With the development of isotope ratio infrared spectroscopy (IRIS) technology, it is now possible for the in situ high temporal resolution and high precision measurement of carbon isotopic composition (δ 13C) and oxygen isotopic composition (δ 18O) of atmospheric CO2, which overcomes the low temporal resolution and labor intensive shortcoming of traditional isotope ratio mass spectrometry (IRMS). The dependence of δ 13C and δ 18O on CO2 concentration (termed as concentration dependence) and the drift due to sensitivity to changing environmental conditions (termed as instrumental drift) are the two main sources of error affecting the IRIS measurements. Therefore, it is important to obtain precise measurements by constructing a proper calibration strategy to solve the concentration dependence and instrumental drift. In this study, we briefly discussed the definition and related theoretical principle of concentration dependence, and elaborated the theoretical and empirical calibration methods of concentration dependence. Moreover, we introduced the calibration methods of instrumental drift, and reviewed the state of the art of calibration methods and its application of IRIS technology. Additionally, we briefly discussed the definition and method of data traceability to the international standard, and reviewed its application of IRIS technology. Finally, we recommend that concentration dependence is corrected by using three standards or above with known CO2 concentration and its δ 13C and δ 18O, bracketing the CO2 concentration of samples. The instrumental drift is corrected by setting appropriate calibration frequency and all dataset are traceable to the international standard. In the future, the comparative study of different IRIS instruments and calibration methods should be enhanced, and the similar methods should be used for measuring CH4, N2O and H2O isotopes by IRIS technique. The IRIS technology combined with other technology will provide a new opportunity for ecological research.
Aims Although acquisition of soil organic nitrogen (N)(mainly amino acids) by plants is a widespread ecological phenomenon in many terrestrial ecosystems, the rate of organic N uptake and their contributions to plant nutrient supply are poorly understood. Our objective was to determine the relative contributions of inorganic N (NO3–-N and NH4+-N) and organic N (amino acids) to plant N uptake in a high-frigid forest ecosystem.Methods The differences in the uptake rate of three different forms of N (NO3–-N, NH4+-N and glycine) were quantified by exposing seedlings of two dominant tree species (Picea asperata and Betula albo-sinensis) in subalpine coniferous forests of western Sichuan, China, to trace quantities of K15NO3,15NH4Cl and (U-13C2/15N) glycine.Important findings Both 13C and 15N were significantly enriched in fine roots 2 h after tracer application, indicating the occurrence of glycine uptake in P. asperata and B. albo-sinensis seedlings. The seedlings of two tree species had a significant preference for NO3–-N compared with glycine and NH4+-N, and the uptake rate of NO3–-N was 5 to 10 times greater than that of glycine and NH4+-N. The roots of seedlings in the two species took up glycine more rapidly than NH4+-N, implying that soil organic N (i.e., amino acids) could be an important N source for the two species in subalpine coniferous forests. The results of this study are of great theoretical significance for understanding N utilization strategies and nutrient regulation processes in plants of the high-frigid forest ecosystems.
Aims We aim to evaluate the water sources of typical riparian arbor species (Populus euphratica) and shrub species (Tamarix ramosissima), and analyze the spatial and temporal dynamics of plant water source in Ejina Delta, the lower reaches of the Heihe River, China.Methods Eight sampling sites were selected in the riparian zones along the East River and West River in Ejina. The plant xylem water, soil moisture, rainwater, stream water and groundwater were taken and pretreated during the growing season in 2015-2016, and the stable oxygen isotope ratio (δ18O) for each water sample was measured. The δ18O of plant xylem water and soil moisture were compared to estimate the dominant depth of root water uptake, and the linear-mixed model called “IsoSource” were applied to determine plant water sources and quantify their proportions.Important findings This study indicated that the main recharge sources for P. euphratica and T. ramosissima were stream water and groundwater. The contributions of rain water to them was negligible due to the limited amount and the shallow infiltration depth of local rainfall. As affected by groundwater level fluctuation, soil physical properties, as well as lateral and vertical recharge of stream water on soil moisture, the dominant depth of root water uptake spatially varied. However, the relative contributions of stream water or groundwater to plant water sources did not change significantly across space. Populus euphratica used more stream water (68%), while T. ramosissima used more groundwater (65%). Plant water sources were sensitive to environmental flow controls. The contributions of stream water to the water sources of the two species went up to 84% and 48% for P. euphratica and T. ramosissima respectively during the discharge period, but dropped to 63% and 30% during the non-discharge period. On the other hand, the contributions of groundwater decreased to 16% and 52% during the discharge period, but increased to 37% and 70% during non-discharge period. It is noteworthy that the high similarity of δ18O between stream water and groundwater due to extensive water exchange in the riparian zone made increase the uncertain in quantifying plant water sources.
