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.

Aims Our objectives were to determine the effect of increased nitrogen deposition on soil organic carbon and nutrients of Pleioblastus amarus plantations.
Methods Beginning in November 2007, we conducted a two-year field experiment of simulated nitrogen deposition in a P. amarus plantation, Rainy Area of West China. The levels of nitrogen deposition were 0, 5, 15 and 30 g N·m^-2·a^-1 for control (CK), low, medium and high nitrogen, respectively. For one year beginning in November 2008, we monthly collected 0-20 cm horizon soil samples and measured soil total organic carbon (TOC), microbial biomass carbon (MBC), extractable dissolved organic carbon (EDOC), liable carbon (LC), total nitrogen (TN), microbial biomass nitrogen (MBN), NH₄⁺-N, NO₃^--N, available phosphorus (AP) and available potassium (AK).
Important findings Nitrogen deposition increased concentrations of TOC, MBC, TN, MBN, NH₄⁺-N and AP in soil and had no effect on the other indicators. MBC and MBN exhibited significant seasonal patterns that were positively related to temperature. AP and AK were significant negatively correlated with MBC and MBN. Nitrogen deposition stimulated availabilities of C, N and P and increased the accumulation of these elements in the soil. Results suggested the P. amarus plantation ecosystem is N-limited and soil organic carbon and nutrients respond positively to nitrogen deposition. Increasing nitrogen deposition may enhance fertility of the soil, stimulate growth of plants and increase future carbon fixation.

Aims Tropical cloud forest, a seldom-studied but important type of tropical forest vegetation, is usually distributed around mountaintops and is characterized by unusual environmental conditions that determine its unique biodiversity and ecosystem functioning. Our objective was to explore the characteristics of solar irradiance, air temperature, air relative humidity, soil conditions and topographic conditions during the rainy season in two tropical cloud forest communities: tropical montane evergreen forest (TMEF) and tropical montane dwarf forest (TMDF).
Methods Based on surveyed environmental data of TMEF and TMDF in Bawangling National Natural Reserve in Hainan Island of South China, we assessed differences in environmental conditions between the two cloud forests, and examined correlations among the environmental conditions, using principal component analysis and Pearson’s correlation analysis.
Important findings Daily photosynthetically active radiation (PAR) showed a unimodal curve both in TMEF and TMDF, but PAR in TMEF was significantly lower than in TMDF. From May to October, mean daily air temperature differed significantly between TMEF and TMDF and showed a unimodal curve in the two forests, with average values of (21.76 ± 2.44) °C and (19.33 ± 1.03) °C, respectively. Additionally, mean daily relative humidity differed significantly between TMEF and TMDF and showed an inverse “S” curve; average values were (88.44 ± 2.90)% and (97.71 ± 0.80)%, respectively. TMEF had higher total nitrogen, total phosphorous, available nitrogen, organic matter, pH and soil thickness, but lower total potassium and available phosphorous than TMDF. Slope, cover of exposed rock and altitude were lower in TMEF than TMDF, and were significantly correlated with other ecological factors. Principal component analysis and Pearson’s correlation analysis indicated that air temperature, available phosphorous, total potassium, total nitrogen and the three topographic factors were predictors of distribution of these tropical cloud forests.

Aims Climatic change has and will continue to decrease summer precipitation in the Dongling Mountain area of Beijing, China. Decreased precipitation impacts trees and hence temperate forest vegetation. Experimental studies suggested that the effects of decreasing summer precipitation on forest were closely related to species-specific characteristics during drought. Our major goals were to project the impact of decreasing summer precipitation on forest dynamics in this region and to analyze long-term consequences of tree-species specific drought response of the temperate forest ecosystem.
Methods We used LPJ-GUESS dynamic vegetation model coupled with different water uptake strategies to investigate drought effects on trees and forests in this temperate region of China.
Important findings Increases in net primary productivity (NPP) and carbon biomass of the predicted area under future climate conditions of increased temperature and elevated CO₂ concentration were independent of summer precipitation. This suggests that precipitation will not be the limiting factor in this area. However, tree diversity strongly depended on the drought response that we assumed. Drought-sensitive tree species (e.g., Juglans mandshurica) were not influenced by long-term drought, whereas the carbon biomass of the most drought-tolerant species (i.e., Quercus liaotungensis) would decrease in the future. Moreover, tree-species specific drought response will affect the water cycle of the temperate forest, including evapotranspiration. Our findings of the species-specific drought response should be considered in future ecosystem models.

Aims Snowpack plant communities in alpine tundra are active in comparison with surrounding vegetation, despite the short growing season due to thick snow cover. Our objective was to understand the growth mechanism of snowpack communities.
Methods The nutrient dynamics and primary productivity of snowpack Sanguisorba sitchensis community in alpine tundra of Changbai Mountain was investigated in different seasons.
Important findings Soil temperature under snowpack during winter was warmer than snow-free areas, and the minimum temperature was significantly higher. Litter decomposition and nitrogen mineralization under snowpack were active even in winter, and nitrogen content was high with rapid mineralization. Annual net primary productivity was 4 046 kg·hm^-2·a^-1. The unique hydro-thermo conditions, nutrient cycling features and high leaf area index were key factors maintaining community structure and primary productivity.

