Non-growing-season soil respiration is controlled by freezing and thawing processes in the summer monsoon-dominated Tibetan alpine grassland

The Tibetan alpine grasslands, sharing many features with arctic tundra ecosystems, have a unique non-growing-season climate that is usually dry and without persistent snow cover. Pronounced winter warming recently observed in this ecosystem may significantly alter the non-growing-season carbon cycle processes such as soil respiration (R_s), but detailed measurements to assess the patterns, drivers of, and potential feedbacks on R_s have not been made yet. We conducted a 4 year study on R_s using a unique R_s measuring system, composed of an automated soil CO₂ flux sampling system and a custom-made container, to facilitate measurements in this extreme environment. We found that in the nongrowing season, (1) cumulative R_s was 82-89 g C m^-2, accounting for 11.8-13.2% of the annual total R_s; (2) surface soil freezing controlled the diurnal pattern of R_s and bulk soil freezing induced lower reference respiration rate (R₀) and temperature sensitivity (Q₁₀) than those in the growing season (0.40-0.53 versus 0.84-1.32 μmol CO₂ m^-2 s^-1 for R₀ and 2.5-2.9 versus 2.9-5.6 for Q₁₀); and (3) the intraannual variation in cumulative R_s was controlled by accumulated surface soil temperature. We found that in the summer monsoon-dominated Tibetan alpine grassland, surface soil freezing, bulk soil freezing, and accumulated surface soil temperature are the day-, season-, and year-scale drivers of the non-growing-season R_s, respectively. Our results suggest that warmer winters can trigger carbon loss from this ecosystem because of higher Q₁₀ of thawed than frozen soils. Key Points Four year continuous hourly monitoring of soil CO₂ flux (R_s) using automated systemNon-growing-season cumulative R_s accounted for 11.8-13.2%