Dominant influence of non-thawing periods on annual CO2 emissions from Zoige peatlands: Five-year eddy covariance analysis

Abstract

Changes in freeze–thaw processes as a result of climate change significantly affect the carbon balance of terrestrial ecosystems, especially peatlands. The Qinghai-Tibetan Plateau contains substantial permafrost and seasonal frozen soils and its peatlands feature a huge carbon stock highly sensitive to global change. In this paper, we used an eddy covariance flux tower to monitor carbon fluxes on the Zoige peatlands with seasonal frozen soils on the eastern Qinghai-Tibetan Plateau from 2013 to 2017. Total net emissions (g C m−2 yr−1) over the monitoring period were 64.1, 106.7, 126.5 and 103.2 in the frozen, frozen-thawing, thawing and thawing-freezing periods; the corresponding weighted mean emission rates (g C m−2 d−1) were 0.36, 0.44, 0.64 and 0.38. Although net CO2 emissions were highest in the thawing period, emissions during the other periods accounted for approximately 68.4% of annual emissions. In fact, emissions during the two freeze–thaw periods were 1.7 times those during the thawing period despite the freeze–thaw periods lasting fewer days. CO2 emission correlated significantly with volumetric water content and soil temperature in the frozen period, with soil temperature and precipitation in the frozen-thawing period, and with precipitation in the thawing and thawing-freezing periods. Annual net CO2 emissions during each of the years were 57.2 g C m−2 yr−1 (2013), 108.3 g C m−2 yr−1 (2014), 41 g C m−2 yr−1 (2015), 62.9 g C m−2 yr−1 (2016) and 131.1 g C m−2 yr−1 (2017), and hydrological conditions were the primary determinant of interannual variations. Long-term continuous monitoring of carbon is essential to understand how freeze–thaw processes affect the ability of alpine peatland ecosystems to act as carbon sources and sinks.