Within the framework of traditional translatory flow, infiltrating precipitation and water at any soil depth is firstly well mixed and eventually enters the stream. Based on the dual stable isotope approach (δD and δ¹⁸O), recent studies showed that ecohydrologic separation occurs during the mixing process between precipitation and soil water. Namely, soil water has two pools: soil bound water which includes unavailable and available water used by plants, and soil mobile water entering the stream. The partial mixing of both water pools is defined as hydrologic connectivity. In this review, the concept and meaning of ecohydrologic separation are explained systematically. We described the mixing process between precipitation and soil water, and water isotopes (δD and δ¹⁸O) of soil bound water and mobile water in detail. We summarized the advantages and disadvantages of the direct and substitute methods to measure δD and δ¹⁸O in soil water, bound water, and mobile water. We reviewed the researches on hydrologic separation and connectivity of soil bound water and mobile water in runoff plot and watershed, including the qualitative research based on the direct and substitute methods, and the quantitative research using the models and control experiments. At last, we proposed that further studies should strengthen the research on the qualitative and quantitative methods of ecohydrological separation, and their influences on traditional ecohydrology models.

To understand the effects of organic fertilizer addition on accumulation and distribution of winter wheat assimilates, we compared organic carbon content in soil and wheat plants, photosynthetic characteristics and assimilation transformation under two different fertilizer treatments, CF (chemical fertilizer only) and OF (organic fertilizer with chemical fertilizer), using ¹³CO₂ pulse labeling method. The results showed that OF increased soil organic carbon content and wheat photosynthetic parameters, resulting in higher organic carbon content and total dry biomass accumulation in wheat plants compared with CF treatment. Compared marking to maturity with that on the 7th day after the same marking, the content and distribution rate of ¹³C in leaves and stems of two treatments decreased; the ¹³C content in spikes increased at jointing stage and filling stage, decreased at anthesis stage, and the ¹³C distribution rate increased at all stages. OF treatment was more beneficial for the transportation of photosynthetic product from leaves to spikes at filling stage, with higher ¹³C distribution rate in spikes compared with CF treatment. The results of correlation analysis showed that dry matter accumulation was significantly positively correlated with the net input of ¹³C content and its distribution rate, but negatively correlated with organic carbon content in plants. The net input of ¹³C content was significantly positively correlated with its distribution rate, and negatively correlated with F_v/F_m and P_n. In conclusion, organic fertilizer application could increase the content of organic carbon in soil, improve wheat plants photosynthetic capacity and transportation of photosynthetic product to spikes, and ultimately increase dry matter accumulation in wheat spikes.

It is necessary to examine the characteristics of hydrological cycle in Gaoyou area of Jiang-su Province in response to climate change and flood disasters. In this study, 121 atmospheric rain samples were collected and environmental factors were recorded from July 2015 to October 2017. We analyzed the hydrogen and oxygen stable isotopic composition of rain and identify moisture sources in this area. Results showed that the hydrogen and oxygen isotopic composition of rain had an seasonal variation, which was positive in the winter but negative in the summer. D-excess value was higher in winter than that in summer. On the annual scale, significantly negative relationships between δD (δ¹⁸O) and temperature and between δD (δ¹⁸O) and precipitation indicated the “anti-temperature effect” and “precipitation amount effect”, respectively. On the seasonal scale, there was no obvious “temperature effect” but “precipitation amount effect” in autumn and winter. Results from the HYSPLIT model showed that the precipitation in this area came mainly from ocean evaporation that was influenced by the Chinese South Sea, Indian Ocean and Pacific Ocean. Precipi-tation in other seasons mainly came from the water vapor mixture from the Eurasian continent, Atlantic Ocean, Arctic Ocean, as well as local evaporation. The seasonal pattern of δD and δ¹⁸O values in precipitation was mainly influenced by monsoon activity and El Niño-Southern Oscillation (ENSO). In addition, the precipitation isotopes clearly indicated the shift in climates from El Niño to La Niña.

Zooplankton plays a mediating role in the food web of aquatic ecosystems, the stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N) of which have been widely used to study the utilization of food resources, material cycling pathways, and trophic relationships. The δ¹³C and δ¹⁵N values of zooplankton have been used to predict primary productivity, sources and sinks of pollutants and environmental changes. To better use δ¹³C and δ¹⁵N of zooplankton as ecological and environmental indicators, it is particularly important to understand their temporal and spatial variations and the influencing factors. Based on related literature, we synthesized spatial and temporal variations in δ¹³C and δ¹⁵N of zooplankton in different aquatic ecosystems and taxa groups, and the use of δ¹³C and δ¹⁵N indicators for ecological processes and environmental changes. The δ¹³C and δ¹⁵N of zooplankton are largely affected by its food sources, and its stable isotope compositions are in turn affected by primary productivity and nitrogen sources. We proposed that the combination of δ¹³C and δ¹⁵N in zooplankton with transportation and transformation of emerging pollutants would form a multi-means, multi-disciplinary and multi-scale research direction in the fields of earth science and biology.

