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Water use efficiency (WUE) reflects the coupling of carbon and water cycles. Analyzing the spatial variability of WUE can improve our understanding on the interaction between carbon and water cycles at a large scale, which also provides a basis for improving the regional carbon budget assessment. Based on China's eddy covariance measurements, we examined the spatial variation of China's WUE and its affecting factors. WUE showed a decreasing trend with the increasing altitude, which was the result of ecosystem type distribution resulting from the climatic gradient. After fully considering the vertical variation of WUE, we found that not only mean annual air temperature (MAT), mean annual precipitation (MAP), and mean leaf area index (MLAI) but also mean annual total photosynthesis active radiation (MAR) affected the spatial variation of WUE. With the increasing MAT, MAP, and MLAI, WUE increased significantly but the increasing MAR decreased WUE. The spatial variation of WUE could be directly depicted by MLAI and altitude, the equation including which explained 65% of the spatial variation of WUE. The effects of MAT and MAP on the spatial variation of WUE may be achieved through altering MLAI, while the mechanism underlying the effect of MAR on the spatial variation of WUE was still unclear, which should be the subject of future investigations. This study reveals the vertical variation of WUE and provides a new approach to generate the spatial variation in WUE, which will benefit the regional carbon budget assessment. [ABSTRACT FROM AUTHOR]

Abstract: Soils of three sites were studied in the Inner Mongolia steppe; one site non-grazed for 26yr (NG26), another site non-grazed for 6yr (NG6) and a third site freely grazed all along (FG). The composition of methantrophic communities was characterized by pmoA gene fragments (coding for a subunit of particulate methane monooxygenase) that were PCR amplified from total soil DNA extracts, using denaturing gradient gel electrophoresis (DGGE) method. Cluster analysis based on the DGGE band patterns indicated that the methanotrophic communities structure of NG6 and FG soils were similar to each other but different from that of NG26 soil. Sequence analysis showed that most bands belonged to the cluster of USCγ. This is the first report that USCγ cluster is dominated in the grassland soil. [Copyright &y& Elsevier]

Abstract: Uptake and release of carbon in grassland ecosystems is very critical to the global carbon balance and carbon storage. In this study, the dynamics of net ecosystem CO2 exchange (F NEE) of two grassland ecosystems were observed continuously using the eddy covariance technique during the growing season of 2003. One is the alpine shrub on the Tibet Plateau, and the other is the semi-arid Leymus chinensis steppe in Inner Mongolia of China. It was found that the F NEE of both ecosystems was significantly depressed under high solar radiation. Comprehensive analysis indicates that the depression of F NEE in the L. chinensis steppe was the results of decreased plant photosynthesis and increased ecosystem respiration (R eco) under high temperature. Soil water stress in addition to the high atmospheric demand under the strong radiation was the primary factor limiting the stomatal conductance. In contrast, the depression of F NEE in the alpine shrub was closely related to the effects of temperature on both photosynthesis and ecosystem respiration, coupled with the reduction of plant photosynthesis due to partial stomatal closure under high temperature at mid-day. The R eco of the alpine shrub was sensitive to soil temperature during high turbulence (u * >0.2ms−1) but its F NEE decreased markedly when the temperature was higher than the optimal value of about 12°C. Such low optimal temperature contrasted the optimal value (about 20°C) for the steppe, and was likely due to the acclimation of most alpine plants to the long-term low temperature on the Tibet Plateau. We inferred that water stress was the primary factor causing depression of the F NEE in the semi-arid steppe ecosystem, while relative high temperature under strong solar radiation was the main reason for the decrease of F NEE in the alpine shrub. This study implies that different grassland ecosystems may respond differently to climate change in the future. [Copyright &y& Elsevier]