Contrasting effects of daytime and nighttime warming on ecosystem carbon uptake in an alpine grassland

As global warming continues, non-uniform diel warming is widely affecting terrestrial ecosystems. Because plants undergo distinct physiological processes during the day and night, daytime and nighttime warming can exert contrasting effects on ecosystem carbon uptake and release. Numerous studies have shown that alpine ecosystems are more sensitive to temperature change than low-elevation ecosystems. Owing to low-temperature constraints, alpine ecosystems may exhibit distinct responses to asymmetric diel warming compared to lowland ecosystems. However, little is known about how the differential warming intensity between day and night would affect alpine ecosystems. Here, we integrated a four-year field warming experiment (daytime warming: +1.11 ℃; nighttime warming: +2.08 ℃) with landscape-scale observations to investigate the effects of asymmetric diurnal warming on carbon cycling in alpine grasslands. We found that daytime warming reduced net ecosystem productivity (NEP) by 32.4%, whereas nighttime warming increased NEP by 14.0%. Soil moisture, the coefficient of variation of soil temperature (CVST), precipitation during phenological periods, and aboveground biomass jointly mediated these responses. Daytime warming suppressed carbon uptake primarily by intensifying soil moisture loss, while nighttime warming enhanced carbon uptake by reducing CVST and creating more favorable thermal conditions. Moreover, precipitation during phenophases modulated the effects of warming-induced phenological shifts on ecosystem carbon exchange. Our results suggest that daytime and nighttime warming influence carbon cycling in alpine grasslands through distinct ecological mechanisms, underscoring the importance of explicitly considering the regulatory roles of soil moisture and temperature variability when assessing ecosystem carbon responses to diurnally asymmetric warming.