We used 10 representative provenances at different latitudes in two 33-year-old Pinus massoniana provenance test stands (Chun’an Laoshan in Zhejiang and Taizi Mountain Shilong in Hubei) to measure the differences among provenances, study the geographical variation, and identify its responses to hydrothermal factors. The results showed that the δ¹³C of the average annual rings was significantly different among the provenances of P. massoniana, being higher in trees from high latitude than from low latitude. The δ¹³C of average annual rings showed a zonal variation trend, indicating an adaptation to the original environment. The δ¹³C of annual rings was negatively correlated with mean annual temperature (MAT), mean temperature in January (T₁), annual precipitation (MAP), precipitation from May to September (P_5-9) and cumulative temperature above 10 ℃ (CT). It was positively correlated with the aridity index (AI). The response function of AI toδδ¹³C annual rings in Chun’an and Taizi Mountain explained 37.5% and 42.5% of the variation, respectively, indicating that AI was the most important limiting factor. The Taizi Mountain test site in Hubei Province located in the central and western region with relatively dry soil and high aridity. Therefore, the δ¹³C of the average annual ring was 1.8% higher than that in the Chun’an test site. The δ¹³C rings of P. massoniana at the Taizi Mountain and Chun’an were sensitive to the temperature in July and August, respectively. Summer precipitation was the main factor for carbon isotope fractionation with stable rings. The sensitivity of different provenances to future climate changes was different.

To enrich niche partition and species coexistence theory in karst seasonal rain forest, and provide reference for species selection and configuration for rocky desertification control, we exami-ned foliar stable carbon isotope composition (δ¹³C) and water use efficiency (WUE) of the representative species in different habitats and the same tree species in different habitats. The results showed that foliar δ¹³C value in karst seasonal rain forest ranged from -34.13‰ to -29.69‰, with a mean value of (-31.40±1.19)‰. WUE ranged from 9.08-58.76 μmol·mol^-1, with a mean value of 41.79 μmol·mol^-1. Both of them were lower than subtropical and warm temperate forests at higher latitude, but higher than tropical rain forests at lower latitude and non-karst seasonal rain forests at the same latitude. The foliar δ¹³C value and WUE of representative species gradually increased with the increases of drought index from the depression to the top of the mountain. The foliar δ¹³C value and WUE of the same tree species increased with altitude. These results indicated that water use efficiency of tree species was not only related to climate factors, but also related to the geological background and water availability of the habitat. The water use efficiency of plants in karst area was higher than that in non-karst area, and was higher in dry habitat than in wet habitat under the same climatic condition. It showed that having different water use efficiencies was one of the strategies for plants in karst area to adapt to different habitats and maintain species coexistence.

We used two types of soil with different physicochemical properties (loam and sand), oven-dried them, and then added the known isotopic composition mineral water that was reference water to compose the soil-water mixture with different soil water contents (loam: 0.15, 0.20, 0.30 g·g^-1; sand: 0.10 g·g^-1). After that, we set up different equilibrium time (loam: 3, 6, 12, 24, 48, 72, 96 h; sand: 96 h) to ensure that the dry soil particles were well mixed with the added water. The soil water was extracted by mechanical centrifugation and cryogenic vacuum extraction after equilibrium, and their isotope composition was analyzed. Results showed that the isotopic values of soil water extracted by mechanical centrifugation method had no significant difference in same water content with different equilibration times, but were more enriched compared with the reference water isotopic value. The maximum enrichment for hydrogen and oxygen isotope was 7.38‰ and 1.24‰, respectively. In contrast, cryogenic vacuum extraction method resulted in more depleted soil water isotopes than reference water, with the maximum depletion for hydrogen and oxygen isotope being 6.27‰ and 1.03‰, respectively. Moreover, the degree of depletion increased with the increases of equilibrium time (less than 24 h) at low water content, and became stable after 24 h. With the increases of soil water content, the isotopic composition of the extracted soil water was less affected by the two extraction methods. The water isotope value of loam that had high clay content, was more sensitive to the extraction method than the sandy soil that had low clay content. The difference of isotopic composition caused by extraction methods did not affect the plant water source segmentation.