Aims Artemisia gmelinii is a dominant specie naturally established after abandonment of cultivated lands in the Loess Plateau, and Caragana korshinskii is one of the main planted shrub species to control soil erosion. Improved understanding of water use strategies of these two species is of great significance to evaluate the sustainable development of the Loess Plateau under the trend of climate warming and increasing drought events. Methods Stable oxygen-18 isotope was used to determine seasonal variations in the water sources of native A. gmelinii communities established after abandonment of cultivated lands for 7 and 30 years and planted C. korshinskii after 30 years. The contributions of soil water from different depths to water uptake were estimated by the MixSIR Bayesian mixing model. The geometric mean regression method was used to fit the line of precipitation to get the local meteoric water line (LWML). Important findings The stable hydrogen isotope rate (δD) and stable oxygen isotope rate (δ18O) of soil water and xylem water plotted to the right side of the LWML, indicating that the isotopic compositions of soil water were enriched due to evaporation. The native A. gmelinii communities established after abandonment of cultivated lands for 7 years and planted C. korshinskii after 30 years showed plasticity in switching water sources from different soil layers, extracting water from shallow soil (0-40 cm) when soil water was available, but deeper soil (40-80 cm) when shallow soil water was dry. In contrast, A. gmelinii growing in site after cultivation abandonment for 30 years mainly relied on water from the surface soil (0-10 cm) throughout the growing season. Our results suggest that the ability of A. gmelinii to compete for soil water reduces with aging of the community while the planted C. korshinskii will have competitive advantage under the condition of increasing frequency of drought events in the future.
Aims The stable isotope fractionation of plant water is an important part for the water cycle in the soil-plant-atmosphere continuum. There is a lack of control mechanisms research of leaf water isotope ratio (δl,b) enrichment based on the field conditions. Because it is tough to get the measured 18O isotope ratio (δ18O l,b) and deuterium (D) isotope ratio (δDl,b) of leaf water (collective name δl,b). Therefore most previous research focuses on model building used the limited number of δl,b. Leaf water δDl,b and δ18O l,b enrichment (collective name Δl,b) is usually represented as the difference of the leaf water isotope ratio (δl,b) and the plant source water isotope ratio (δDx and δ18O x,collective name δx), that is Δl,b = δl,b - δx.Methods A field experiment with spring maize (Zea mays) was conducted in the middle reaches of Heihe River Basin to investigate the characteristics of leaf water δ18O and δD enrichment and their abiotic control mechanisms on seasonal and daily scales. Leaf and stem samples were collected and analyzed according to different time scales, and the δ18O and δD of atmospheric water vapor (collective name δv) were determined based on the in situ and continuous water vapor isotope ratio measurement system at the same time.Important findings The results showed that: δl,b and Δl,b of leaf water varied little during the experimental season while largely at daily scale, which enrichment was found at the daytime but depletion at night. Atmospheric water vapor isotope ratio (δv) and relative humidity were main factors to D on both seasonal and daily scales; for 18O, only relative humidity was the key control factor on both seasonal and daily scales. Differences of D and 18O came from the equilibrium fractionation because equilibrium fractionation factor for D was over 8 times than for 18O. The analysis of these differences help us distinguish the environmental factors of leaf water enrichment for D (ΔDl,b) from leaf water enrichment for 18O (Δ18Ol,b), and improve our understanding of leaf water enrichment process and develop the related models as well.
Aims The optimal patterns of plant community for water use and nutrient utilization, the responses of soil carbon and nitrogen turnover processes to forest succession, and the mechanisms of soil organic carbon accumulation, are three critical issues in forest ecosystem study. It is difficult to accurately detect these ecological processes with conventional methodologies in the short term, yet the application of 13C and 15N natural abundance technique may yield important information about these processes.Methods This study was conducted in Dinghushan Biosphere Reserve. We investigated the natural isotopic abundance of both 13C and 15N of plant-soil continuum along a successional gradient from Pinus massoniana forest (PF) to coniferous and broad-leaved mixed forest (MF), and monsoon evergreen broad-leaved forest (BF). We also analyzed the correlations of foliar stable carbon isotope ratio (δ13C) and stable nitrogen isotope ratio (δ15N) with foliar elemental contents and the variations of soil δ13C and δ15N along soil profiles at different successional stages.Important findings A significant positive correlation between foliar δ13C and foliar C:N was observed. In both litter and soil, the δ13C values tended to decrease along the forest succession, with the order as PF > MF > BF. Foliar δ15N was positively correlated with foliar N content. The δ15N values of litter and upper soil (0-10 cm) increased with successional status. Both soil δ13C and δ15N values increased with increasing soil depth at all three forests. Our results imply that 1) trade-off between water use efficiency and nitrogen use efficiency did not necessarily exist in subtropical forests of China; 2) the application of isotopic technique could assist understanding of the mechanisms of soil carbon accumulation in subtropical forests, especially in old-grow forests; 3) the 15N natural abundance of plant-soil continuum could be a potential indicator of soil nitrogen availability and ecosystem nitrogen saturation status.
