Search

• The alpine meadow ecosystem in northeast Tibetan Plateau was a CO 2 sink. • The flux exchanges of the ecosystem were energy-limited. • The CO 2 fluxes of the ecosystem were mainly controlled by soil temperature. • Warming will reduce the CO 2 sink because of a larger increase in R e than in GPP. The alpine meadow in the Tibetan Plateau (TP) is a fragile ecosystem and very sensitive to climate change and human activities. However, the flux (energy, water, and carbon dioxide) exchanges (FEs) and their controls in this ecosystem are not well understood. Using two years (2015–2016) of observations from three eddy covariance towers and different grazing management types, we quantified the FEs of an alpine meadow ecosystem in the northeastern TP. A boosted regression trees (BRT)-based method was used to determine the environmental controls of FEs and clarify the factors causing the differences in FEs among the sites. The mean annual latent heat flux (LE), sensible heat flux (H), and evapotranspiration (ET) of the sites were 36.3 W m−2, 23.2 W m−2, and 458.2 mm, respectively. The mean annual gross primary productivity (GPP) and ecosystem respiration (R e) of the sites were 504.8 and 317.2 g C m−2, respectively. The mean annual net ecosystem CO 2 exchange (NEE) of the sites was –187.6 g C m–2, indicating that the ecosystem was a CO 2 sink, not accounting for grazing losses. The BRT-based analysis showed that, (i) The LE (ET) and H were mostly determined by soil temperature (ST) and downward shortwave radiation (DR), while the NEE, GPP, and R e were primarily correlated with ST. As a result, the FEs of the ecosystem were energy-limited; (ii) The high spatial heterogeneity in multiple environmental variables jointly determined the differences in FEs of the ecosystem. The differences in LE (ET) and H among the sites resulted from multiple environmental variables, including DR, ST, precipitation, and SM; and differences in NEE, GPP, and R e were mainly caused by precipitation, SM, and RH. The study highlights that future warming will enhance GPP and R e of the ecosystem, while reducing the CO 2 sink because of the larger increases in R e than in GPP. [ABSTRACT FROM AUTHOR]

• Wet alpine meadow ecosystems (WAME) act as a significant carbon sink. • Ecosystem respiration was significantly decreased when soil was nearly-saturated. • Shallow groundwater inhibits soil respiration and favors carbon uptake of WAME. • Warming affects carbon dynamics of WAME through changing soil hydrology. Wet alpine meadows generally act as a significant carbon sink, since their low rate of soil decomposition determines a much smaller ecosystem respiration (Re) than photosynthesis. However, it remains unclear whether the low soil decomposition rate is determined by low temperatures or by nearly-saturated soil moisture. We explored this issue by using five years of measurements from two eddy-covariance sites with low temperature and significantly different soil water conditions. The results showed that both sites were carbon sinks. However, despite a smaller annual gross primary productivity, the wet site with a shallow groundwater showed a much higher carbon use efficiency and larger carbon sink than the dry site (which had a deeper water table) due to its much lower Re. Our analyses showed that Re of the wet site was significantly decreased under the nearly-saturated soil condition during the unfrozen seasons. This effect of nearly-saturated soil water on Re increased with soil depths. In contrast, at the dry site the high soil water content favored Re. The corresponding soil temperature at both sites expectedly showed large and positive effects on Re. These results demonstrated that the high carbon sink of the wet alpine meadow was mainly caused by the inhibiting effects of the nearly-saturated soil condition on soil respiration rather than by the low temperatures. Therefore, we argue that a warming-induced shrinking cryosphere may affect the carbon dynamics of wet and cold ecosystems through changes in soil hydrology and its impact on soil respiration. In addition, our study highlights the different responses of soil respiration to warming across soil depths. The thawing of frozen soil may cause larger CO 2 emission in the top soil, while it may also partially contribute to slowing down soil carbon decomposition in the deep soil through decreasing metabolic activity of aerobic organisms. Image, graphical abstract [ABSTRACT FROM AUTHOR]