Recently photosynthesized carbon (C) constitutes an important portion of C cycling in plant systems. The quantification of the partitioning of recently photosynthesized C to plant tissues, soils and respirations is essential for understanding the global C cycle. The cycle of recently photosynthesized C is difficult to study due to the rapid turnover of recently metabolized C and that stable organic C compounds are not immediately synthesized. Carbon isotope techniques can be used to study the allocation and turnover of recently photosynthesized C. The following research topics should be addressed in future studies. 1) The fraction of recently photosynthesized C used in respiration of living roots, rhizosphere, and soil organic matter, and photosynthate turnover in the rhizosphere needs to be quantified. 2) The amount of recently photosynthesized C that is lost to atmospheric fluxes of CO₂ and CH₄ should be quantified. 3) Because of significant difference in the physiologies C₃ and C₄ plants, global climate change will produce profound effects on the distribution and productivities of C₃ and C₄ species and further influence the global C cycle pattern. We believe that studies on differences in C allocation and turnover in C₃ and C₄ species are very valuable. 4) The effects of human activities, such as livestock farming and land use, on the cycle patterns of recently photosynthesized C. In order to further understand C partitioning patterns, it is vital that more ecosystem level studies of C cycling are conducted. Little information in photosynthesized C fluxes in China's grasslands is available and this region should be a research priority.

Recently developed in recent decades, the carbon isotope tracing technology is one of the most reliable methods, which has been widely used in the study of carbon (C) cycling in terrestrial ecosystems due to its high specificity and sensitivity. Here, the principle, analysis method and application process of C isotope tracing technology in C cycling in terrestrial ecosystem have been reviewed. Four different methods are currently being used in laboratory or field conditions, including natural abundance method, Free-Air Concentration Enrichment (FACE) technology coupling with ¹³C dilution method, pulse and continuous labeling with ¹³C enriched CO₂, and labeling with ¹³C enriched substrates. Results of field experiments and lab incubation experiments employing carbon isotope tracing technology were combined in order to quantify the transformation and distribution of photosynthetic C in plant-soil system. Furthermore, these techniques also help to understand the contribution of plant photosynthetic C to soil organic matter, the stabilization of soil organic matter and its microbial mechanism, to illustrate the dynamic changes of soil organic carbon (SOC), evaluate the contribution of new and old organic C to soil C storage, and estimate the micromechanism of SOC input, conversion and the stabilization in terrestrial ecosystems. Carbon cycle is affected by climate, vegetation, human activities and other factors, and therefore it is imperative to further develop a sensitive, accurate, multiscale and multidirectional isotope tracing system by combining carbon isotopes with mass spectrometry, spectroscopy and molecular biological technology. We have summarized the coupled application of carbon isotope tracing technology and the insitu detection involving molecular and biological approaches, and discussed the existing issues of carbon isotope tracing technology.

Changing the allocation of carbohydrates to various organs is a central mechanism by which plants cope with temporally or spatially varying environments. Hence, a primary objective of eco-physiological research is to understand when and how this process is affected by specific external conditions. Chinese fir (Cunninghamia lanceolata) is one of the most important timber species in Southern China due to its fast growth and good timber quality. Due to large-scale afforestation/reforestation activities, Chinese fir stands, most of which are plantations, have expanded rapidly since the 1950s, and, in particular, since the 1980s, with both the area and standing volume having more than doubled. Therefore, to understand the carbon allocation in Chinese fir in response to varying soil moisture conditions, we studied the photosynthetic response and changes in photosynthate allocation of 2-year-old Chinese fir seedlings under different water treatments. The experiment was conducted in the Subtropical Forestry Experimental Center of the Chinese Academy of Forestry located in Fengyi of Jiangxi Province. Eighty potted 2-year-old Chinese fir seedlings were grown for one growing season under two water treatments: a water stress treatment in which one third the normal water supply was applied and a control (normal water management). The net photosynthetic rate (P n) in response to photosynthetic photo flux density (PPFD) were measured using a LI-6400 portable photosynthesis system from 8∶00 to 11∶00 on clear days in late June. Air temperature and relative humidity in the chamber were maintained at (25±1) ℃ and 70%±5%. Carbon allocation in seedlings under the two water treatments was measured in the morning of early July using a 13 C pulse labeling technique. The initial CO 2 concentration in the labeling chamber was about 1 000 μmol·mol -1 and each labeling lasted 40 minutes. The current-needles, 1-year old needles, branches, stems, fine roots and coarse roots were sampled on day 1, day 3, day 7 and day 21 following labeling. Samples were dried, grinded to a powder, combusted, and then analyzed on an isotope ratio mass spectrometer to measure the stable carbon isotope ratio. Our results showed that water stress did not alter the photosynthetic characteristics of Chinese fir seedlings. The δ 13 C values and net 13 C ratio (N 13 CR), i.e., the ratio of the net increment of 13 C to the natural total carbon, of the seedlings decreased in the water-stress treatment. The effect of water deficit on the δ 13 C values and N 13 CR in shoots was more significant than in roots. The shoot biomass under water stress was reduced remarkably, while little changes were found for root biomass. Water stress had a more significant effect on current-needles than other organs. The δ 13 values, N 13 CR and dry weight of current-needles under water stress were lower than in the control. The more rapid decline of N 13 CR in current-needles of water-stressed seedlings 21 days following labeling indicated that there was an increase in the export of photosynthetic products. The growth decline of the current needles under water stress caused a decrease in leaf area resulting in a reduction in total photosynthesis. Under water stress, more photosynthetic products were transferred to belowground biomass, especially to the fine roots. As a result, carbon allocation patterns were altered and higher root:shoot ratios were found in seedlings experiencing water stress!in comparison to seedlings under conditions of normal water management.

We determined the δ¹³C values of leaf samples collected from plants belonging to 6 genera in the Rhizophoraceae family grown in four locations in China, including three coastal areas (Dongzhaigang of Hainan Province, Xiamen of Fujian Province, Beihai of Guangxi Zhuang Autonomous Region), and one inland area (Xishuangbanna of Yunan Province) in May and September, 2002. The nine plant species included Carallia brachiata, C. diphopetala, Pellacalyx yunnanensis, Rhizophora apiculata, R. stylosa, Bruguiera sexangula, B. gymnorhiza, Kandelia candel, and Ceriops tagal. The results indicated that leaf δ¹³C values, corresponding to integrated wateruse efficiency (WUE) (ratio of CO₂ assimilation rate to leaf transpiration), ranged from -32‰ to -26‰, but did not vary significantly between May and September in most of those plant species. However, there were significant differences in leafδ¹³C values among growing environments with lower foliar δ¹³C values for the plants grown in the inland than those in the coastal areas on two sampling dates, indicating a higher intercellular CO₂ concentration and thus a lower water use efficiency for the plants grown in the inland. In addition, among those plants living in the coastal areas, the leaves of plants grown in the Beihai had the highest δ¹³C values indicating the lowest intercellular CO₂ concentration and thus highest WUE, which may be caused by the high salt content in the seawater or high water stress in Beihai. Our results suggest that the foliar δ¹³C values of the plants in the Rhizophoraceae family could be highly affected by intermittent environmental factors, although their genetic characteristics may play a significant role in determining their foliar δ¹³C values.