Aims Understanding the interspecific water relations is important for designing agroforestry systems. The objective of this study was to determine the water use strategies of component species in a walnut (Juglans regia)-woad (Isatis tinctoria)/sicklepod (Senna tora) agroforestry system.Methods Water sources of component species in a walnut-woad/sicklepod agroforestry system were investigated with the technique of stable deuterium isotope tracing at a site of hilly area in Northern China during 2012-2013.Important findings Results showed that the soil water content in the agroforestry system was 26.74% and 7.93% greater than in the pure woad field in the first half year, and 17.39% and 13.65% greater than in the pure sicklepod field in the second half year (sicklepod growth period), in 2012 and 2013, respectively. The lowest water content was found in the middle of tree rows, and the highest water content was found in the northern side of tree rows or under the trees. In the soil layers measured, the pure woad and pure sicklepod systems had greater hydrogen stable isotope ratios (δ D value) of soil water than in the agroforestry system. During the period of woad growth, more than half of the water absorbed by walnut was from the deeper soil layer (30-80 cm). In contrast, the walnut trees mainly utilized shallow layer (0-30 cm) soil water during the period of sicklepod growth. These findings suggest that walnut has a two-state root system: during the period of woad growth, shallow roots of walnut are not active when soil is dry whereas the sicklepod growth occur in rainy season, and the shallow roots of walnut are active and utilize more shallow soil water supplemented by rainwater. More than 85% of water used by both the woad and the sicklepod were from the shallow layer soil. At the seedling stage, the roots of woad, cannot grow into the deeper soil layer, and the absorbed water is completely from the shallow layer in the pure woad system. However, 5.7% of the water absorbed by the intercropped woad was from the deeper soil layer in 2012, and the proportion increased further (9.7%) in the following year when there was less precipitation. The results confirmed that hydraulic lift effect of walnut occurred on shallow layer crop in dry season, and this effect become greater under drier conditions. Therefore, deeper roots of walnut improved water condition in the walnut- woad/sicklepod agroforestry systems compared to pure crop systems. The walnut mainly utilized water from the deeper layer to avoid water competition with the shallow layer. In the dry season, crops benefited from the water provided by walnut roots through hydraulic lift. Walnut and intercropped plants exhibited water facilitation in the agroforestry systems, suggesting that this configuration is a suitable practice in this area.
Aims Phragmites australis marshes in Tianjin play an important role in ecosystem functioning. Wetlands of Tianjin municipality have been suffering from serious nitrogen loading, salinization and water shortage. The foliar stable carbon isotope ratio (δ13C) is a good parameter which records environmental change information associated with the plant growth process, and reflects physiological and ecological responses of plants to environment changes. The objective of this study is to investigate the effects of environment stress on the leaf δ13C of P. australis in marsh wetlands in Tianjin municipality.Methods This study was conducted in Qilihai, Beidagang, and Dahuangpu marsh wetlands. We investigated the foliar δ13C of P. australis and sediment properties, and evaluated the relationships between the foliar δ13C and sediment environmental factors. Important findings 1) Foliar δ13C ranged from -26.3‰ to -23.6‰, with an average value of -25.8‰. 2) Sediment water and nitrogen status were the important factors affecting reed foliar δ13C. Foliar δ13C was negatively correlated to sediment relative water content, and positively correlated to sediment total nitrogen and available nitrogen content. In contrast, foliar δ13C was not significantly correlated to sediment salinity and phosphorus content. 3) Leaf δ13C were significantly positively correlated with leaf nitrogen content, and negatively correlated with leaf carbon and nitrogen ratio across all site. However, these relationships were not detected due to the wetland drainage at Qilihai site in August. Wetland drainage changed the plant water and nitrogen balance, and further affected water and nitrogen utilization strategies of P. australis. Moreover, wetland drainage had stronger effects on these processes than nitrogen loading and salinization.
Spartina alterniflora was introduced into China because of its strong sedimentation promotion ability, currently, it became one of the most invasive species along coastal areas. Most researches focused on its high productivity which directly increased soil carbon (C) input. However, little is known about its indirect contribution to soil carbon via increased sedimentation.
Spartina alterniflora patches with different invasion history (4, 6, and 10 years) was selected in Chongming Dongtan wetland, and Phragmites australis and mudflat were chosen as control respectively. The plant, soil and water samples were collected for C and nitrogen (N) analysis. Based on the stable isotope ratio of C and N in plant and soil organic carbon, the contribution of soil organic carbon pool from S. alterniflora patches were estimated by using 2 and 3 sources mixing model, respectively.
