Carbon dioxide (CO₂) exchange between the atmosphere and grassland ecosystems is very important for the global carbon balance. To assess the CO₂ flux and its relationship to environmental factors, the eddy covariance method was used to evaluate the diurnal cycle and seasonal pattern of the net ecosystem CO₂ exchange (NEE) of a cultivated pasture in the Three-River Source Region (TRSR) on the Qinghai−Tibetan Plateau from January 1 to December 31, 2008. The diurnal variations in the NEE and ecosystem respiration (R_e) during the growing season exhibited single-peak patterns, the maximum and minimum CO₂ uptake observed during the noon hours and night; and the maximum and minimum R_e took place in the afternoon and early morning, respectively. The minimum hourly NEE rate and the maximum hourly R_e rate were −7.89 and 5.03 μmol CO₂ m⁻² s⁻¹, respectively. The NEE and R_e showed clear seasonal variations, with lower values in winter and higher values in the peak growth period. The highest daily values for C uptake and R_e were observed on August 12 (−2.91 g C m⁻² d⁻¹) and July 28 (5.04 g C m⁻² day⁻¹), respectively. The annual total NEE and R_e were −140.01 and 403.57 g C m⁻² year⁻¹, respectively. The apparent quantum yield (α) was −0.0275 μmol μmol⁻¹ for the entire growing period, and the α values for the pasture’s light response curve varied with the leaf area index (LAI), air temperature (T_a), soil water content (SWC) and vapor pressure deficit (VPD). Piecewise regression results indicated that the optimum T_a and VPD for the daytime NEE were 14.1°C and 0.65 kPa, respectively. The daytime NEE decreased with increasing SWC, and the temperature sensitivity of respiration (Q₁₀) was 3.0 during the growing season, which was controlled by the SWC conditions. Path analysis suggested that the soil temperature at a depth of 5 cm (T_soil) was the most important environmental factor affecting daily variations in NEE during the growing season, and the photosynthetic photon flux density (PPFD) was the major limiting factor for this cultivated pasture.

The Qinghai-Tibetan Plateau (QTP) is generally considered to be the water source region for its surrounding lowlands. However, there have only been a few studies that have focused on quantifying alpine meadow evapotranspiration (ET) and its partitioning, which are important components of water balance. This paper used the Shuttleworth–Wallace (S–W) model to quantify soil evaporation (E) and plant transpiration (T) in a degraded alpine meadow (34°24′ N, 100°24′ E, 3963 m a.s.l) located at the QTP from September 2006 to December 2008. The results showed that the annual ET estimated by the S–W model (ET_SW) was 511.5 mm (2007) and 499.8 mm (2008), while E estimated by the model (E_SW) was 306.0 mm and 281.7 mm for 2007 and 2008, respectively, which was 49% and 29% higher than plant transpiration (T_SW). Model analysis showed that ET, E, and T were mainly dominated by net radiation (R_n), while leaf area index (LAI) and soil water content at a 5 cm depth (SWC_5cm) were the most important factors influencing ET partitioning. The study results suggest that meadow degradation may increase water loss through increasing E, and reduce the water conservation capability of the alpine meadow ecosystem.

Terrestrial ecosystem water balance refers to the state between precipitation inputs, and outputs via evapotranspiration, groundwater recharge and runoff. The Three River Source Region (TRSR) is known as the “water tower of Asia” and is an important water source region for much of southeast Asia. Spatial and temporal distributions of water balance across the TRSR have been affected by recent changing climatic conditions. However, the status and variation of water balance under the influence of environmental factors remain unclear. We employed SOILWAT2, a daily time step ecosystem water model, along with 35 years of climate data from 29 meteorological stations to characterize the hydrological dynamics of the region. Our results revealed that 15 of the sites exhibited evidence of a net water source (precipitation > actual evapotranspiration) between 1981 and 2015. These stations were predominantly located at intermediate elevations (3,400–4,100 m) and low latitudes (<34°N), while stations at lower and higher elevations as well as higher latitudes were not sources because of moisture limitations by a dry climate. In contrast to a strong spatial pattern, we found no obvious temporal trend of the net water source. The interannual and seasonal variability of the net water source is dependent on the temporal fluctuations in potential evapotranspiration and precipitation. We therefore conclude that there is an elevation and latitude dominated spatial pattern of the water balance on the TRSR, with important implications for the availability of water resources.

We examined the response of soil CO₂emissions to warming and environmental control mechanisms in an alpine swamp meadow ecosystem on the Tibetan Plateau. Experimental warming treatments were performed in an alpine swamp meadow ecosystem using two open-top chambers (OTCs) 40 cm (OA) and 80 cm (OB) tall. The results indicate that temperatures were increased by 2.79°C in OA and 4.96°C in OB, that ecosystem CO₂efflux showed remarkable seasonal variations in the control (CK) and the two warming treatments, and that all three systems yielded peak values in August of 123.6, 142.3, and 166.2 g C m⁻² month⁻¹. Annual CO₂efflux also showed a gradual upward trend with increased warming: OB (684.1 g C m⁻² year⁻¹) > OA (580.7 g C m⁻² year⁻¹) > CK (473.3 g C m⁻² year⁻¹). Path analysis revealed that the 5 cm depth soil temperature was the most important environmental factor affecting soil CO₂emissions in the three systems.