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

Abstract

Abstract 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 CO 2 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 ( R 0 ) and temperature sensitivity ( Q 10 ) than those in the growing season (0.40–0.53 versus 0.84–1.32 µmol CO 2 m −2 s −1 for R 0 and 2.5–2.9 versus 2.9–5.6 for Q 10 ); 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 Q 10 of thawed than frozen soils.