Da Wei

Revisiting the role of CH4 emissions from alpine wetlands on the Tibetan Plateau: Evidence from two in situ measurements at 4758 and 4320 m above sea level

The alpine wetlands on the Tibetan Plateau (TP) constitute 30% of Chinas wetlands, and previous studies have considered these wetlands to be important sources of CH4, based on several swamp measurements from the eastern edges of the plateau. However, the alpine wetlands consist of both swamps (9.5%) and swamp meadows (79.8%). In this study, the CH4 fluxes of a swamp meadow and a swamp were determined. The results showed that the swamp meadow emitted much less CH4 (130.8 ± 123.9 µg m−2 h−1) than the swamp (2795.2 ± 796.4 µg m−2 h−1). The CH4 fluxes within the swamp meadow showed distinct microscale spatial heterogeneity: the hollow terrain released CH4, while the hummocks absorbed CH4; this pattern was explained well by soil moisture. The CH4 emissions in the swamp meadow were highly sensitive to soil temperature variation (Q10 = 3.62), while they were more sensitive to soil moisture in the swamp. By summarizing existing measurements, and considering the differences in CH4 emissions from swamp meadows and swamps, the emissions of CH4 from alpine wetlands across the TP were recalculated to range from 0.215 to 0.412 Tg CH4 a−1, lower than previous studies. By comparison, the CH4 uptake by nonwetland ecosystems ranges from −0.68 to −0.53 Tg CH4 a−1. Therefore, this study conveys a notion that the alpine wetlands on the TP may not be significant CH4 sources. However, further studies are needed to reduce the uncertainty regarding CH4 emissions.

Considerable methane uptake by alpine grasslands despite the cold climate: in situ measurements on the central Tibetan Plateau, 2008–2013

The uptake of CH4 by aerate soil plays a secondary role in the removal of tropospheric CH4 , but it is still highly uncertain in terms of its magnitude, spatial, and temporal variation. In an attempt to quantify the sink of the vast alpine grasslands (1,400,000 km(2)) of the Tibetan Plateau, we conducted in situ measurements in an alpine steppe (4730 m) and alpine meadow (4900 m) using the static chamber and gas chromatograph method. For the alpine steppe, measurements (2008-2013) suggested that there is large interannual variability in CH4 uptake, ranging from -48.8 to -95.8 μg CH4 m(-2) h(-1) (averaged of -71.5 ± 2.5 μg CH4 m(-2) h(-1)), due to the variability in precipitation seasonality. The seasonal pattern of CH4 uptakes in the form of stronger uptake in the early growing season and weaker uptake in the rainy season closely matched the precipitation seasonality and subsequent soil moisture variation. The relationships between alpine steppe CH4 uptake and soil moisture/temperature are best depicted by a quadratic function and an exponential function (Q10 = 1.67) respectively. Our measurements also showed that the alpine meadow soil (average of -59.2 ± 3.7 μg CH4 m(-2) h(-1)) uptake less CH4 than the alpine steppe and produces a similar seasonal pattern, which is negatively regulated by soil moisture. Our measurements quantified--at values far higher than those estimated by process-based models--that both the alpine steppe and alpine meadow are considerable CH4 sinks, despite the cold weather of this high-altitude area. The consecutive measurements gathered in this study also highlight that precipitation seasonality tends to drive the interannual variation in CH4 uptake, indicating that future study should be done to better characterize how CH4 cycling might feedback to the more extreme climate.

CH4 exchanges of the natural ecosystems in China during the past three decades: The role of wetland extent and its dynamics

