Soil Moisture but Not Warming Dominates Nitrous Oxide Emissions During Freeze–Thaw Cycles in a Qinghai–Tibetan Plateau Alpine Meadow With Discontinuous Permafrost

Abstract

Large quantities of organic matter are stored in frozen soils (permafrost) within the Qinghai–Tibetan Plateau (QTP). The most of QTP regions in particular have experienced significant warming and wetting over the past 50 years, and this warming trend is projected to intensify in the future. Such climate change will likely alter the soil freeze–thaw pattern in permafrost active layer and toward significant greenhouse gas nitrous oxide (N 2 O) release. However, the interaction effect of warming and altered soil moisture on N 2 O emission during freezing and thawing is unclear. Here, we used simulation experiments to test how changes in N 2 O flux relate to different thawing temperatures (T 5 –5°C, T 10 –10°C, and T 20 –20°C) and soil volumetric water contents (VWCs, W 15 –15%, W 30 –30%, and W 45 –45%) under 165 F–T cycles in topsoil (0–20 cm) of an alpine meadow with discontinuous permafrost in the QTP. First, in contrast to the prevailing view, soil moisture but not thawing temperature dominated the large N 2 O pulses during F–T events. The maximum emissions, 1,123.16–5,849.54 μg m –2 h –1 , appeared in the range of soil VWC from 17% to 38%. However, the mean N 2 O fluxes had no significant difference between different thawing temperatures when soil was dry or waterlogged. Second, in medium soil moisture, low thawing temperature is more able to promote soil N 2 O emission than high temperature. For example, the peak value (5,849.54 μg m –2 h –1 ) and cumulative emissions (366.6 mg m –2 ) of W 30 T 5 treatment were five times and two to four times higher than W 30 T 10 and W 30 T 20 , respectively. Third, during long-term freeze–thaw cycles, the patterns of cumulative N 2 O emissions were related to soil moisture. treatments; on the contrary, the cumulative emissions of W 45 treatments slowly increased until more than 80 cycles. Finally, long-term freeze–thaw cycles could improve nitrogen availability, prolong N 2 O release time, and increase N 2 O cumulative emission in permafrost active layer. Particularly, the high emission was concentrated in the first 27 and 48 cycles in W 15 and W 30 , respectively. Overall, our study highlighted that large emissions of N 2 O in F–T events tend to occur in medium moisture soil at lower thawing temperature; the increased number of F–T cycles may enhance N 2 O emission and nitrogen mineralization in permafrost active layer.

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