(1) The soil organic C content and stable carbon isotope ratio of S. alterniflora patches increased with time. Spartina alterniflora invasion cumulatively enhanced soil organic carbon pool. The soil C:N ratio decreased with invasion history and became close to the redfield ratio, indicating the important role of sedimentation input. (2) The contribution of invasive S. alterniflora to soil organic carbon pool increased with time, but the sedimentation contribution dropped gradually. In patch with 4 years invasion history, the contribution ratio of sedimentation was more than 90.0%. While in patch invaded 10 years ago, the sedimentation contribution reduced by 18.4%, and in comparison, S. alterniflora contributed up to 73.5% to soil organic carbon pool. These findings suggested that S. alterniflora contributed to soil organic carbon pool mainly by promoting sedimentation in early invasion period, while gradually relied on its own productivity with invasion time.
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.
Caragana microphylla is a dominant shrub widely distributed in the grasslands of Nei Mongol. Its water utilization pattern has an important effect on local plant community composition. However, the water source for C. microphylla remains poorly quantified, which is vital in understanding the adaptation of this specie to arid environment. This study aims to investigate the dynamics of water use in C. microphylla and its response to soil water fluctuations resulting from precipitation pattern including summer rains and winter snow.
Seasonal variations of hydrogen stable isotope ratios (δD) in the xylem of C. microphylla and potential sources of water (summer rains, winter snow and soil water) were analyzed by LGR hydrogen stable isotope tracer method. The contribution of soil water to C. microphylla was calculated by IsoSource model and the contribution of rainfall to soil water was calculated by a two-source model.
The xylem of C. microphylla (xylem) showed no seasonal differences in δD, whereas there were significant differences in the water source of plants between two contrasting hydrological environments. In habitats where rainfall was shielded off, C. microphylla mainly used the soil water at depth of 0-20 cm, which accounted for 73.30% ± 16.14% of the total water usage; whilst under natural conditions, water uptake by C. microphylla was more or less similar at the depths of 0-20 cm, 20-60 cm and 60-100 cm, accounting for 34.66% ± 7.83%, 32.44% ± 7.42%, 32.90% ± 4.14% of the total water usage, respectively. Competition for water between shrubs and grasses gave a possible explanation for different water utilization patterns of the plants. In addition, rainfall accounted for 42.65%-63.92% of the water sources for C. microphylla. Resultsin this study demonstrated that C. microphylla used both summer rains and winter snow, and dynamics of water use in C. microphylla reflects its adaptation to the water-limiting arid environment.
Aims Alfalfa (Medicago sativa) is considered as an elite forage with high economic and ecological values. In semi-arid areas of Northwest China, the low rainfall is far from satisfying water demand of alfalfa. The northern part of Yanchi County is adjacent to Mu Us desert and has a shallow groundwater table. In this area, groundwater could be the potential water sources for growing alfalfa. Our objective was to investigate the growth performance and potential water sources of alfalfa grown in four slope positions with different altitudes in the bottomland of northern Yanchi County.
Method Stable 18O and D isotope compositions (δ18O and δD) of different water sources and xylem water were analyzed in April, June, July and August 2013. IsoSource was used to calculate the probable contribution of different water sources to the total plant water uptake. Stomatal conductance, carbon isotope discrimination (Δ13C) of whole plant, soil volumetric water content in the 0–300 cm soil profile were also determined.
Important findings Slope position had a significant effect on water content in the 0–300 cm soil profile. Highest soil water content was found in the slope position of lowest altitude. The δ18O-δD coordinates of soil water and plant stem xylem water were distributed on the right of Northwest China local meteoric water line (LMWL), indicating that oxygen and hydrogen isotopic compositions in the water sources of alfalfa were subjected to enrichment due to evaporation. The δ18O values of soil water in the 0–450 cm profile increased with altitude. Soil water δ18O values decreased with the depth of soil profile for a given slope position. Soil water δ18O values in deep profile were similar to those in groundwater, implying that groundwater would move to the upper soil layer via soil capillary. Seasonal fluctuation was observed in soil water δ18O in the 0–40 cm profile, while soil water δ18O in profile below 270 cm was stable seasonally. Plant stem xylem water δ18O value was significantly lower (p < 0.001) in the slope position 1 than in other three slope positions in April, July and August. Highest water utilization rate from deep soil profile (below 270 cm) was recorded for alfalfa grown in the slope position 1 in April, June and July. In August, alfalfa grown in the slope positions 1, 3 and 4 mainly used soil water in the 150–270 cm and 270–450 cm profiles and groundwater; highest dependence on soil water in the shallow profile (0–20 cm) was found in alfalfa grown in the slope position 2. Higher yield, whole plant Δ13C value and stomatal conductance were observed in alfalfa grown in the slope position 1 than in other three slope positions. These results suggest that bottomland with lower altitude and shallow groundwater table should be adopted when planting alfalfa without irrigation in the semi-arid areas of Northwest China where average annual rainfall is around 280 mm. Thus, groundwater could contribute to better growth performance of alfalfa, leading to higher ecological and economic returns.
