nan Xu-Ri

Modelling terrestrial nitrous oxide emissions and implications for climate feedback

Ecosystem nitrous oxide (N2O) emissions respond to changes in climate and CO2 concentration as well as anthropogenic nitrogen (N) enhancements. Here, we aimed to quantify the responses of natural ecosystem N2O emissions to multiple environmental drivers using a process-based global vegetation model (DyN-LPJ). We checked that modelled annual N2O emissions from nonagricultural ecosystems could reproduce field measurements worldwide, and experimentally observed responses to step changes in environmental factors. We then simulated global N2O emissions throughout the 20th century and analysed the effects of environmental changes. The model reproduced well the global pattern of N2O emissions and the observed responses of N cycle components to changes in environmental factors. Simulated 20th century global decadal-average soil emissions were c. 8.2-9.5 Tg N yr(-1) (or 8.3-10.3 Tg N yr(-1) with N deposition). Warming and N deposition contributed 0.85±0.41 and 0.80±0.14 Tg N yr(-1), respectively, to an overall upward trend. Rising CO2 also contributed, in part, through a positive interaction with warming. The modelled temperature dependence of N2O emission (c. 1 Tg N yr(-1) K(-1)) implies a positive climate feedback which, over the lifetime of N2O (114 yr), could become as important as the climate-carbon cycle feedback caused by soil CO2 release.

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.

Estimating N2O emissions from soils under natural vegetation in China

BackgroundNatural and managed soils have been identified as the largest sources of atmospheric nitrous oxide (N2O). However, the quantification of N2O emissions from soils under natural vegetation in China and their possible responses to changing climate and atmospheric nitrogen deposition remains uncertain. In particular, information regarding N2O emissions from Chinese shrublands is lacking.MethodThis study used 28 sets of N2O field measurements in China to validate a process-based dynamic nitrogen cycle model (DyN-LPJ), which was then used to investigate the N2O fluxes from soils under natural vegetation in China from 1970 to 2009.ResultsN2O emissions from Chinese forests, grasslands, and shrublands in the 2000s were estimated to be 0.10 ± 0.06 Tg N yr.−1, 0.09 ± 0.09, Tg N yr.−1 and 0.14 ± 0.07 Tg N yr.−1, respectively. Monthly N2O fluxes were linearly correlated with precipitation, and exponentially (Q10 = 3) with air temperature. The total N2O fluxes from natural terrestrial ecosystems in China increased from 0.28 ± 0.03 Tg N yr.−1 in the 1970s to 0.46 ± 0.03 Tg N yr.−1 in the 2000s. Warming and atmospheric nitrogen deposition accounted for 37% (or 0.07 ± 0.03 Tg N) and 63% (0.11 ± 0.01 Tg N) of this increase respectively.ConclusionsOur results indicate that when compared to grassland ecosystems, N2O emissions from forest and shrubland ecosystems contain larger uncertainties due to either their uncertain areal extent or their emission rates. Long-term and continuous field measurements should be conducted to obtain more representative data in order to better constrain shrubland N2O emissions.