CH4 is the second largest contributor to human-induced global warming. However, large uncertainties still exist regarding the magnitude and temporal variation of CH4 exchanges in Chinas natural ecosystems, especially under climate changes. In this study, we assessed its uncertainty and temporal variation during 1979-2012, by integrating a biogeochemical model, extensive in situ measurements, and various sources of wetland maps. Uncertainty analyses suggested that previous studies might have underestimated CH4 emissions, primarily due to bias in wetland extents in NE China. After that, 1km resolution wetland maps were used to drive the model, together with a 0.1 degrees resolution climate data set. The model showed that Chinas natural wetlands emitted 4.561.24TgCH(4)yr(-1) during the 1980s, which decreased to 3.861.09TgCH(4)yr(-1) in the 2000s, mainly due to wetland drainage in NE China. However, recent glacier-melt-induced wetland expansion has enhanced CH4 emissions by 28% on the Tibetan Plateau since the 1980s. The magnitude of CH4 uptake by the natural ecosystems has remained relatively stable, e.g., -2.570.18 and -2.700.19 Tg CH4 yr(-1) in the 1980s and 2000s, respectively. In summary, the net CH4 balance of Chinas natural ecosystems has shown a decreasing pattern, i.e., 1.99 +/- 1.42 and 1.16 +/- 1.28TgCH(4)yr(-1) in the 1980s and 2000s, respectively, despite distinct regional differences between NE China and the Tibetan Plateau. Furthermore, this study emphasizes the correct representation of wetland extent and its dynamics, i.e., wetland drainage in populated regions and wetland expansion in glacier-fed regions, in driving the decadal CH4 exchange magnitude.

Lakes on the Tibetan Plateau as Conduits of Greenhouse Gases to the Atmosphere

Lakes play an important role in the global carbon cycle, and littoral zones of lakes are potential hotspots of greenhouse gas production. In this study, we measured the partial pressures of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in the littoral zones of 17 lakes on the Tibetan Plateau. The littoral zones of lakes on the Tibetan Plateau were supersaturated and acted as sources of CO2, CH4, and N2O to the atmosphere. The average partial pressures of CO2, CH4, and N2O in the surface lake water were 664.8 ± 182.5, 139.8 ± 335.6, and 0.3 ± 0.1 μatm, respectively. The average diffusive fluxes (and uncentainty intervals) of these three gases were 73.7 (0.9–295.3) mmol · m 2 · day , 5.2 (0.0008–45.9) mmol · m 2 · day , and 6.5 (0.07–20.9) μmol · m 2 · day , respectively. The diffusive fluxes of CO2 in lakes were significantly correlated with dissolved organic carbon, dissolved organic nitrogen, salinity, and water temperature. The diffusive fluxes of N2O were significantly correlated with lake water depth. However, no relationships were found between environmental factors and the CH4 diffusive flux at the scale of this study. CO2 exchange with the atmosphere from saline lakes was found to be higher than from freshwater lakes with equivalent CO2 concentrations by a factor of 2.5 due to chemical enhancement of the gas transfer velocity. Therefore, further study with enhanced spatiotemporal resolution and breadth is needed to better understand the important role played by lakes on the Tibetan Plateau in both regional and global carbon cycles.

Strengthening Hydrological Regulation of China's Wetland Greenness Under a Warmer Climate: Natural Wetland Greenness in China

Natural wetlands are permanently or seasonally inundated with water, and the growth of vegetation in these wetlands is assumed to be sensitive to a warming climate. Chinas natural wetlands are mostly found in cold, high-latitude (>40 degrees N in NE China) and high-altitude (>4,000 m in average on the Tibetan Plateau (TP)) areas. Rapid warming of regional climate (>0.30 degrees C decade(-1) since the 1960s) is thought to have promoted the growth of vegetation in these cold wetlands. However, using three independent greenness data sets, we show that the wetlands in the TP and NE China experienced significant browning between 1999 and 2007, rather than a straightforward increase in greening. The interannual variation in wetland greenness on the TP was regulated by both the temperature and the amount of solar radiation. In NE China, however, the effect of temperature and solar radiation was unable to explain the temporal variation in greenness and the changes were mostly regulated by the soil moisture content and drought. There was a decrease in the dependence of vegetation growth on temperature in NE China, but no significant change on the TP, which has experienced a warmer and wetter climate in recent decades. The effect of drought and the soil moisture content on the interannual variation in the greenness of wetlands consistently increased across the TP and NE China. Our results highlight the hydrological regulation of the growth of vegetation in a warmer climate, even in wetland environments.