Abstract

Predicted intensified climate warming will likely alter the ecosystem net carbon (C) uptake of the Qinghai-Tibet 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 water-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 consistent and strong C sink of CO2 (−168.0 ± −62.5 gC m−2 y−1, average ± standard deviation) across the entire 4-year study period. A random forest machine-learning analysis suggests that the diurnal, seasonal, and annual variations of net ecosystem exchange (NEE) and gross primary productivity (GPP) were controlled by temperature and solar 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 identical temperatures and significantly differences on SWC and vice versa. Our data suggest that, despite the relatively abundant water supply, periods with a substantial decrease of SWC or increase of temperature produced higher Re lowering the C sink strength. Our results reveal that nearly saturated soil conditions during the warm seasons can help to maintain lower ecosystem respiration rates and thus enhance the overall C sequestration capacity in this alpine swamp meadow. We argue that changes in soil hydrological conditions induced by a warming climate near permafrost (or seasonal frozen layers) may affect the C sink magnitude of wet and cold ecosystems through changes in soil hydrology and the subsequent effect on respiration losses.

Highlights

  • Wetlands play a significant role in the global carbon (C) cycle due to the large amount of C stored in their soils

  • The objectives of this study are to (i) quantify the diurnal, seasonal, and annual variations of net ecosystem exchange (NEE), gross primary productivity (GPP), and ecosystem respiration (Re), (ii) quantify the relative importance of different key environmental drivers contributing to the variability observed of NEE, Re, and GPP, and (iii) analyse how these C fluxes respond to key environmental drivers such as temperature or soil moisture variation in a Qinghai-Tibet Plateau (QTP) alpine swamp meadow

  • The lower precipitation regime led to a marked decline in soil water content (SWC), making the late growing season of 2015 the driest period among all growing seasons during the study period

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Summary

Introduction

Wetlands play a significant role in the global carbon (C) cycle due to the large amount of C stored in their soils. The Qinghai-Tibet Plateau (QTP), with an average altitude of over 4,000 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). Hydrological regimes have an important role in controlling wetland functioning (Bohn et al, 2007; Christensen et al, 2003). The changes of hydrological regimes may put the wetland functioning of the QTP under pressure (Hruby, 1999; Woodward and Wui, 2001; Foti et al, 2013).

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