With the rise of water level, the construction of the Three-Gorges Dam may have significant impacts on plant carbon-water relation and ecosystem properties in this region. To understand how the plants in this region adapt to the changes in water environments, we measured gas exchange, water potential and δ ¹³C of sapling and mature trees of three co-occurring coniferous and deciduous species: Pinus massoniana, Quercus variabilis and Q. aliena, all are dominant tree species in this region. The two deciduous broad-leaf trees (Q. variabilis and Q. aliena) exhibited higher photosynthetic (P_n) and stomatal conductance (G_s) than the evergreen conifer species (P. massoniana). The predawn water potential (ψ_pd) of P. massoniana was lower than that of the two broad-leaf species. Intrinsic water use efficiency (WUE_i, P_n/G_s) of P. massoniana was higher than those of Q. variabilis and Q. aliena. However, the differences in carbon isotope ratio (δ¹³C) of leaves among species, which gives integrative information of WUE in growing season period, were not statistically significant. We also compared eco-physiological parameters between saplings and mature trees of these three species. The P_n and G_s values of the mature trees were significantly lower than those of the saplings. The mature trees showed lower ψ_pd value, but the difference between the mature trees and the saplings was not statistically significant. However, the WUE_i values of all mature trees were significantly higher than those of the saplings. The δ¹³C values of mature trees showed more positive than those of corresponding saplings, indicating also higher WUE in the mature trees. From these results, we concluded that 1) P. massoniana showed different water use strategies from two Quercus trees species, and 2) the mature trees of these three dominant tree species showed lower photosynthetic rate but higher WUE than those of their corresponding saplings.

Water uptake is generally accepted as the most important function of plant roots, while the fact that roots can also release water to soil and its ecological consequences and significance to ecosystem function are not well understood. Studies in the past 20 years confirmed the existence of hydraulic redistribution (HR), which describes the passive movement of water from moist soil to dry soil via roots when transpiration ceases (usually at night). The direction of HR can be upward, downward or lateral in soil along water potential gradients. Because this newly recognized small cycle of soil-root-soil exists within the Soil-Plant-Atmosphere Continuum (SPAC), the mechanism of dynamic movement and storage in the soil-root system has interested many functional ecologists. Continuous measurement of soil water potentials or contents and root sap flow and the use of stable isotopes techniques have been extensively used in HR research. HR acts as a good feedback mechanism between plant water status and soil water dynamics. When deep soil water is plentiful, HR could enhance the efficiency of water uptake via roots, helping plants to make full use of water to improve transpiration and photosynthesis rates and hence maintain physiological function and hydraulic conductivity of the SPAC system. When rainfall comes after the dry season, more precipitation could be transferred to deep soil as an effective storage because of HR, thus increasing availability of rainfall to plant growth. HR was significant for water flux in some ecosystems, for example arid-semiarid sandlands and grasslands, as well as seasonal dry forests. Experiments on HR in Chinese ecosystems are expected or are already underway. On the other hand, integrating HR into ecosystem and hydrology models to study the water relationships between neighboring plants and predict the dynamics of ecosystems with HR is also important. The implications of HR to designing agro-forestry, vegetation recovery, calculating ecological water demand and water conservation in agriculture also deserve more attention.

Stable isotopes are used as both natural integrators and tracers of complicated biological, ecological and biogeochemical processes, and their responses to environmental changes at different spatial and temporal scales. In this article, the application of stable isotopes and the Keeling plot approach to carbon and water exchange studies of terrestrial ecosystems were reviewed. We focused mainly on the current applications and potential development of stable isotope techniques and the Keeling plot approach in conjunction with concentration and flux measurements of CO₂ and water in terrestrial ecosystems. For these applications it is critical to know the isotopic identities of specific ecosystem components, such as the isotopic compositions of CO₂, organic matter, liquid water, and water vapor, as well as the associated isotopic fractionations, in the soil-plant-atmosphere continuum. Based on the principle of mass conservation, the Keeling plot approach combines measurements of stable isotope ratios and concentrations of CO₂, water or other trace gases, and allows the identification of the contributions of various ecosystems, or ecosystem components, to the net exchange fluxes between the terrestrial biosphere and atmosphere, and the estimation of net ecosystem isotopic discrimination and disequilibrium effect. Net ecosystem carbon fluxes can be partitioned into C uptake during photosynthesis and C release during respiration or evapotranspiration into leaf transpiration and soil evaporation by the Keeling plot technique. This approach also allows partitioning urban CO₂ sources into gasoline combustion, natural gas combustion and biogenic respiration. Recent modifications of the Keeling plot approach permit examination of CO₂ recycling in forest ecosystems. At the global scale, we can estimate relative contributions of terrestrial and ocean ecosystems to the global carbon cycle by combining stable isotope techniques, the Keeling plot approach and terrestrial ecosystem models. However, applications of stable isotope techniques and the Keeling plot approach to ecological research are sometimes constrained by the heterogeneity of terrestrial ecosystems. In addition, selection of suitable isotopic sampling protocols is another factor that we should consider in its application. Nevertheless, with new improvements in analytic protocols in the near future, stable isotope techniques and the Keeling plot approach will become one of the most effective techniques for understanding carbon and water relationships in terrestrial ecosystems.

Aims In arid and semiarid regions, precipitation is the most important water source for plants. Our objective was to investigate the water-use strategies of two dominant desert plants along a precipitation gradient in northwestern China.

Methods We determined stable hydrogen and oxygen isotope compositions of the stem water from Artemisia ordosicaand Nitraria tangutorum and potential water sources (rain water, groundwater and soil water) at three study sites with different annual precipitation (Hanggin Banner and Dengkou County of Inner Mongolia Autonomous Region and Minqin County of Gansu Province). The IsoSource model was then used to calculate probable contributions of potential water sources to total plant water uptake. We also determined foliar carbon isotope ratios and free proline contents of both species to indicate water use efficiency and osmotic-adjustment ability.

Important findings At the Hanggin Banner site (highest annual precipitation), both species obtained the highest proportion of water from shallow soil water and A. ordosicatook up water mostly from the 0-50 cm soil layer. However, they depended mainly on deep soil water or groundwater at the Dengkou and Minqin sites with lower annual precipitation. The water use efficiency of both species decreased with increasing annual precipitation. There was a positive correlation between carbon isotope ratio and free proline content in A. ordosica. These results suggest that desert plants can adjust their capabilities for up-take from different water sources and other physiological properties with variation in natural precipitation, but the strategies are species-specific.

The stable oxygen isotope, δ¹⁸O, is a very useful tracer tool to study the impacts of forests on the water cycle. The Wolong Natural Reserve is a key conservation area focusing on the protection of Panda, rare animals and alpine ecosystems. The Wolong Natural Reserve is located in the Mingjiang River headwaters and plays an important role in the conservation of soil and water in this watershed as well as the upper reaches of the Yangtze River. Although much research has been conducted on the water cycle, including changes in precipitation intensity, rainfall frequency, runoff dynamics and subsurface flow, little has been devoted to understanding rainfall water allocation in sub-alpine dark coniferous forests. Using a stable oxygen isotope technique, we studied changes in precipitation, throughfall and soilwater flow over time under different rainfall intensities in three plots in a sub-alpine dark conifer forest. The results showed that 1) there were no significant differences between rainfall δ¹⁸O and daily precipitation (r=0.612, p≥0.05,n=20). There was a significant linear relationship between the rainfall δ¹⁸O and throughfall δ¹⁸O defined by the following equations for the three plots: plot A: y₁=0.970 6x₁-2.254 2,R₁²=0.796 2, p<0.05,F=42.97; plot B: y₂=0.997 6x₂-1.632 1,R₁²=0.729 7, p<0.05,F=29.60; plot C: y₃=0.920 3x₃-2.728 9,R₃²=0.729 7, p<0.05,F=26.47. 2) The δ¹⁸O exceeded 0 when precipitation ≥ 3.20 mm, and the maximum δ¹⁸O value occurred when precipitation was 12.65 mm due to the interactions between canopy evaporation and precipitation. 3) There was limited soil water flow when rainfall intensity, daily precipitation or rainfall continuity was low resulting in a slow response of the precipitation δ¹⁸O signature on soil water. When soil water flow was strong and continuous, the precipitations δ¹⁸O signature showed up rapidly in the soil water flow. The δ¹⁸O of soil water showed the same rise and fall patterns of the rainfall δ¹⁸O when precipitation was less than 10 mm, but with 4-day time lag. When precipitation was between 10 mm to 20 mm, the time lag in the increase in the soil water δ¹⁸O occurred 2-3 days after raining, and, when precipitation was from 20 mm to 30 mm, the time lag was only 1-2 days after the rain fall event. The well-developed sub-alpine dark coniferous forest could effectively regulate rainfall by delaying the time, and rainfall became part of the subsurface water flow that helping to prevent downstream flooding. There were no significant differences in the δ¹⁸O between rainfall and throughfall (p=0.491>0.05), but there were a signficant differences between throughfall and soil waterδ¹⁸O (p=0.025<0.05). These results suggest that soil water flow was not directly supplied by the current rainfall event but was supplemented by a pre-rainfall event.