Aims Plant water use is an important aspect of plant-water relations in desert regions, and is vital in understanding the adaptation of desert species to arid environment. Haloxylon ammodendron is a dominant plant species in the Gurbantünggüt Desert, China. Its water use pattern has an important effect on water balance of the local ecosystem and plant community composition. This study aims to investigate the dynamics of water usage in H. ammodendron and its response to soil water fluctuations resulting from precipitation pulses.
Methods Oxygen stable isotope ratios (δ18O) were measured for xylem water, soil water in different soil layers (0–300 cm depth), precipitation and groundwater. Four potential water sources were classified: shallow (0–40 cm), middle (40–100 cm) and deep soil water (100–300 cm), and groundwater. The possible ranges of potential water sources used by H. ammodendron were calculated using the IsoSource model.
Important findings Main water sources for H. ammodendron shifted from the shallow soil water in April to the groundwater during May to September. In April, the contributions of shallow soil water were in the range of 62%–95%, and the possible ranges of middle and deep soil water and groundwater were 0–8%, 0–15% and 0–38%, respectively. However, during May to September, the contribution of shallow soil water decreased drastically while that of groundwater increased rapidly. Contributions of groundwater were in the range of 68%–100%. Haloxylon ammodendron responded differently to two similar precipitation pulses occurred in different months. After the 6.7 mm precipitation pulse on May 22, the usage of soil water increased from 9.8% prior to the event to maximum at 40.4% one day after rainfall (May 23), while the usage of groundwater decreased from 83%–98% to 42%–81%. After 7 mm precipitation pulse on August 31, the usage of soil water was almost unchanged and the usage of groundwater was still up to 71%–98%, implying that roots of H. ammodendron in the upper soil layer are inactive due to the long-lasting low soil water content. Hence, H. ammodendron seemed to be insensitive to the August precipitation pulse. The shallow soil water recharged by snowmelt and precipitation in spring and groundwater are two important water sources for H. ammodendron. Dynamics of water usage in H. ammodendron reflects its adaptation to this water-limiting desert environment.
Hydraulic redistribution (HR), one of the common bio-physical processes, plays key roles in mediating plant drought stress, regulating plant interspecific relations and community composition, as well as in influencing water and carbon balance of ecosystems. Great improvements have been achieved in HR research with fast development of isotopic labelling and tracing technologies. This paper summarizes the effects of HR on nutrient cycling of soil-plant system, based on studies over the past decade. HR increases soil water content in dry soil layers, thus helping to prevent embolism in roots, increasing the survival rate and the growth rate of fine roots and improving microbial activities. Such effects improve plant nutrient uptake, i.e., hydraulic lift promotes nitrogen uptake from upper soil layers and hydraulic descent promotes phosphorus uptake from deep soil layers. HR may facilitate nutrient exchange between upper and lower soil nutrient pools, improve nutrient flows and regulate the N:P ratio in both plants and soil. These effects may ultimately affect global ecosystems. Under the global change scenarios (e.g. nitrogen deposition), it is necessary to further explore the effects of HR on biogeochemical cycles. HR should be taken into account when using ecosystem models for future predictions.
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 (Tr), instantaneous water use efficiency (WUEi), 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 characteristics 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 WUEi. 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 WUEi 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.
Aims Resource-use differentiation among species, which can reduce species competition for the same resources, is the main mechanism to maintain species diversity. Changes in soil temperature and moisture conditions, in the context of global change, may affect nitrogen (N) nutrition of plants of alpine meadow ecosystems. Our objective is to compare the characteristics of N uptake and resource allocation of dominant species of alpine meadow with changes in soil N and water.
Methods An alpine meadow was treated with N and water addition for three years using the method of 15N isotope injection. We determined the growth responses of dominant species to the N and water additions, as well as the features of N uptake capacity, N allocation and root to shoot ratio.
Important findings The species showed significantly different responses to the N and water treatments, with respect to functional traits of species in N absorption capacity, root N content and root to shoot ratio. There was no significant relationship between N absorption capacity and root N content, whereas N absorption capacity was negatively correlated with root to shoot ratio across all plant species. These results indicated there was ecological niche differentiation in N uptake and a trade-off between the N absorption capacity and resource allocation strategies among species.
Aims Little study has been conducted to quantify plant water sources for Myricaria squamosa, which is a dominant alpine riparian shrub in the Qinghai Lake basin and plays a key role in maintaining riverine wetland system. The objective of this study was to quantify water sources for M. squamosa growing under different hydrological conditions.
Methods We collected the water samples from the xylem of M. squamosa, groundwater, river, and soils in the Qinghai Lake basin from June through September, and analyzed the seasonal variation of hydrogen stable isotope ratio (δD) in the xylem and potential water sources using stable hydrogen isotope tracer method. We then compared the differences in water sources for M. squamosa growing under two contrasting hydrological conditions.