Aims Our objectives are to compare soil respiration rate and its temperature sensitivity at different land use types and discuss soil respiration response to soil temperature (T_s) and soil water content at different soil depths.
Methods Periodic measurements of soil respiration rates (R_s) were made during August-October 2009 with a LI-8100 portable automated soil CO₂ flux system in three agro-pastoral ecotone land use types: cropland, abandoned cultivated land and grazing enclosure. Soil temperature and soil water content at 0-5, 5-10 and 10-15 cm depths were measured simultaneous adjacent to the soil collar.
Important findings R_s is significantly different among the three land use types and greatest in cropland. R_s exhibited a unimodal curve during 6:00-18:00, with a maximum during 12:00-15:00. R_s decreased with T_s, so R_s was significantly higher in August than in September and October. With the Van’t Hoff model, we concluded there is a positive, exponential relationship between measured T_s and R_s. In addition, temperature sensitivity of soil respiration (Q₁₀), which is derived from the Van’t Hoff model, was largest in cropland. In contrast, R_s was negatively related to soil water content in different soil depths at the sites.

Aims In the Qinghai-Tibet area, high levels of UV-B radiation reach the earth’s surface as part of strong solar radiation. It is therefore important to examine the response and adaptation of native alpine plants to strong UV-B radiation. Our objective was to determine the physiological response of the photosynthetic apparatus to current ambient UV-B intensity.
Methods UV-B-exclusion experiments were conducted on a field site with UV-B-excluding and UV-B-transmitting filters. The two filters transmitted similar levels of photosynthetically active radiation, and there were no differences in air temperature and relative humidity under these metal-frame suspended plastic filters. The experiments were performed in alpine Kobresia humilis meadow with the native alpine species Saussurea superba. Pulse-modulated in vivo chlorophyll fluorescence was mainly used to obtain rapid information on UV-B effects on photosynthetic processes. The trials lasted 15 or 16 days during the growing season of July and August in 2008 and 2009. We measured the 3-min dark-adapted quantum efficiency of PSII photochemistry (F_(v)/F_(m)), PSII photochemistry efficiency and photosynthetic gas exchange parameters under natural sunlight. The contents of photosynthetic pigments and UV-B-absorbing compounds were analyzed in terms of both leaf area and leaf fresh weight units.
Important findings Net photosynthetic rate, significantly increased after removal of UV-B components from natural sunlight. Although there was no significant difference, the F_(v)/F_(m) was increased in low UV-B treatment compared with the ambient UV-B control. Both the coefficient of photochemical quenching and actual photo- chemical efficiency of PSII in the light were higher in low UV-B treatment compared to ambient UV-B during the 16 days of continuous measurement during clear days. Variation of PSII maximum efficiency in the light and nonphotochemical quenching further confirmed that the decrease in PSII photochemistry efficiency and increase in nonphotochemical quenching were the results of a strong solar UV-B component in natural sunlight. There was only a slight increase in photosynthetic pigments based both on leaf area and fresh weight after removing UV-B radiation, and these phenomena indicated that strong solar UV-B radiation could photo-oxidize photosynthetic pigments and further accelerate the maturity and senescence of plant cells. UV-B-absorbing compounds were not altered after removal of UV-B radiation, which suggested that high levels of UV-B-absorbing compounds in the epidermal cell layer could protect photosynthetic function from UV-B.

Aims Liquidambar formosana is an important canopy tree species widely distributed in the plains and mountains from the north-subtropical zone to the tropical zone in China. Full understanding of the natural regeneration processes of tree seedling and limiting factors is important for plant population conservation and forest management. Our objective is to investigate the factors that limit seedling regeneration of L. formosana under canopies of Formosan sweetgum forests in hilly regions of Southeast Hubei, China.
Methods Seedling regeneration of L. formosana occurs sporadically under canopies of mixed pine forests but poorly under Formosan sweetgum canopy. In 2008, we carried out a series of experiments to study factors limiting seedling regeneration of L. formosana under mixed pine forests and Formosan sweetgum forests and tested: (1) effects of lower temperature and soil moisture on seed longevity, (2) effects of forest type, soil type and litter cover on seed survival, (3) litter allelopathy effects on seed longevity and germination, and (4) effects of light intensity under canopies on seedling survival.
Important findings Dispersed seeds of L. formosana could germinate immediately at lower temperatures (4-8 °C), although such temperatures were fatal to seedling establishment. Contrary to our prediction, soil type, litter leachate and litter cover had no effects on L. formosana seed viability and could not be used to explain poor seedling regeneration under canopies of Formosan sweetgum forest. We also did not find any litter leachate inhibition or promotion effects on seed germination, although a short germination lag effect was found with leachate of pine needles and sweetgum leaves. Shading first-year seedlings of L. formosana indicated that about 90% would die under the canopy of the Formosan sweetgum forest (with its 3% transmittance of open-sky light). In general, our experiments suggested that non-dormant seed and shade-intolerant seedlings of L. formosana were key factors limiting seedling regeneration under canopies of Formosan sweetgum forest.

