Shifts in soil freeze-thaw cycle and their climate impacts along the alpine wetland-grassland continuum

Abstract

Climate change and human activity have profoundly altered soil hydrology, reshaping the areal extent and boundaries of wetland ecosystems. However, the impact of these shifts on the soil freeze-thaw cycle and their subsequent influence on greenhouse gas emissions remains poorly understood. This knowledge gap is particularly critical in high-latitude and high-altitude regions, which harbor substantial carbon stocks and exhibit distinct seasonal soil freeze-thaw cycles. Here, we conducted year-round field monitoring of the soil freeze-thaw cycle along an alpine wetland-grassland continuum on the Tibetan Plateau. We found that as the landscape transitioned from mesic meadow to wet meadow and then to fen, soils tended to freeze later (with delays of 4 and 24 days, respectively), more slowly (with reductions of 55.8% and 45.7%, respectively), and at shallower depths (with reductions of 59.7% and 57.8%). By combining high-resolution greenhouse gas carbon flux and energy exchange monitoring, we further found that soil thawing increased CO₂ and CH₄ emissions, inducing pronounced positive climate-carbon feedbacks, which were stronger in magnitude than the negative climate-carbon feedbacks associated with soil freezing (+632.1 vs -183.1 g CO₂-eq m^-2). In contrast, both soil thawing and freezing resulted in significant negative climate feedbacks due to net reductions in sensible and latent heat fluxes. These findings underscore the critical role of hydrological-driven shifts in the soil freeze-thaw cycle, highlighting their interactive effects on climate-carbon and climate-energy feedbacks.