Aims Stable N isotope signature (δ¹⁵N) in both plant and soil is potentially an important parameter to evaluate N cycling in grassland ecosystems. Grazing is a dominant land use in northern China grasslands and might have greatly changed N cycling. Our objective was to determine the¹⁵N signature of leaves and soils in grazed and fenced plots of a typical steppe to investigate how δ¹⁵N values in an ecosystem are affected by grazing.

Methods We collected foliar and soil samples in a nearly 20 years fenced plot and an adjacent freely grazed plot. The foliar samples came from eight dominant species, including three perennial legumes (Caragana microphylla, Astragalus galactites and Melilotoides ruthenica), two grasses (Leymus chinensis and Stipa grandis), two forbs (Heteropappus altaicus and Potentilla acaulis) and a semi-shrub (Kochia prostrata). We also sampled 0-10 and 10-20 cm rhizosphere soils of C. microphylla and K. prostrata. Nitrogen isotope ratios were determined with a Thermal Finnigan MAT Delta^plus XP isotope-ratio mass spectrometer (IRMS).

Important findings Although the δ¹⁵N value of leaves of different plant species varied greatly, the observed pattern of variability is consistent in both grazed and fenced plots, i.e., semi-shrub > forbs and grasses > legumes. Long-term grazing reduced foliar δ¹⁵N values of forbs, semi-shrub and S. grandis significantly while changing little or even increasing that of potential biological N fixers (legumes and L. chinensis). As the only arbuscular mycorrhizae non-infected plant species, K. prostrata is enriched in¹⁵N (with δ¹⁵N values of 4.34‰ ± 0.35‰ and 2.04‰ ± 0.20‰ in fenced and grazed plots, respectively) compared with the depletion of other species. Mycorrhizal association may be considered to play an important role in plant nutrient transfer in those N-limited grassland ecosystems. In contrast to results of previous studies, soil δ¹⁵N values decreased significantly with grazing.

Spiraea pubescens, a common shrub in the warm-temperate deciduous forest zone, is distributed in the Dongling Mountain area of Beijing, was exposed to ambient and enhanced ultraviolet-B (UV-B, 280-320 nm) radiation by artificially supplying a daily dose of 9.4 kJ·m^-2 for three growing seasons, a level that simulated a 17% depletion in stratospheric ozone. The objective of this study was to explore the effects of long-term UV-B enhancement on stomatal conductance, leaf tissue δ¹³C, leaf water content, and leaf area. Particular attention was paid to the effects of UV-B radiation on water use efficiency (WUE) and leaf total nitrogen content. Enhanced UV-B radiation significantly reduced leaf area (50.1%) but increased leaf total nitrogen content (102%). These changes were associated with a decrease in stomatal conductance (16.1%) and intercellular CO₂ concentration/air CO₂ concentration (C_i/C_a) (4.0%), and an increase in leaf tissue δ¹³C (20.5‰), leaf water content (3.1%), specific leaf weight (SLW) (5.2%), and WUE (4.1%). The effects of UV-B on the plant were greatly affected by the water content of the deep soil (30-40 cm). During the dry season, differences in the stomatal conductance, δ¹³C, and WUE between the control and UV-B treated shrubs were very small, whereas differences became much greater when soil water stress disappeared. Furthermore, the effects of UV-B became much less significant as the treatment period progressed over the three growing seasons. Correlation analysis showed that enhanced UV-B radiation decreased the strength of the correlation between soil water content and leaf water content, δ¹³C, C_i/C_a, stomatal conductance, with the exception of WUE that had a significant correlation coefficient with soil water content. These results suggest that WUE would become more sensitive to soil water variation due to UV-B radiation. Based on this experiment, it was found that enhanced UV-B radiation had much more significant effects on morphological traits and growth of S. pubescens than hydro-physiological characteristics.

Aims Reaumuria soongoricais a major dominant of desert shrub vegetation in arid regions of China. Our objectives were to investigate correlations between foliar characteristics of this desert plant and meteorological factors and to identify the major factor influencing variations in different environments.

Methods We collected 407 individuals of R. soongorica from 21 natural populations in its major area of distribution in northwestern China and measured the nitrogen, phosphorus, potassium and leaf water content and stable carbon isotope composition in leaves. Meteorological data, including mean annual precipitation and temperature, evaporation, mean relative humidity and duration of sunshine, were collected from the Cold and Arid Environmental and Engineering Research Institute, Chinese Academy of Sciences. The relationships between foliar characteristics and meteorological factors were analyzed by simple linear regression and Pearson's correlation.

Important findings Variation of leaf nitrogen content in R. soongorica was due to the complex effect of meteorological factors. Mean relative humidity was the limiting factor that affected leaf potassium content and stable carbon isotope composition. Evaporation and duration of sunshine were the key factors influencing leaf phosphorus content and water content, respectively. The impacts of climate conditions on foliar characteristics of R. soongorica were notable, and the contributions of different meteorological factors to foliar characteristics differed significantly. This pattern of variation in foliar characteristics responded to different meteorological conditions, reflecting the environmental status and the stable extent of desert ecosystems dominated by R. soongorica.

Aims Light, temperature and humidity conditions differ among individual leaves and may influence leaf morphology, anatomy, stomatal conductance, chlorophyll content and distribution and leaf photosynthetic processes. Our objective was to determine the influence of local microenvironment on structure and biophysical characteristics of leaves through research on changes of leaf morphology and biophysical indices along the lateral distribution of leaves within tree crowns and with orientation differences.

Methods We selected five tree species with wide crowns (Sophora japonica, Platanus orientalis, Ginkgo biloba, Ficus microcarpa and F. lacor) and measured leaf photosynthetic pigments (including chlorophyll a, chlorophyll b and carotenoids), leaf mass per area (LMA) and carbon isotope ratio (δ¹³C) along the horizontal distance from the sample point to the tree trunk and with different orientations in each tree species. Pigment content was measured by colorimetry, LMA as the ratio of leaf dry mass to fresh area and δ¹³C with a mass spectrograph.

Important findings Leaf δ¹³C and LMA increased and photosynthetic pigment decreased with horizontal distance from the sample point to the trunk. Leaf δ¹³C and LMA also differed by orientation, with southward leaves having the highest values, followed by westward leaves, and eastward leaves having the lowest values. Photosynthetic pigments had a more complex relation with orientation, although chlorophyll a, chlorophyll b, total chlorophyll content, chlorophyll a to b ratio and carotenoids were the highest in eastward leaves. These results suggest that southward, westward and leaves on the outside of the crown have increased LMA, reduced stomatal aperture and photosynthetic pigment content and, as a result, decreased photosynthesis and increased δ¹³C values in response to stronger irradiance, higher temperature and lower humidity. Therefore, local microenvironmental differences within an individual tree greatly influence leaf morphological and biophysical characteristics.

