Coinciding spring and autumn frosts have a limited impact on carbon fluxes in a grassland ecosystem

Frosts, increasingly prevalent due to climate warming, can offset the carbon storage benefits of an extended growing season, potentially exacerbating climate warming. However, existing research primarily focus on species, with limited evidence on carbon fluxes at the ecosystem scale. Using a manipulative experiment simulating 7-day frosts in a temperate grassland, we find that ongoing frosts, whether in spring or autumn, have limited effects on gross ecosystem productivity, ecosystem respiration, and net ecosystem productivity during the frost measurement periods. However, frosts profoundly impact net ecosystem productivity over the entire growing season outside the frost measurement periods. Specifically, spring frosts significantly increase net ecosystem productivity, autumn frosts marginal decrease it, and the combined effect of both frosts neutralize net ecosystem productivity. The early-year (2018-2020) impacts of frosts on net ecosystem productivity may be driven by plant eco-physiological changes, whereas the late-year impacts (2021-2023) were attributed to shifts in plant community structure. Our findings suggest that frequent frosts in both seasons may not stimulate ecosystem carbon release in temperate grasslands. Understanding these patterns is crucial for predicting carbon balance and developing effective climate-change mitigation strategies in response to the future warmer climate.