Effects of soil water content on carbon sink strength in an alpine swamp meadow of the northeastern Qinghai-Tibet Plateau

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.