There is considerable interest in the use of atmospheric C¹⁸O¹⁶O and ¹⁸O¹⁶O as a tracer for resolving the role of the terrestrial biosphere in the global carbon cycle. Leaf transpiration will result in the enrichment of the heavy H₂¹⁸O isotopes. The δ¹⁸O of leaf water at the evaporating site in the stomatal cavity directly influences the C¹⁸O¹⁶O and ¹⁸O¹⁶O exchanges, instead of that of the bulk leaf water. How to best quantify this enrichment effect remains an active area of research. In the past, a closed form of the Craig-Gordon model was obtained by invoking the steady-state assumption (δ¹⁸O of the transpired water is identical to δ¹⁸O of the xylem water). For the purpose of verification, the predictions of Craig-Gordon model are compared with δ¹⁸O of the bulk leaf water after appropriate corrections for the Péclet effect. On small time scales of minutes to hours,δ¹⁸O of the transpired water is variable in field conditions, implying that the steady state assumption is invalid. Recently, in-situ δ¹⁸O and δD measurement technology has been developed that has potential for improving our understanding of isotopic exchanges between the Earth’s surface and the atmosphere. The precision of hourlyδ¹⁸O and δD is comparable to the precision of mass spectrometry. It has the potential to improve prediction of δ¹⁸O of leaf water at the evaporating site within the stomatal cavity for the temporal dynamics of atmospheric water vapor δ¹⁸O and the δ¹⁸O of the transpired water, especially if its measurement is made in a non-destructive manner and on a continuous basis. Because the isotopic flux of δ¹⁸O and δD is influenced by a similar set of biological and meteorological variables, simultaneous observations of δ¹⁸O and δD will provide additional constraints on the hydrological and ecological processes of the ecosystem. We review the theory and measurement techniques for the enrichment of H₂¹⁸O in leaves and focus on the recently developed in-situ measurement technology and its potential for improving our understanding of H₂¹⁸O enrichment in leaf water and C¹⁸O¹⁶O and ¹⁸O¹⁶O exchanges between the ecosystem and atmosphere.

Aims We investigated the new and senescent tissues of moss and the soil rhizosphere to determine 1) the elemental and isotopic composition characteristics during senescence and 2) the final contribution to soil.

Methods C and N contents and isotopic composition (δ¹³C and δ¹⁵N) of Haplocladium microphyllum and its soil rhizosphere were analyzed at Guiyang, China.

Important findings The C and N contents were higher in new growing tissues than in senescent tissues, and there were significant correlations between the two types of tissues, indicating physiological malfunction (e.g., loss of photosynthesis) and internal element redistribution during moss senescence. No significant difference was found between new and senescent tissues in isotopic composition, but their δ¹³C values or δ¹⁵N values were correlated, suggesting special isotopic regulation in the simple and nonvascular structure of mosses. In contrast, there was no C or N correlation between the moss tissues and soil. This was attributed to long-term nutrient retention and slow deposition to form soil, indicating the moss layer in this area has a small contribution to soil carbon and nitrogen.

Aims Fast, efficient breeding of fast-growth and high water use efficiency (WUE) in Populus tomentosa clones is a pressing need for afforestation in arid and semi-arid regions. There is much research on the correlation between stable carbon isotope and WUE in crops and a few trees, but little in different clones of P. tomentosa. Our objective was to explore the application of δ¹³C on selection of clones with high photosynthesis and WUE.

Methods We examined 13 clones of Populus-grafted seedlings in Beijing Forestry University (39°46′ N, 116°19′ E) Nursery in Beijing, China. We studied leaf δ¹³C of the seedlings by MAT-251-MS in two different growth periods (mid-July and September) and gas exchange parameters (net photosynthetic rate (P_n), transpiration rate (T_r), instantaneous water use efficiency (WUE_i), stomatal conductance (G_s) and intercellular CO₂ concentration (C_i)) by LI-6400 portable photosynthesis system between 9:30 and 11:30 a.m. in the same periods.

Important findings Leaf δ¹³C, T_r, WUE_i, G_s and C_i significantly differed between the periods and among clones. Leaf δ¹³C and WUE_i were higher in July than September; however, T_r, G_s and C_i were lower in July than September and P_n showed no significant difference between periods. The main reason for the difference of leaf δ¹³C was seasonal variation. Leaf δ¹³C and WUE_i of the clones were consistent during the same period, i.e, the higher WUE_i appeared in clones of 30, 42, 46, 83, BL2 and BL5 with higher δ¹³C and the lower WUE_i occurred in clones of B331 and TG34 with lower δ¹³C. There was strong positive correlation between δ¹³C and WUE_i (r = 0.739 0 and 0.545 8 in July and September, respectively); therefore, high δ¹³C can be used as an effective indicator of high WUE_i in Populus. Clones with higher WUE_i may have moderate or low G_s and C_i, but not necessarily high P_n.

Aims There have been many studies of carbon isotope composition (δ¹³C) of C₃ plants in China, and δ¹³C has been widely used as an index of water use efficiency (WUE); however, most studies focused on single sites or small regions. Therefore, our objective was to study the spatial pattern of δ¹³C, the relationships between δ ¹³C and climate factors and whether δ ¹³C can represent WUE in large regions.
Methods We obtained leaf δ ¹³C for 478 C₃ species from 187 sites in China based on the literature.
Important findings The range of δ¹³C was from -33.50‰ to -22.00‰, and the mean was -(27.10‰ ± 1.70)‰. There were significant differences among δ¹³C of grasses, shrubs and trees, with grasses having the highest value and trees the lowest. The result was different from studies in single sites and small regions. For different phylogenic plants, δ ¹³C of seed plants was significantly higher than ferns, the difference between gymnosperms and angiosperms was not statistically significant and monocotyledons had significantly higher values than dicotyledons. Leaf δ ¹³C had irregular variation with increasing longitude, but significantly increased with increasing latitude. Leaf δ ¹³C significantly increased with mean annual temperature and decreasing mean annual precipitation. The relationship between δ ¹³C and precipitation was similar to that of WUE and precipitation, so we conclude that δ ¹³C of C₃ plants can be used as an index of WUE in large regions as well as in single sites or small regions.

