetic carbon in a plant-soil system. We used an in situ 13 CO₂ pulse labelling in an alpine meadow to compare the dynamics of photosynthetic carbon in leaves, roots (0-10 cm), and rhizosphere soils (0-10 cm) as well as particle organic matter (POC) and mineral-associated organic matter (MAOC) (0-10 cm) in the thawing and freezing periods on the QTP. The results revealed that the total assimilation of 13 C in the freezing period (0.572 g m⁻²) was only 30% of that in the thawing period (1.955 g m⁻²) after 21 days of labelling.The largest proportion of assimilated carbon was allocated belowground in the freezing period (86% vs. 73%), and photosynthetic C represented much of the aboveground carbon allocation (27% vs. 14%) in the thawing period, which peaked rapidly but exhibited fast turnover. The amount of 13 C in the roots was greater in the freezing period than that in the thawing period immediately after labelling (Day 0), and the amount of 13 C in the roots was greater in the thawing period than that in the freezing period from 1-21 days after labelling. A large proportion of photosynthetic carbon was allocated to soil MAOC during the freezing period, and the proportion of photosynthetic carbon allocated to soil POC was high during the thawing period. These findings highlight that under climate warming, the extension of the thawing period and shortening of the freezing duration may temporarily increase soil organic carbon stocks. However, the decline in MAOC proportions could reduce soil carbon stability. This study provides critical insights into the fate of photosynthetic carbon during seasonal freezethaw processes and the impacts of climate warming on soil carbon turnover.