Aims Our objective was to elucidate the physiological causes of the influence of soil water deficit on grain-filling and rice (Oryza sativa) yield by evaluating enzyme activities involved in sucrose-starch synthesis during the grain-filling period.
Methods Two rice cultivars (‘Shanyou 63’ and ‘Pc311/Zaoxiandang’) were grown in cement containers. Controlled soil water deficit was imposed at 9th day after flowering because the division of endosperm cells is sensitive to water deficit. The activities of key enzymes involved in the metabolism of sucrose to starch during grain-filling were evaluated.
Important findings The 1000-grain weight and rice yield under soil water deficit were significantly decreased. The key enzyme activities, including sucrose synthase, adenine diphosphoglucose pyrophosphorylase, soluble starch synthase and starch branching enzyme, involved in the metabolism of sucrose to starch were significantly increased under soil water deficit during 10-20 days after anthesis. However, the activities of these enzymes under soil water-deficit were decreased rapidly 20-30 days after flowering compared to well-watered treatments. Active grain-filling periods of the water-deficit treatments were shortened as a result of the decrease of starch accumulation. Therefore, the rapid decline of the key enzyme activities involved in sucrose to starch metabolism and the shortage of assimilation supply at 20-30 days after anthesis are reasons for decreases in grain weight and yield under soil water deficit conditions.

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.

Aims Degradation of forestland productivity by continuously cropped poplar plantations is closely related to phenolic acid accumulation in the soil. Soil nitrogen is an essential factor affecting the growth and development of poplars. Nitrate was one of the main forms of nitrogen in soil, and the impact of phenolic acids on absorption and utilization of nitrate is vital to revealing the allelopathic mechanism of phenolic acids and proposing measures for improving forestland productivity.
Methods We cultured hydroponic cuttings of black poplar cultivar ‘I-107’ (Populus × euramericana ‘Neva’) in improved Hoagland nutrient solution to which we added gradations of phenolic acids (0, 0.5X, 1.0X, 1.5X, 2.0X) according to actual content (1.0X) in the soil of a second-generation poplar plantation. We observed the effects of phenolic acids on nitrate absorption and utilization by the hydroponic cuttings. We tested several physiological characteristics closely related to nitrate absorption and utilization at 1, 5, 10, 20, 30 and 40 days after treatment with phenolic acids.
Important findings Degree of root injury was aggravated by phenolic acids, root activity was inhibited by phenolic acids and both indexes of phenolic acid treatments were significantly different from the control at 40th day (p < 0.05). Absorption of nitrate by poplar hydroponic cuttings was significantly diminished by phenolic acids at 48 h (p < 0.05). Low levels of phenolic acids (0.5X and 1.0X) led to accumulation of nitrate in cuttings, but high levels (1.5X and 2.0X) led to decline in nitrate content. Nitrate activity of poplar hydroponic cuttings was inhibited by phenolic acids, and treatment values were significantly different from the control at 40th day (p < 0.05). Nitrate content increased in later periods with phenolic acids treatments. Relationship of effects of phenolic acids concentration and duration time on nitrate metabolism could be described by multiple linear regression.

Aims Our objective was to assess the phytotoxicity of atrazine residue in water on the emergent aquatic plant Scirpus tabernaemontani.
Methods We conducted hydroponics experiments to study the effects of atrazine stress on peroxidase (POD) and root activities, malonaldehyde (MDA) and chlorophyll contents, relative water content (RWC) of leaf and fresh weight of S. tabernaemontani. The original dosages of atrazine in the culture solution based on Hoagland solution were 1, 2, 4, 8 and 16 mg·L^-1. Plant samples were collected 15, 30, 45 and 60 days after the onset of the experiment.
Important findings POD and root activities, chlorophyll content, RWC and fresh weight of S. tabernaemontani decreased, and MDA contents increased. All results indicated that elevated concentration of atrazine (>8 mg·L^-1) has significant effects on biomass and physiological characteristics of S. tabernaemontani. With the exception of chlorophyll contents, the harmful effects weakened with prolonged treatment exposure due to degradation of atrazine. There were no significant differences in investigated variables between the 1 mg·L ^-1 atrazine treatment and the control during the growing season.

Glomalin (or glomalin-related soil protein) is thought to be produced by arbuscular mycorrhizal fungi. It is a thermotolerant glycoprotein that contains metal ions and has special physical and chemical characteristics. The structure and function of glomalin have been intensively studied. We review progress in the characterization and properties of glomalin as well as its environmental functions in the rhizosphere (especially its chelation of heavy metals). Research has shown that glomalin is a putative homolog of heat shock protein 60 and can carry different metal ions because of different kinds of soil. Further research on methods for its measurement is urgently needed.