Atmospheric CO 2 concentrations are expected to double around the middle part of the 21 st century. Plant growth might be favored by CO 2 enrichment, but water limitation is a common stress for plant growth and productivity. At present, only a few studies have looked at the combined effects of CO 2 enrichment and drought on plant ecophysiology. This experiment was conducted to investigate the responses of two dominant desert shrubs, Caragana intermedia and Hedysarum mongolicum, in western China to the interaction of doubled CO 2 levels and soil drought in large environmental growth chambers (19 m 2). In this paper, we employed different methods, including allometry and carbon isotope discrimination, to examine the effects of water availability on carbon allocation and stable carbon isotope composition (δ 13 C) of the two desert shrubs under two CO 2 concentrations. The objectives included the following: 1) to investigate the effects of soil drought and CO 2 enrichment on plant biomass and δ 13 C; 2) to investigate the effects of soil drought and CO 2 enrichment on the allocation of dry matter and carbohydrates; and 3) to elucidate the adaptive strategies of C. intermedia and H. mongolicum to soil drought under doubled atmospheric CO 2 concentrations. Compared to ambient CO 2 concentrations, doubled CO 2 concentrations did not improve the leaf water status, but soil drought significantly reduced the leaf relative water content (RWC). Doubled CO 2 concentrations enhanced plant growth under well-watered conditions but increased root growth under drought conditions resulting in an increase in root to shoot ratios. Soil drought significantly reduced plant biomass and increased root to shoot ratios, especially for H. mongolicum. The δ 13 C values were reduced at doubled CO 2 concentrations but increased under drought conditions. By plotting the leaf δ 13 C values against the root δ 13 C values, it was possible to assess carbon allocation and incorporation into roots in relation to present biomass. There was a significant and linear relationship between leaf δ 13 C and root δ 13 C values, and the slope of H. mongolicum was greater than that of C. intermedia indicated a higher plasticity in the ability to change carbon allocation patterns. This resulted in higher root to shoot ratios in H. mongolicum under drought conditions. The results indicated that both C. intermedia and H. mongolicum had a higher tolerance to severe water deficits under doubled CO 2 conditions. Decreases in precipitation might accompany with future increases in atmospheric CO 2 concentrations in the region dominated by these two species, suggesting that distribution ranges of C. intermedia and H. mongolicum might be constrained. Our results suggest that H. mongolicum has a higher tolerance to environmental stress than C. intermedia. Future work should emphasize how to enhance the drought tolerance of plants in semiarid region under conditions of CO 2 enrichment.

Increases in the world population combined with accelerated rates of industrialization has led to increases in green-house gases in the atmosphere, particularly CO₂ concentrations, resulting in an increase in global temperatures. Water use efficiency (WUE) is a measure of a plant’s performance growing under different environmental conditions, and is an important index that reflects a plant’s ability to adapt to climate change. Carbon isotopes provide a long-term, integrative measure of WUE with high δ¹³C indicating high WUE. Stable carbon isotopes have become an important technique in ecophysiology studies in recent years for analyzing the long-term WUE of plants growing in different habits and to study the response of plant to different environmental variables, such as light intensity, precipitation, temperature, CO₂, and mineral nutrient availability. The Loess Plateau is a critical ecosystem in China but suffers from serious soil erosion problems. Water is the most important limiting environmental factor in this region and research on plant WUE responses to climate change is of particular relevance. However, only a few studies have examined foliar δ ¹³C value in response to climate change in this ecosystem. Four C₃ plants that are typical of the Loess Plateau were chosen as materials: Quercus liaotungensis, a tree, and three species of shrubs, Ostryopsis davidiana, Zizyphus jujuba var. spinosa and Sophora viciifolia. Dry leaves were collected from herbarium specimens collected over a 70 year period from the 1930’s to 2002. A total of 25 samples from about 160 plant specimens were analyzed that were collected from the hilly and gully region of the Plateau ranging from the northern Tongchuan to the southern Yanan. The carbon isotope composition (δ¹³C) of leaves was analyzed using a MAT-251 mass-spectra photometer. The results showed that the variation in the δ ¹³C values differed among the species tested: :Ostryopsis davidiana (-25.05‰ to -29.75‰) > Quercus liaotungensis (-25.51‰ to -29.20‰) > Sophora viciifolia (-25.12‰ to -28.80‰) > Zizyphus jujuba var. spinosa (-26.69‰ to -28.69‰). The mean δ13C value of four plants was -27.04‰ ranging from -25.05‰ to -29.75‰. Comparing the mean δ¹³C value among four species in the age of 30, 50, 70, 80 and the year 2002 indicated that the following order of Sophora viciifolia (-26.54‰) > Ostryopsis davidiana (-26.99‰) > Quercus liaotungensis (-27.14‰) > Zizyphus jujuba var. spinosa (-27.49‰) is obtained. A decrease in the foliar δ¹³C value with time was found in all four species, indicating that the WUE of the four species all declined over time. However, the decrease in foliar δ ¹³C values varied among the four species with significant decreases measured in two species, Sophora viciifolia and Quercus liaotungensis, a strong but not significant decrease in the leaves of Ostryopsis davidiana, and only a slight decrease in Zizyphus jujuba var. spinosa. The decrease in the δ ¹³C values in the four species were: 14.65‰, 14.46‰, 11.99‰ and 2.44‰, respectively. It was shown that different species have different sensitivities to climate change, and Zizyphus jujuba var. spinosa was the most drought-tolerant of the four species with a high WUE.

With the increase of anthropogenic SO₂ emission, behavior of sulfur isotopes during biogeochemical processes has been the focus of numerous studies. We reviewed the potential contributions of studies using sulfur stable isotope, which include studies on 1) sources of sulfur in atmospheric processes, 2) sulfur dynamics in forest, farmland and aquatic ecosystems, 3) additions of sulfur stable isotope in ecological systems and 4) acid rain research in China. Furthermore, we made suggestions on future research, proper analysis methods and source partitioning models using sulfur stable isotopes.

Aims Stable isotopes technique and Keeling Plot relationship offer great promise for partitioning evapotranspiration (ET), which can help us better understand the hydrologic cycle within terrestrial ecosystems. Our objectives are to evaluate the Keeling Plot method in ET partitioning using in situ continuous δ¹⁸O data and find the fractional contribution of crop transpiration to total ET in a winter wheat (Triticum aestivum) field.

Methods Field experiments were conducted at Luancheng Agro-ecology Station, Chinese Academy of Sciences. A hydrogen and oxygen isotopes in situ measurement system based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) was used to obtain the continuous atmospheric vapor δ¹⁸O data. Other measurements were made with the eddy covariance technique, cryogenic vacuum distillation and stable isotope ratio mass spectrometry.

Important findings An analysis on the Keeling Plot relationships based on data from different time intervals in one daytime showed that the Keeling Plot would be better when using the midday time interval data to build Keeling Plot, which inferred that the plant transpiration isotopic steady-state (ISS) can be more easily obtained during midday when plant transpiration flux is generally largest. ISS was not always satisfied in field conditions, especially when mature wheat suffered from water stress. Using isotopic partitioning, we estimated transpiration contributed roughly 94%-99% to the total ET during the field measurement period, which indicated plant transpiration dominated local ET.

Aims Sabina przewalskii and Picea crassifolia, widely distributed in the Qilian Mountains, were employed to analyze the response of leaf δ¹³C of subalpine trees to altitude and its mechanism.

Methods Leaf samples were taken at 2 600-3 600 m elevation in October 2007. The δ¹³C was determined using an isotope mass spectrometer, carbon content was estimated with the potassium dichromate method and nitrogen content was measured with the micro-Kjeldahl method. Data were evaluated with one-way analysis of variance and correlation analysis using SPSS version 11.5.

Important findings Foliar δ¹³C in both species was positively related to elevation (p < 0.000 1) and annual mean precipitation, but negatively related to annual temperature (p < 0.000 1). The δ¹³C value of S. przewalskii increased as leaf and soil water content increased, while that of P. crassifolia was not significantly related to soil water content, leaf water content, leaf N content or C/N ratio. Altitude, which can lead to changes of hydrothermal conditions, is the main factor influencing carbon isotope fractionation in these subalpine trees; however, the combined effects of various factors on the mechanism of action maybe more complicated and need further study.

