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Abstract.Continental-scale estimations of terrestrial methane (CH 4 ) and nitrous oxide (N 2 O) fluxes over a long time period are crucial to accurately assess the global balance of greenhouse gases and enhance our understanding and prediction of global climate change and terrestrial ecosystem feedbacks. Using a process-based global biogeochemical model, the Dynamic Land Ecosystem Model (DLEM), we quantified simultaneously CH 4 and N 2 O fluxes in North America's terrestrial ecosystems from 1979 to 2008. During the past 30 years, approximately 14.69 ± 1.64 T g C a −1 (1 T g = 10 12 g) of CH 4 , and 1.94 ± 0.1 T g N a −1 of N 2 O were released from terrestrial ecosystems in North America. At the country level, both the US and Canada acted as CH 4 sources to the atmosphere, but Mexico mainly oxidized and consumed CH 4 from the atmosphere. Wetlands in North America contributed predominantly to the regional CH 4 source, while all other ecosystems acted as sinks for atmospheric CH 4 , of which forests accounted for 36.8%. Regarding N 2 O emission in North America, the US, Canada, and Mexico contributed 56.19%, 18.23%, and 25.58%, respectively, to the continental source over the past 30 years. Forests and croplands were the two ecosystems that contributed most to continental N 2 O emission. The inter-annual variations of CH 4 and N 2 O fluxes in North America were mainly attributed to year-to-year climatic variability. While only annual precipitation was found to have a significant effect on annual CH 4 flux, both mean annual temperature and annual precipitation were significantly correlated to annual Correspondence to: H. Tian (tianhan@auburn.edu) N 2 O flux. The regional estimates and spatiotemporal patterns of terrestrial ecosystem CH 4 and N 2 O fluxes in North America generated in this study provide useful information for global change research and policy making.

[1] The magnitude, spatial, and temporal patterns of the terrestrial carbon sink and the underlying mechanisms remain uncertain and need to be investigated. China is important in determining the global carbon balance in terms of both carbon emission and carbon uptake. Of particular importance to climate-change policy and carbon management is the ability to evaluate the relative contributions of multiple environmental factors to net carbon source and sink in China's terrestrial ecosystems. Here the effects of multiple environmental factors (climate, atmospheric CO 2 , ozone pollution, nitrogen deposition, nitrogen fertilizer application, and land cover/land use change) on net carbon balance in terrestrial ecosystems of China for the period 1961-2005 were modeled with newly developed, detailed historical information of these changes. For this period, results from two models indicated a mean land sink of 0.21 Pg C per year, with a multimodel range from 0.18 to 0.24 Pg C per year. The models' results are consistent with field observations and national inventory data and provide insights into the biogeochemical mechanisms responsible for the carbon sink in China's land ecosystems. In the simulations, nitrogen deposition and fertilizer applications together accounted for 61 percent of the net carbon storage in China's land ecosystems in recent decades, with atmospheric CO 2 increases and land use also functioning to stimulate carbon storage.

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[1] Land-cover changes in China are being powered by demand for food for its growing population and by the nation's transition from a largely rural society to one in which more than half of its people are expected to live in cities within two decades. Here we use an analysis of remotely sensed data gathered between 1990 and 2000, to map the magnitude and pattern of changes such as the conversion of grasslands and forests to croplands and the loss of croplands to urban expansion. With high-resolution (30 m) imagery from Landsat TM for the entire country, we show that between 1990 and 2000 the cropland area increased by 2.99 million hectares and urban areas increased by 0.82 million hectares. In northern China, large areas of woodlands, grasslands and wetlands were converted to croplands, while in southern China large areas of croplands were converted to urban areas. The land-cover products presented here give the Chinese government and international community, for the first time, an unambiguous understanding of the degree to which the nation's landscape is being altered. Documentation of these changes in a reliable and spatially explicit way forms the foundation for management of China's environment over the coming decades. Citation: Liu, J., H. Tian, M. Liu, D. Zhuang, J. M.Melillo, and Z. Zhang (2005), China's changing landscape during the 1990s: Large-scale land transformations estimated with satellite data, Geophys. Res. Lett., 32, L02405,

The high-mountain lakes on the Yungui Plateau in China are exposed to high-intensity ultraviolet radiation, and contain low concentrations of chromophoric dissolved organic matter (CDOM). We determined CDOM absorption, fluorescence, composition, and source in 38 lakes on the Yungui Plateau at altitudes of 1516 to 4591 m above sea level. Total nitrogen (TN), total phosphorus (TP), and chlorophyll a (Chl a) concentrations significantly increased with increasing trophic state, and decreased with altitude. The CDOM absorption coefficient a CDOM (280) significantly increased with increasing trophic state, but not with altitude. There were significant and negative correlations between altitude and TN, TP, Chl a concentrations, and a CDOM (280). Parallel factor analysis identified two humic-like and two protein-like fluorescent components. Humic-like component 1 was terrestrially derived and positively correlated to CDOM absorption. Component 2 was similar to a marine humic-like substance originating from biological degradation of phytoplankton. Components 3 and 4 were autochthonous tryptophanlike and tyrosine-like fluorophores. CDOM was, thus, a mixture of material from the catchment and autochthonous material produced by biota in the lake. CDOM fluorescence characteristics of oligotrophic and mesotrophic lakes were dominated by the spectral signatures of protein-like components, but marine and terrestrial humic-like components dominated in eutrophic lakes. The fluorescence indices FI 255 , FI 310 , and FI 370 were useful tools for readily defining and classifying CDOM characteristics in the Yungui Plateau lake waters.

[1] China's terrestrial ecosystems have been recognized as an atmospheric CO 2 sink ; however, it is uncertain whether this sink can alleviate global warming given the fluxes of CH 4 and N 2 O. In this study, we used a process-based ecosystem model driven by multiple environmental factors to examine the net warming potential resulting from net exchanges of CO 2 , CH 4 , and N 2 O between China's terrestrial ecosystems and the atmosphere during 1961-2005. In the past 45 years, China's terrestrial ecosystems were found to sequestrate CO 2 at a rate of 179.3 Tg C yr −1 with a 95% confidence range of (62.0 Tg C yr −1 , 264.9 Tg C yr) while emitting CH 4 and N 2 O at rates of 8.3 Tg C yrwith a 95% confidence range of (3.3 Tg C yr −1 , 12.4 Tg C yr −1 ) and 0.6 Tg N yr −1 with a 95% confidence range of (0.2 Tg N yr −1 , 1.1 Tg N yr −1 ), respectively. When translated into global warming potential, it is highly possible that China's terrestrial ecosystems mitigated global climate warming at a rate of 96.9 Tg CO 2 eq yr −1 (1 Tg = 10 12 g), substantially varying from a source of 766.8 Tg CO 2 eq yr −1 in 1997 to a sink of 705.2 Tg CO 2 eq yr −1 in 2002. The southeast and northeast of China slightly contributed to global climate warming; while the northwest, north, and southwest of China imposed cooling effects on the climate system. Paddy land, followed by natural wetland and dry cropland, was the largest contributor to national warming potential; forest, followed by woodland and grassland, played the most significant role in alleviating climate warming. Our simulated results indicate that CH 4 and N 2 O emissions offset approximately 84.8% of terrestrial CO 2 sink in China during . This study suggests that the relieving effects of China's terrestrial ecosystems on climate warming through sequestering CO 2 might be gradually offset by increasing N 2 O emission, in combination with CH 4 emission.