Abstract
Abstract. Freezing and thawing action of the active layer plays a significant role in soil respiration (Rs) in permafrost regions. However, little is known about how the freeze–thaw processes affect the Rs dynamics in different stages of the alpine meadow underlain by permafrost in the Qinghai–Tibet Plateau (QTP). We conducted continuous in situ measurements of Rs and freeze–thaw processes of the active layer at an alpine meadow site in the Beiluhe permafrost region of the QTP and divided the freeze–thaw processes into four different stages in a complete freeze–thaw cycle, comprising the summer thawing (ST) stage, autumn freezing (AF) stage, winter cooling (WC) stage, and spring warming (SW) stage. We found that the freeze–thaw processes have various effects on the Rs dynamics in different freeze–thaw stages. The mean Rs ranged from 0.12 to 3.18 µmol m−2 s−1 across the stages, with the lowest value in WC and highest value in ST. Q10 among the different freeze–thaw stages changed greatly, with the maximum (4.91±0.35) in WC and minimum (0.33±0.21) in AF. Patterns of Rs among the ST, AF, WC, and SW stages differed, and the corresponding contribution percentages of cumulative Rs to total Rs of a complete freeze–thaw cycle (1692.98±51.43 g CO2 m−2) were 61.32±0.32 %, 8.89±0.18 %, 18.43±0.11 %, and 11.29±0.11 %, respectively. Soil temperature (Ts) was the most important driver of Rs regardless of soil water status in all stages. Our results suggest that as climate change and permafrost degradation continue, great changes in freeze–thaw process patterns may trigger more Rs emissions from this ecosystem because of a prolonged ST stage.
Highlights
Soil respiration (Rs) is a significant source in estimating the terrestrial carbon budget under climate change
We assumed that the soils began to freeze when the temperature dropped lower than 0 ◦C and to thaw when the temperature was continuously greater than 0 ◦C, based on the fact that the effects of the surface energy of soil particles and salinity in soil on freezing temperatures are negligible
Our observations clearly demonstrated that the freeze–thaw process of the active layer strongly affected the Rs dynamics and that Rs emission models were significantly different for each of the freeze–thaw stages (P < 0.01)
Summary
Soil respiration (Rs) is a significant source in estimating the terrestrial carbon budget under climate change. It is the second-largest source of carbon emissions to the atmosphere from the terrestrial ecosystem on a global scale (BondLamberty and Thomson, 2010; Schlesinger and Andrews, 2000). The Arctic tundra ecosystem is becoming a consistent source of CO2 because CO2 emission in winter offsets its uptake in the growing season with progressive permafrost thaw and active-layer thickening (Celis et al, 2017). In Alaska, emissions of CO2 from tundra during early winter seasons have increased by about 73 % since 1975, and the Arctic ecosystem has been a net source of CO2 due to rising temperatures (Commane et al, 2017). For the subarctic tundra ecosystem, the wintertime CO2 loss has been in-
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