An important grassland ecosystem management strategy is the application of nitrogen fertilizer; however, the fate of applied nitrogen is highly correlated with risks and benefits associated with the fertilization of grasslands. The fate of nitrogen tracers applied to a typical Inner Mongolia steppe ecosystem has not been studied previously. We examined the fate of 15N-labelled fertilizer in a Leymus chinensis site at the Inner Mongolia Grassland Ecosystem Research Station, Chinese Academy of Sciences. The results showed that after one growing season an average of 31.61% (range from 25.33% to 38.65%) of the applied 15N was recovered in the plant pool. 15N recovered by aboveground and belowground organs significantly increased with increasing nitrogen rates suggesting that nitrogen fertilizer significantly affected the recovery of 15N by plants. About 2.92% (range from 2.58% to 3.16%) of the applied 15N was recovered in the litter, with the percent 15N in the belowground litter significantly higher than in the aboveground litter pool. About 36.16% of the labelled nitrogen was retained in the soil pool, mostly in the 0-40 cm soil layer. 15N retained by the soil increased significantly with increases in the nitrogen application rate. The loss of 15N was about 21.77%-43.38% of the labelled nitrogen. Risk/benefit analysis showed that, under the climatic and soil conditions prevailing during this experiment, fertilizer application rates of 5.25 g N•m-2 and 28 g N•m-2 were associated with high risk/benefit ratios, whereas the 17.5 g N•m-2 fertilizer treatment achieved the lowest risk/benefit ratio among the nitrogen rate treatments. These results provide a reference for future grassland ecosystem management strategies.

Aims Plants with high water use efficiency could contribute to an overall lower water-using landscape. Our objective was to analyze differences in water use efficiency among landscaping plants in Beijing, China.

Methods We used stable carbon isotope techniques to investigate δ¹³C values of 75 species belonging to 35 families and 65 genera in spring, summer and autumn in 2006 and analyzed variations in water use efficiency of different plant species and different life forms (evergreen tree, deciduous tree, evergreen shrub, deciduous shrub, forb and liana).

Important findings Leaf δ¹³C values varied between -30.7‰ and -23.4‰ in spring, -31.5‰ and -25.1‰ in summer and -31.4‰ and -23.9‰ in autumn. Differences of δ¹³C values were not significant in deciduous shrub species (p = 0.114), but were significant in evergreen trees (p = 0.005), deciduous trees (p < 0.001), evergreen shrubs (p = 0.022), forbs (p < 0.001) and lianas (p = 0.001). Leaf δ¹³C values were significantly different among seasons, being greater in spring than in summer and autumn (except for evergreen trees). The δ¹³C values for life forms were significantly different in different seasons, with a sequence of trees>shrubs>lianas>forbs, and evergreen plants>deciduous plants. Results showed that variation of leaf δ¹³C is dependent on species and life form, especially life form, and that water use efficiency differed greatly among life forms.

Photosynthesis is responsible for one of the most significant carbon isotopic fractionations in nature and thus forms the basis for applying stable carbon isotopes to ecological research. Although stable isotope techniques were used originally for disting

Aims Our objectives were to analyze differences of grain yield, water use efficiency (WUE) and stable carbon isotope ratio (δ¹³C) in various genotypes of dryland winter wheat (Triticum aestivum) and the correlation between δ¹³C value and grain yield and WUE under two different ecological conditions. Findings will help clarify the reliability of using δ ¹³C to evaluate WUE and provide a basis for breeding water-saving types.
Methods We studied 15 winter wheat genotypes (12 from north China and 3 from Texas, USA) on the Loess Plateau of East Gansu under dryland and supplemental irrigation treatments at the jointing stage.
Important findings Different genotypes had considerable differences in grain yield, WUE and δ ¹³C value whether under dryland or irrigation, and the δ ¹³C value increased with grain filling proceeding. Moreover, the δ ¹³C value under dryland condition was higher than under limited irrigation. The correlation between δ ¹³C with grain yield and WUE is significant during the grain filling stages whether under dryland or irrigation, and the correlation under dryland is better than that under limited irrigation. With irrigation of 100 mm water at jointing stage, various genotypes of winter wheat had significant compensation or super compensation effects. The δ ¹³C value can indicate WUE well whether under dryland or irrigated condition. Therefore, the δ ¹³C value can be used by breeding programs as a potential selection criterion for grain yield and WUE in wheat.

Leaf anatomical structure, photosynthetic enzyme activity, glycollate oxidase activity and stable carbon isotopic composition (δ13C) in leaves from different habitat reed distributed over the Hexi corridor of Gansu province were comparatively investigated. The results indicate that the bundle sheath cells of swamp reed do not contain chloroplasts, the ratio of RUBPcase activity/PEP case activity, activity of glycollate oxidase and δ13C value in leaves of swamp reed fall in ranges typical for C3 photosynthetic plant (wheat). The typical Kranz structure and the bundle sheath cells containning dimorphological chloroplasts were observed in leaves from sand dune reed, the ratio of RUBPcase activity/PEPcase activity, activity of glycollate oxidase and δ13C value are quite similar to that of typical C4 photosynthetic plant(Maize). Salt meadow reed and salt-meadow-sand-dune reed compared with swamp, although there are Kranz structure and bundle sheath cells containning chloroplasts in the former leaves, and the ratio of RUBPcase activity/PEPcase activity in the former leaves are lower than in the latter leaves, activity of glycollate oxidase, especially δ13C values in the former leaves are similar to that of the latter leaves. These results may suggest that, in the Hexi corridor of Gansu province, environmental factors induce an active evolution of the pathway of photosynthetic carbon metabolism within reed species.

Because destruction of the ozone layer is becoming increasingly more serious, the amount of ultraviolet radiation reaching the earth's surface, esp. UV-B radiation that is harmful to the DNA of organisms, has increased. The increase in UV-B radiation has altered ecological systems on the earth and has emerged as one of the most noticeable forces of global change. Interest on the potential dangers and injuries from enhanced UV radiation on marine organisms (esp. marine plankton) has increased. A lot of work has been done on UV-B radiation effects on marine microalgae at the molecular, cellular, physiological and biochemical levels; however, there are few reports on red-tide microalgae. In this study, the effects of UV-B radiation on the protein and nucleic acid synthesis of three red-tide microalgae species, Heterosigma akashiwo, Alexandrium tamarense and Skeletonema costatum, were investigated to better understand the influence of UV-B radiation on marine ecological systems and the mechanism and occurrence of red tides. The microalgae were cultured in Erlenmeyer flasks with f/2 medium. Salinity of the seawater was (30.0±1.0) ‰ and the initial pH of the culture was ( 8.0 ±0.1). Cultures were grown at (19±1) ℃ under a 12 h∶12 h dark-light cycle at an illumination intensity of 3 000 Lx. Ultraviolet B radiation was provided by two UV-B tubes (Philips TL 40 w/12 uv) covered by a film of cellulose acetate (0.12 mm) to remove all radiation below 280 nm. In order to minimize the change of the filter properties of the film, the cellulose acetate was pre-burned for 48 h at a distance of 1 m from two UV-B lamps. Algae were exposed to UV-B radiation treatments of 0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4, 2.7, and 3.0 J·m -2, respectively, for 4 days. All experiments were carried out in triplicate. The responses of protein and nucleic acid synthesis in H. akashiwo, A. tamarense and S. costatum to UV-B radiation were studied using isotope-tracing methods. The results showed that the order of sensitivity from high to low in three red-tide microalgae to UV-B radiation was H. akashiwo, A. tamarense and S. costatum. The growth and DNA synthesis of H. akashiwo were inhibited. Whereas lower doses of UV-B radiation stimulated the growth and DNA synthesis of A. tamarense and S. costatum, higher doses had an inhibitory effect. The RNA and protein synthesis in all three species decreased with increased levels of UV-B radiation, but decreases in H. akashiwo were greater than in A. tamarense and S. costatum. Therefore, the sensitivity of RNA and protein synthesis to UV-B radiation enhancement in H. akashiwo was higher than that in A. tamarense and S. costatum.