Important findings Myricaria squamosa plants growing on the river bank mainly used groundwater and water from the river stream in June and July, with groundwater contributing up to 89% of water use in June and 55% in July and river stream contributing up to 86% in June and 65% in July, respectively; whilst in August, they used water mainly from the 0–20 cm soil layer; the water source in September was identifiable. In contrast, Myricaria squamosa plants growing approximately about 100 m away from the river bank mainly accessed the groundwater and river water in June (91% and 70%, respectively), and used water from the 0–60 cm soil layer during the rainy months July, August and September. Results suggest that M. squamosa plants on the river bank use mainly groundwater and river water; soil water was more important for those far away from the river bank. These are resulted from the responses of this shrub species to specific water conditions when growing under contrasting water regimes.
Aims Water is the most important limiting factor for plant growth in desert ecosystems. Our objective was to investigate the water sources of three co-existing desert plants and illustrate seasonal variation characteristics in southeastern Junggar Desert in China.
Methods We investigated three kinds of desert plants (Tamarix ramosissima, Nitraria sibirica and Reaumuria soongorica) in the same habitat and measured hydrogen and oxygen stable isotope ratio (δD and δ18O) values of their xylem water and potential water sources (precipitation, soil moisture and ground water). The IsoSource model was then used to calculate probable contributions of potential water sources to total plant water uptake.
Important findings The water sources of the three desert plants had obvious seasonal characteristics. Reaumuria soongorica and N. tangutorum obtained a high proportion of water from shallow soil water (0–100 cm) in spring. However, during the summer and autumn, R. soongorica tended to use deeper soil water, and N. tangutorum tended to use ground water. Tamarix ramosissima obtained 90% of its water from deep soil water and ground water, and there were no seasonal variations. The three kinds of plants had different water sources closely related to their water use strategies. This shows desert shrubs, through self-regulation, could tend to their optimal phenotypes and maximize water uptake.
Aims As a key element in plant tissue, nitrogen plays an important role in the growth and development of plants. Our objective is to determine (1) how both leaf nitrogen and its allocation between photosynthetic and non-photosynthetic systems have responded to an altitudinal gradient and (2) what role their response has played in the adaptation of Rumex dentatus to its changed environment along the altitudinal gradient.
Methods We measured foliar parameters of photosynthesis, diffusional conductance to CO2, stable carbon isotope ratio (δ13C), nitrogen content and specific leaf area (SLA) of the forb R. dentatus in four sites with different altitudes (2 350, 2 700, 3 150 and 3 530 m) in the Wolong Reserve. One-way ANOVA was used to find the differences for all parameters among R. dentatus plants from different altitudes, and standardized major axis (SMA) was used to determine the relationships among some main parameters.
Important findings Leaf nitrogen content per area (Narea) increased as maximum photosynthetic capacity (Amax) increased with altitude in R. dentatus. Increased diffusional conductance also had a positive effect on increased photosynthetic capacity with altitude. These may be the result of adaptation of plants to a shortened leaf lifespan caused by low temperature at high altitude. Along with the altitudinal gradient, nitrogen and diffusional conductance of R. dentatus have an indirect effect on foliar δ13C through acting on the ratio of chloroplast partial pressure of CO2 to ambient CO2 partial pressure (Pc/Pa). Compared with diffusional conductance, nitrogen (or carboxylation capacity based on nitrogen) played a more important role in the process, in order to increase foliar δ13C with altitude. Rumex dentatus allocated more nitrogen to build defensive structural tissue with increased altitude. This is why SLA and photosynthetic nitrogen use efficiency (PNUE) decreased with altitude. In the photosynthetic system, more nitrogen was allocated to light-harvesting components in order that enhanced light resource was used preferably by R. dentatus, and then photosynthetic capacity was increased with altitude. Nitrogen and its allocation among systems (especially between photosynthetic and non-photosynthetic systems) are the keys to the adaptation and the response of R. dentatus to the gradient in altitude.
Aims We assess the relationship among the carbon isotopic signatures of earlywood (EW), transitional wood (TW) and latewood (LW) from tree rings. Our aims were to investigate variation in the intra-annual stable carbon isotope ratio (δ13C) in Pinus sylvestris var. mongolica and determine the relationship between them and homologous ring width.
Methods Based on two tree discs of Pinus sylvestris var. mongolica sampled from the northern part of Daxing’an Mountains in China, the EW, TW and LW were obtained with different stripping and pooling programs. After performing ring widths measurement and cross-dating, the periods analyzed were the maximum growth periods for one sample and different growth periods for the other. The holocellulose fractions were extracted and the intra-annual δ13C of samples were measured.
