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

Zhe Chen,Huakun Zhou,Zhenhua Zhang,Shidong Ge,Huichun Xie,Pan Liu,Buqing Yao,Yufang Zhang,Wenying Wang,Yangong Du

doi:10.3389/fevo.2021.676027

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 (N2O) release. However, the interaction effect of warming and altered soil moisture on N2O emission during freezing and thawing is unclear. Here, we used simulation experiments to test how changes in N2O flux relate to different thawing temperatures (T5–5°C, T10–10°C, and T20–20°C) and soil volumetric water contents (VWCs, W15–15%, W30–30%, and W45–45%) 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 N2O 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 N2O 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 N2O 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 W30T5 treatment were five times and two to four times higher than W30T10 and W30T20, respectively. Third, during long-term freeze–thaw cycles, the patterns of cumulative N2O emissions were related to soil moisture. treatments; on the contrary, the cumulative emissions of W45 treatments slowly increased until more than 80 cycles. Finally, long-term freeze–thaw cycles could improve nitrogen availability, prolong N2O release time, and increase N2O cumulative emission in permafrost active layer. Particularly, the high emission was concentrated in the first 27 and 48 cycles in W15 and W30, respectively. Overall, our study highlighted that large emissions of N2O in F–T events tend to occur in medium moisture soil at lower thawing temperature; the increased number of F–T cycles may enhance N2O emission and nitrogen mineralization in permafrost active layer.

Highlights

The global permafrost and seasonally frozen ground cover about 70% of all terrestrial ecosystems (Lawrence et al, 2012)
The N2O emissions of W30 treatment were higher than W15 and W45, and its patterns were similar even at different thawing temperatures
The results showed that soil moisture, thawing temperature, and the number of F–T cycles all had significant impacts on N2O flux, and their interactions were significant (P < 0.01) (Table 1)

Summary

Introduction

The global permafrost and seasonally frozen ground cover about 70% of all terrestrial ecosystems (Lawrence et al, 2012). Soil freeze–thaw (F–T) events are a key natural process driving soil aggregate fragmentation (Chai et al, 2014), organic matter activization (Chen L. et al, 2016), root death (Kreyling et al, 2012), changes in microbial community structure and function (Yang et al, 2018; Mao et al, 2019), and available nitrogen (N) transformation (Jiang et al, 2020; Mao et al, 2020) These process changes in a warmer world are further intensifying nitrous oxide (N2O) release from permafrost and seasonally frozen zones (Henry, 2008; Brooks et al, 2011; Risk et al, 2013; Chen et al, 2018; Lv et al, 2020). The magnitude of F–T effects on the key process of N transformation, N2O emissions, in high latitude and/or high-altitude regions remains highly uncertain

Objectives

Methods

Results

Conclusion