Aims Pot experiments were conducted to investigate effects of water deficit and duration of the deficit (0, 15, 36 and 14 days recovery) on four hybrid Populus: 15-29 (P. trichocarpa × P. deltoids), DN-2 (P. deltoids × P. nigra), DN-14274 (P. deltoids × P. nigra) and R-270 (P. deltoids × P. nigra). Our objective was to examine the responses of Populus plants to soil water deficit by analyzing eco-physiological, morphological, and growth characteristics, as well as several parameters of plant performance.

Methods Seedlings were exposed to four treatments: 100%, 70%, 50% and 30% of soil field water capacity (treatments T1-T4, respectively).

Important findings The four hybrids were sensitive to water deficit. All developed physiological adaptive mechanisms as well as configurational strategies to cope with water shortages to different degrees by closing stomata and reducing leaf number and leaf area to regulate water loss, by depressing net photosynthetic rate (P_n), transpiration rate (T_r) and leaf water potential (ψ) to enhance water use efficiency (WUE), or by changing allocation of biomass productivity (B_p). Under water stress, R-270 only decreased its leaf dry weight but the other three hybrids decreased their dry weight of leaf, stem and root. With declining soil moisture, root/shoot of 15-29 and R-270 increased, implying the roots obtained more carbohydrates, which favors water absorption. Carbon isotope composition (δ¹³) of DN-2 was significantly positively correlated to WUE, but δ¹³ of R-270 was significantly negatively correlated to WUE. P_n, stomatal conductance (G_s), T_r,ψ, biomass and canopy areas of the seedlings in T1 and T2 are higher than those in T3 and T4, suggesting that the four hybrids can obtain high production in arid areas under sufficient-moderate irrigation. LowerP_n, G_s, T_r,ψ, biomass, canopy areas and higher WUE of the seedling in T3 and T4 indicate that the four hybrids can develop survival strategies under water stress, but biomass production was negatively affected. Clone 15-29 and R-270 showed a stronger adaptive response than DN-2 and DN-14274 under water stress, implying they have greater drought-resistance ability.

Tree root respiration is a major contributor to soil CO2 pools, and 2/3 of total forest soil respiration is from root respiration. Information about root respiration is important for understanding the implications of environmental change on soil carbon cycling and carbon sequestration, and for the development of carbon models of forest ecosystem dynamics. Furthermore, the global CO2 flux from root respiration is estimated to be about 18 Pg C·a-1, which is an order of magnitude larger than that produced by anthropogenic sources of CO2. Therefore, changes in forest tree root respiration could have a significant impact on the future global carbon balance. Ecological research on tree root respiration is relatively recent and still very little research on this topic has been conducted in China. In this paper, We review the characteristics, methodologies and factors affecting tree root respiration for the purpose of stimulating new, domestic research on this topic. Tree root respiration is composed of maintenance respiration and growth respiration. Maintenance respiration, the dominant component of total of total root respiration, is used to maintain the living biomass, and growth respiration, which is used to construct new biomass, is proportional to the amount of new dry matter synthesized. Root respiration rates vary significantly among forest types. The proportion of the total soil carbon flux that is attributable to live root respiration appears to be very high in cold, northern biomes, ranging from 50% to 93% in the arctic tundra and from 62% to 89% in boreal forests. In temperate zones, estimated proportions of the total soil respiration flux that is derived from live root respiration range from 33% to 50% in broad-leaved forests and from 35% to 62% in pine forests. Root respiration typically is 40%-60% of total soil respiration in most forests. Tree root respiration has significant seasonal dynamics with respiration greatest during the growing season and lowest during the dormant periods of the year. . Methods used to measure tree root respiration include root exclusion methods, in vitro root techniques, stable or radioactive isotope methods and in situ cuvette methods. Each approach has advantages and disadvantages. The first two methods are relatively simple and inexpensive and are commonly used in forest ecosystems. Isotope based methods provide quantitative answers with the least amount of disturbance to the soil and roots, but the complexity of the experimental setup and the high costs associated with the analysis of radioactive or stable C isotopes are major disadvantages. The in situ cuvette method is considered an important method for future studies. Critical factors influencing rates of tree root respiration include soil temperature, root diameter size, ambient CO2 concentration, soil moisture, and nutrient availability. . Despite intensive research in recent years, many uncertainties remain in this dynamic and important field of research. Topics of particular importance include: 1) Discussion and comparison of the appropriate methods for accurately measuring tree root respiration; 2) Application of effective methods for separating root respiration and rhizosphere respiration in the field; 3) Long-term research on the dynamics of tree root respiration in forest ecosystems; 4) Studies of tree root respiration in different ecosystems and climatic zones; 5) How to scale up from small chambers to the stand level, ecosystem level, regional or global level; 6) Understanding the mechanistic response of tree root respiration to global climate change; and, 7) Inter-disciplinary research on tree root respiration to understand its role in the global carbon cycle.

The carbon isotope composition (δ13C value) of plants is a useful index for assessing intrinsic water use efficiency (WUE) and can also provide information on long term WUE, because the δ13C value integrates photosynthetic activity throughout the entire life span of the leaf tissue. Water is the limiting environmental factor for growth and reproduction of steppe plants in the Xilin River Basin, Inner Mongolia. Carex korshinskyi, a perennial forb, is widely distributed throughout the Xilin River Basin and shows strong adaptive characteristics enabling it to survive in habitats with widely varying nutrient and water conditions. In this study, six plant communities were selected which differed in floristic composition and soil water status but had similar climatic conditions, such as temperature and precipitation. Foliar δ13C values, leaf water content (LWC) and population characteristics (including height, density and aboveground biomass) of C. korshinskyi were measured in each of the six communities. Our objectives were to study the variations in foliar δ13C values, LWC and population characteristics of C. korshinskyi along a soil water gradient to better understand the adaptive strategies of C. korshinskyi to water stress. Results showed that: 1) There were significant variations in foliar δ13C values of C. korshinskyi in different habitats (changing range 1.8‰). The foliar δ13C values of different C. korshinskyi populations tended to increase with decreasing soil water content (SWC). A significant negative correlation was found between foliar δ13C values and SWC in different soil layers, indicating that C. korshinskyi populations could change WUE in response to water availability. 2) A significantly negative correlation was found between foliar δ13C values and LWC of C. korshinskyi. Only small variations in LWC were found among the six different C. korshinskyi populations indicating that the WUE of C. korshinskyi was sensitive to changes in leaf water status. 3) There were significant differences in height, density, aboveground biomass and frequency of occurrence among C. korshinskyi communities along the soil water gradient. The C. korshinskyi populations with higher δ13C values had higher occurrences in the plant community and contributed more to total aboveground biomass and community productivity. Our results suggest that C. korshinskyi can adjust its water use pattern (such as enhancing WUE) to adapt to habitats with different soil water availability increasing its competitive ability across a wide range of habitats.

Stable isotope technique has been widely used in ecology research with the increasing concern on global change. Our objectives are to better understand the impacts of nitrogen addition and other environment changes on the nitrogen cycling of terrestrial ecosystem, predict the consequent changes in environmental conditions, and provide a reference for policy making to help ensure the sustainable development of terrestrial ecosystems. Based on the relationship between nitrogen (N) isotope composition (δ ¹⁵N) in ecosystem N status and soil N cycle, we summarized the effects and mechanisms of N input and other environment changes on δ ¹⁵N of plant and soil. Most studies show significant positive relationships between N input and δ ¹⁵N values of plant and soil. Higher N input increases soil N availability, which leads to ¹⁵N enrichment in soil because of mass discrimination during soil N cycling processes. Foliar δ ¹⁵N also will be higher as plants take up the relatively ¹⁵N-enriched soil available N. ¹⁵N natural abundance can be a useful tool for assessing nitrogen saturation and N cycling.

