Abstract
Abstract. Predicted intensified climate warming will likely alter the ecosystem net carbon (C) uptake of the Qinghai–Tibetan Plateau (QTP). Variations in C sink–source responses to climate warming have been linked to water availability; however, the mechanisms by which net C uptake responds to soil water content in saturated swamp meadow ecosystems remain unclear. To explore how soil moisture and other environmental drivers modulate net C uptake in the QTP, field measurements were conducted using the eddy covariance technique in 2014, 2015, 2017, and 2018. The alpine swamp meadow presented in this study was a persistent and strong C sink of CO2 (−168.0 ± 62.5 g C m−2 yr−1, average ± standard deviation) across the entire 4-year study period. A random forest machine-learning analysis suggested that the diurnal and seasonal variations of net ecosystem exchange (NEE) and gross primary productivity (GPP) were regulated by temperature and net radiation. Ecosystem respiration (Re), however, was found mainly regulated by the variability of soil water content (SWC) at different temporal aggregations, followed by temperature, the second contributing driver. We further explored how Re is controlled by nearly saturated soil moisture and temperature comparing two different periods featuring almost identical temperatures and significant differences on SWC and vice versa. Our data suggest that, despite the relatively abundant water supply, periods with a substantial decrease in SWC or increase in temperature produced higher Re and therefore weakened the C sink strength. Our results reveal that nearly saturated soil conditions during the growing seasons can help maintain lower ecosystem respiration rates and thus enhance the overall C sequestration capacity in this alpine swamp meadow. We argue that soil respiration and subsequent ecosystem C sink magnitude in alpine swamp meadows could likely be affected by future changes in soil hydrological conditions caused by permafrost degradation or accelerated thawing–freezing cycling due to climate warming.
Highlights
Wetlands play a significant role in the global carbon (C) cycle due to a large amount of C stored in their soils
The alpine swamp meadow from the Qinghai–Tibetan Plateau (QTP) presented in this study has been found to act as a consistent and strong sink of CO2 (−168.0 ± 62.5 g C m−2 yr−1)
The results from a novel machine-learning technique revealed that air temperature is the most important variable driving net ecosystem exchange (NEE) and gross primary productivity (GPP) on a diurnal scale, while net radiation has a stronger importance controlling the seasonal variability of the same fluxes
Summary
Wetlands play a significant role in the global carbon (C) cycle due to a large amount of C stored in their soils. The Qinghai–Tibetan Plateau (QTP), with an average altitude of over 4000 m a.s.l., has approximately 10 × 104 km of natural wetlands, of which ∼ 50 % (4.9 × 104 km2) are alpine swamp meadows. These ecosystems are predominantly located in permafrost areas and are typically soil-nutrient-rich and water-logged (Bai et al, 2019; Zhao et al, 2005). The potential increase in CO2 emissions due to warming in alpine regions could be partially offset by enhanced C uptake (Schuur et al, 2009), triggering different net C uptake responses to climate warming. An increase in temperature in the QTP has been associated with net C sinks in the Zoige alpine wetlands (Kang et al, 2014) and with net C sources in the Damxung alpine swamp meadow (Niu et al, 2017)
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