Important findings In general, the δ13C values of TW are the highest, EW come second and LW are the lowest. The intra-annual trend of δ13C is fluctuateing prominently from the juvenile period to the fast-growing period and is smoother from the maturation period to the senescence period. The variation amplitude of LW is almost greater than EW at the same period. The δ13C of LW is always prominently higher than EW for the juvenile period. The difference between EW and LW is indistinctive for the maturation period and is negligible for the senescence period. The intra-annual variability of δ13C concentrates on the middle and later phase of the growing season. The correlation relationship between the intra-annual δ13C sequences and homologous detrended ring width sequences (dRWS) decreases with the seasons, which implies that environmental factors play a dominant role in cell formation and carbon fractionation during the middle and later phase of the growing season in each year. The ring width of EW of the current year is positively correlated with LW of the previous year (pLW). Also the δ13C of EW is negative correlated with the incorporative dRWS of EW + pLW. But the correlation between δ13C of EW and δ13C or dRWS of pLW is statistically insignificant. The growing season could be divided as: EW (from late April to middle June, with greater soil moisture and rapidly increasing temperature), TW (from late June to middle July, with lower soil moisture and maximum temperature) and LW (from late July to middle September, with greater soil moisture and decreased temperature).
We add research content and several hypotheses and also amend some scientific questions in the field of seed geography. Three research areas that we expound upon are the geographies of soil seed banks, seed rain and seed dispersal syndromes. We summarize two hypotheses: 3-dimensional (latitude, longitude and altitude) variation hypotheses on soil seed banks and on seed dispersion modes. We also supplement the geography of seed chemical elements with seed isotope content. In the geography of seed mass, we mention elaiosomes. In the geography of seed morphology, we reaffirm use of length and width of seed and their ratio, absence or presence of seed (or fruit) wings, length and width of seed wings and their ratio, etc. In the geography of fruits and flowers, we add length and width of flower sepal and petal, as well as length, width and height of fruits and also percentage of fruit ripe in summer and autumn. In the geography of seed genetic characteristics, we add seed DNA content. In the geography of seed cell characteristics, we include seed starch grain. Overall, seed geography is a constantly developing field that should incorporate additional research from seed science and technology.
Aims Shallow groundwater table is the main cause of salinization in the northern Ningxia Plain of China. The uptake of groundwater by halophytes can lower the groundwater table and reduce the demand for irrigation. Our objective was to investigate the potential water sources (different soil profile and groundwater) of four shrubs, i.e., twenty-year-old Tamarix ramosissima and three-year-old T. ramosissima, Lycium barbarum and Atriplex canescens.
Method Stable 18O and D isotope compositions (δ18O and δD) of different potential water sources and xylem water were analyzed before and after irrigation. The IsoSource mode was used to calculate the probable contribution of different water sources to the total plant water uptake. The photosynthetic gas exchange parameters, contents of salt and water and pH in 0–200 cm soil profile were also determined.
Important findings Soil water δ18O and δD in different depths differed consistently. The soil water δ18O and δD values decreased with depth. More negative soil water δ18O and δD values were recorded after irrigation. There were significant effects of evaporation and irrigation on soil water δ18O and δD values. δ18O and δD values of twenty-year-old T. ramosissima were lower than that of the three-year-old shrubs. Before irrigation, the three-year-old T. ramosissima, L. barbarum and A. canescens mainly tapped water in the upper soil layer (70.1%, 52.3% and 48.9%); highest groundwater uptake rate (21.5%) was observed for twenty-year-old T. ramosissima. After irrigation, higher water uptake rates from 80–140 cm soil profile were recorded for three-year-old T. ramosissima (59.5%) and L. barbarum (58.8%). Highest groundwater uptake rate was found for twenty-year-old T. ramosissima (18.3%). Before irrigation, the net photosynthetic rate, stomatal conductance and transpiration rate of twenty-year-old T. ramosissima were higher than those of other shrubs. Highest net photosynthetic rate were observed for three-year-old A. canescens after irrigation. Irrigation had significant effects on net photosynthetic rate and stomatal conductance of three-year-old T. ramosissima and L. barbarum. The intrinsic water use efficiency of three-year-old A. canescens was higher than other shrubs. These results suggested that different shrubs have different water use strategies, which is presumably related to species and age of shrubs. Young T. ramosissima mainly extracted soil water from upper un-saturated profile based on its strong tolerance to drought, and it switched its water use to middle soil profile after irrigation. Mature T. ramosissima turned out to bephreatophytic with growth being heavily dependent on groundwater and deep soil water and unresponsive to irrigation.
Aims The δ18O of soil evaporation (δE) is an important factor controlling the variations of atmospheric δ18O (δv), and it is also one of the key challenges of partitioning evapotranspiration into evaporation and transpiration components. δE is mostly simulated by the Craig-Gordon model, which is constrained by the δv of water vapor, the relative humidity (h), the equilibrium and kinetic factors and the δ18O of soil water (δs) at the evaporating front. Our objective is to investigate the diurnal variations of δE and factors affecting it.
Methods We determined the δ18O of water vapor in a winter wheat-summer maize cropland based on the in-situ and continuous water vapor isotope ratio measurement system. We sampled soil water at different depths and analyzed it using the cryogenic vacuum distillation technique to acquire the δ18O of soil water at the evaporating front.