Aims Woody plants in karst regions often grow on the continuous rock outcrops. However, the source of water for these plants is unclear. This study aimed at investigating the seasonal variation of water sources for plants growing on continuous limestone outcrops in southwestern China.
Methods Stable isotope techniques were used to access plant water uptake patterns for five different species in both wet and dry seasons. Then the IsoSource model was used to determine the probable contribution of each potential water source to total plant water uptake.
Important findings In the wet season, an average of >80% of water for four of the five species came from rainwater received in the previous 15 days and stored in fissures in shallow layers. Water accessed by the other species came mostly from recent rainwater; however, the species simultaneously absorbed water from fissures in deep layers that had been recharged from previous precipitation. In the dry season, all five species utilized both recent and previous rainwater, and the two tree species and one large evergreen shrub species used more previous rainwater (>50%).

Aims Our objective was to analyze relationships between foliar stable carbon isotope composition (δ¹³C) and environmental factors and leaf element contents in Pinus tabulaeformis, which is widely distributed in northwestern China.

Methods Leaf samples were taken in ten natural populations of P. tabulaeformis from Gansu, Ningxia and Inner Mongolia. The δ¹³C was determined using an isotope mass spectrometer. Environmental factors were recorded by Magellan GPS320 and from nearby weather stations. Leaf element contents were measured with low vacuum scanning electron microscope and X-ray energy dispersive spectroscopy. Carbon content was estimated with the potassium dichromate method, and nitrogen content was measured with the micro-Kjeldahl method. Data were evaluated with Excel 2003 and SPSS 13.0 analysis of correlation and regression.

Important findings Foliar δ¹³C ranged from -28.68‰ to -25.02‰, with an average value of -26.82‰. Foliar δ¹³C did not closely correlate with altitude and longitude, but was significantly positively correlated to latitude and negatively correlated with annual mean precipitation and temperature. Therefore, δ¹³C in different natural populations of P. tabulaeformis is significantly influenced by environmental factors, and annual mean temperature and precipitation are essential factors in δ¹³C’s capability of fractionation and growth. Leaf N, P and K content were significantly negatively correlated with δ¹³C; however, Si, Ca and Fe content were significantly positively correlated with δ¹³C. Different δ¹³C in different natural populations reflects changes in plant nutrient element content. Response mode of climatic factors and element content under different habitat conditions reflected and affected properties of δ¹³C change in natural populations of P. tabulaeformis.

Data obtained from pot experiments was analyzed using a theoretical model of carbon isotope discrimination (△) for the effects of soil moisture and nitrogen phosphorus nutrient on carbon isotope discrimination. The results showed that △ increased with increasing soil relative water content (SRWC),with the highest △ values found between 60% and 70% SRWC. △ also increased with increasing phosphorus levels under conditions of water deficit. Correlation between water use efficiency (WUE) and △ could be dependent on the status of soil moisture and nutrient availability. Stable △ was negatively associated with WUE under water stress, and positively associated with WUE under optimum soil moisture. However, there was no correlation between △ and WUE under nitrogen deficit and correlations were significant when nitrogen was applied at 150 kg·hm-2.

Pure Chinese fir (Cunninghamia lanceolata) plantations, a major commercial tree species in South China, have been extensively replanted on the same site in successive rotations, in response to the growing need for timber. The resultant soil degradation ha

In the past several decades, the development of nitrogen (N) stable isotope techniques has improved the understanding of N cycling in terrestrial ecosystems. This review briefly introduced the history of N stable isotope techniques in studying N cycling in terrestrial ecosystems and summarized typical studies focusing on different aspects of ecosystem N cycling in recent years, including using 1) ¹⁵N natural abundance to identify plant N sources, indicate N status of ecosystems, and quantify N transformation rates; 2) ¹⁵N enriched tracers to study N fates, redistribution and gaseous loss from ecosystems. In the end, this review points out challenges and future applications of N stable isotope techniques on studying N cycling in terrestrial ecosystems.

Leaf traits reflect the highly adaptable and self-regulatory capacity of plants to complex environmental conditions. That how they respond to climate change is one of key topics in studies of plant adaptability. This review synthesizes the current understanding on the responses of leaf size, specific leaf mass, leaf nitrogen content and carbon isotopes to climate change. The responses of leaf traits to climate change vary with different leaf structures and ecological properties. Thus, a single leaf trait cannot be used to fully reflect the responses of plants to climate change. There are still a lot of uncertainties concerning the effects of climate change on leaf traits under different scales. Studies are relatively lacking in the alpine region. This review helps us to better understand the relationships between leaf traits and climate as well as the responses and adaptation of plants to climate change. It is critical to predict the variations and evolutionary strategies of plants in response to future climate change.

Aims Providing indispensably theoretical evidence for establishing indices of evaluation on drought tolerance in shrubs, and screening for tree species that are drought tolerant for afforestation in arid regions or for matching their characteristics with suitable habitat conditions are the key to vegetation restoration in the Qinghai-Xizang Plateau. However, these issues are not adequately addressed in recent research due to lack of systematic methods. Therefore, our objective was to make a comprehensive evaluation on drought tolerance in 20 shrub species collected from different areas in Qinghai-Xizang Plateau, and to study their underlying mechanisms in drought tolerance.
Methods We made measurements on variables depicting root characteristics, including the root length (TRL), surface area (TRSA), volume (TRV), and tips number (TRTN) of all roots, the root length (FRL), surface area (FRSA), volume (FRV), and tips number (FRTN) of fine roots (d≤2 mm), and derived plant characteristic indices including thickness of cuticle (CT), thickness of palisade tissue (TPT), thickness of spongy tissue (TST), TPT/TST, thickness of leaf (LT), palisable tissue cell density, and tissue structural tense ratio (CTR = TPT/LT × 100%) and spongy tissue loosened ratio (SR = TST/LT × 100%) of leaf anatomical structure, root to shoot ratio (RSR), leaf transpiration rate (T_r), instantaneous water use efficiency (WUE_i), and carbon isotopic composition (δ¹³C) of the 20 shrub species through field experiments. Correlation analysis and principal component analysis were performed on the 19 variables and indices.
Important findings Different shrubs had different mechanisms of drought tolerance. In this study, the character- istics of drought tolerance were mainly categorized into 6 types, involving modifications of (1) root systems, (2) leaf anatomical structure, (3) leaf pattern, and (4) biomass allocation, or via (5) low water-consumption and (6) high WUE_i. Different genera or different tree species within the same genus clearly differed in drought tolerance. The species of the genus Hippophae were relatively poorly tolerant to drought, whereas several shrubs including Potentilla fruticosa, Berberis julianae, Caragana arborescens, Spiraea salicifolia and Hippophae rhamnoides ssp. mongolica occurred to be more drought tolerant than other shrub species investigated in this study. On the other hand, there were highly significant correlations among the characteristics of root systems and among characteristics of leaf anatomical structure. The results of principal component analysis on 19 variables and indices showed that TRL, TRSA, TRV, TRTN, FRL, FRSA, FRV, FRTN, CT, TPT, TST and WUE_i could be effective indicators of drought tolerance of shrubs in the Qinghai-Xizang Plateau. In addition, the drought tolerance of shrubs had a close connection with their origin of collections; the shrubs collected from Xining prefecture in Qinghai Province were more drought tolerant than those from Tianshui Prefecture in Gansu and Lasa Prefecture in Xizang.