Important findings During the growing period of winter wheat-summer maize, the diurnal variation of δE exhibited a bimodal pattern with peaks at 6:00 and 15:00. The h has a significant effect on the diurnal variation of δE in cropland ecosystems, and causes the Craig-Gordon model to be invalid under high humidity condition of h > 95%. The in-situ and high resolution measurement of δv overcomes the uncertainty of using the local precipitation equilibrium method to evaluate δv, which improves the accuracy of δE. Different equilibrium factors have no significant influence on the accuracy of δE. Different kinetic factors, especially the canopy scale kinetic factor, influence the accuracy of δE significantly. The location of the evaporating front determines the h normalized to soil temperature and the δ18O of soil water directly and also influences the accuracy of δE significantly. Further research is needed to attain direct measurement of δE by combining isotope ratio infrared spectroscopy (IRIS) with the static chamber or dynamic chamber.
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 (δ15N) in ecosystem N status and soil N cycle, we summarized the effects and mechanisms of N input and other environment changes on δ15N of plant and soil. Most studies show significant positive relationships between N input and δ15N 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 δ15N 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 The study was to explore the effects of different ratios of straw to N-fertilizer on growth of Malus hupehensis seedlings and 15 N absorption, distribution and utilization in them.
Methods Using 15N trace technique, we measured growth parameters (plant height, basal stem diameter and dry weight) and 15N parameters (absorption, N derived from fertilizer, distribution and utilization) of two-year-old M. hupehensis seedlings and C/N ratio in soil organic matter under different straw-nitrogen fertilizer treatments.
Important findings Seedlings grew best under the condition of 45 : 1 of soil to straw and 300 mg·kg–1 N addition, showing the greatest height, basal stem diameter and total dry weight: 85.33 cm, 8.05 mm and 74.68 g, respectively. Total nitrogen, the 15N uptake and 15N utilization were also greatest: 0.938 g, 29.2 mg and 9.7%, respectively. The root/shoot ratio of seedlings without addition of straw (the control treatment) was 1.54, higher than those of other treatments. The 15N derived from fertilizer (Ndff) values of the aboveground parts for all the treatments were higher than those of the belowground parts, and the control showed the highest Ndff values in both aboveground and belowground parts, which were 7.94% and 4.69%, respectively. The 15N distribution ratios of the aboveground parts were higher than those of belowground parts in all treatments except the control. Straw additions could obviously increase the contents of organic matter and total nitrogen in soil and increase C/N ratio of soil organic matter. The correlation analysis showed that the C/N ratio of soil organic matter was significantly negatively correlated with the Ndff value of the belowground parts, as well as in the whole plant. Our findings suggest that it is better to apply nitrogen at rates between 200 and 300 mg·kg–1 when corn straw is added into orchard soil.
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 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 ordosica and 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. ordosica took 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.
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.
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 δ13C 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 δ13C value whether under dryland or irrigation, and the δ13C value increased with grain filling proceeding. Moreover, the δ13C value under dryland condition was higher than under limited irrigation. The correlation between δ13C 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 δ13C value can indicate WUE well whether under dryland or irrigated condition. Therefore, the δ13C value can be used by breeding programs as a potential selection criterion for grain yield and WUE in wheat.
Aims There were many studies about carbon isotope compositions (δ13C) of C3 plant in China, and δ13C have been widely used as the index of water use efficiency (WUE). But most of the studies focused on sites or small-scale regions, so the spatial pattern of δ13C, the relationships between δ13C and climate factors, and if δ13C can represent water use efficiency (WUE) in large-scale regions should be further studied. Methods Based on the collection of references, we studied leaf δ13C of 478 C3 species from 187 sites in China. Important findings The results showed that the range of δ13C was from -33.50‰ ~ -22.00‰, and the mean was - (27.10±1.70) ‰. There were significant difference between δ13C of grass, shrub and tree. δ13C of grass was highest, and that of tree was lowest. The result was different from other studies in sites and small-scale regions. For different phylogenic plants, δ13C of seed plant was significantly higher than fern, the difference between gymnosperm and angiosperm wasn’t significant, but monocotyledon was significantly higher than dicotyledon. Leaf δ13C had irregular variation with longitude increasing, but significantly increased with latitude increasing. Leaf δ13C significantly increased with mean annual temperature and mean annual precipitation decreasing. The relationship between δ13C and precipitation was similar to that of WUE and precipitation, so we can conclude that δ13C of C3 plant can be used as the index of WUE in large-scale regions as well as in sites or small-scale regions.
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 (Pn), transpiration rate (Tr) and leaf water potential (ψ) to enhance water use efficiency (WUE), or by changing allocation of biomass productivity (Bp). 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. Pn, stomatal conductance (Gs), Tr,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. Lower Pn, Gs, Tr,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.
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