Ammonium salts, including ammonium nitrate, ammonium sulfate and ammonium hydrogen sulfate, are the main components of secondary inorganic aerosols and play an important role in the formation of haze events. The sources and transformation processes of atmospheric ammonium have received more and more attention. In this study, we modified the previous stable isotope analysis technique by improving the injection volume and adding a pH adjustment step, which gave a rapid and accurate measurement of ammonium nitrogen isotope ratio in atmospheric aerosol samples. Firstly, we added alkaline hypobromite to the extracted solution of the atmospheric aerosol filter samples (0.25 μg·mL^-1 ammonium nitrogen in 4 mL) to oxidize ammonium (NH₄⁺) to nitrite (NO₂^-). Then, after adjusting the pH, nitrite (NO₂^-) was reduced to nitrous oxide (N₂O) by hydroxylamine hydrochloride under pH <0.3. Finally, nitrous oxide (N₂O) was analyzed by Precon-GasBench-IRMS system to measure ammonium nitrogen isotope ratio. Our approach required low amount of NH₄⁺ and avoided the use of highly toxic and explosive reagents. Meanwhile, the precision of our method could reach as high as 0.2‰ (n=10). This method could increase the NH₄⁺ reduction efficiency to 100% at a condition of pH <0.3 and satisfy the demands of precision and accuracy for determination of ammonium nitrogen isotope in atmospheric aerosol samples. This method would help us better understand the sources, evolutions, chemical and deposition processes of atmospheric ammonium.

To clarify the effects and underlying mechanism of ABA on sugar accumulation in apple fruits, ¹³C trace technique was used to examined the effects of different ABA levels (0, 50, 100 and 150 mg·L^-1) and fluoridone (ABA biosynthesis inhibitor) on translocation of photosynthate to fruit during late stage of fruit rapid-swelling in five-year-old ‘Yanfu3’/M26/Malus hupehensis Rehd. The results showed that the activities of related enzyme in sugar metabolism, the relative expression of sucrose transporter gene MdSUT1, MdSUT2.2 and sorbitol transporter gene MdSOT3 tended to increase first and then decrease with increasing ABA concentration, with a peak in 100 mg·L^-1ABA treatment. Fluridone treatment significantly inhibited the enzymes activities of sugar metabolism and the relative expression of sugar transporters. The treatment of 100 mg·L^-1ABA significantly reduced leaf ¹³C content, increased fruit ¹³C content and increased the transport rate of photosynthate from leaves to fruits compared with other treatments. Our results indicated that exogenous ABA enhanced sink strength of fruit and promoted the transportation of more photosynthate to fruits, which increased the soluble sugar content in fruits.

Hydroponic experiment was carried out on M9T337 seedlings using ¹⁵N and ¹³C isotope tracer technology to study the effects of different potassium supply levels (K₀, K₁, K₂, K₃ and K₄ were equivalent to 0, 3, 6, 9 and 12 mmol·L^-1, respectively) on the growth, photosynthetic characteristics and ¹⁵N and ¹³C absorption and utilization of M9T337 seedlings. The results showed that dry mass, root length, root surface area, number of tips and root activity of M9T337 seedlings under the K₂ level were significantly higher than those under other levels. The net photosynthetic rate (P_n) of leaves increased at low K⁺ concentration and then decreased with the increases of potassium supply level, and reached the maximum value at K₂ treatment (15.5 μmol CO₂·m^-2·s^-1). At the 30th day after treatment, the activities of nitrate reductase (NR) and carbon metabolism enzyme were highest in K₂ treatment, and lowest in K₀ treatment. With the increases of potassium application rate, the ¹³C accumulation of seedlings were first increased and then decreased, with the ¹³C distribution rate of each organ being the most balanced at K₂ treatment. There were significant differences in ¹⁵N uptake and utilization rate among treatments. ¹⁵N uptake and utilization rates of seedlings under K₂ treatment were the highest, which were 16.1 mg and 17.9%, respectively. Therefore, too low or too high potassium supply could inhibit seedling root growth and leaf photosynthesis, which was not conducive to carbon and nitrogen absorption. Appropriate potassium supply could improve root activity and net photosynthetic rate, enhance nitrate reductase (NR) and carbon metabolic enzyme activity, and promote carbon and nitrogen metabolism.

Priming effect is one of the important mechanisms regulating soil organic matter decomposition. However, the variation of priming effects in different soil layers remains unclear. In this study, we conducted a 30-day incubation experiment using no-tillage black soil from northeastern China. ¹³C-glucose and dynamic CO₂ trapping methods were employed to investigate soil organic carbon (SOC) mineralization rates and the priming effect of the added ¹³C-glucose in the upper soil layer (0-10 cm) and the lower soil layer (30-40 cm). Our results showed that the cumulative SOC-specific mineralization rate in the upper layer was similar to that in the lower layer soil without glucose addition. Glucose addition significantly altered the mineralization rates in both layers, resulting in a positive priming effect (36.7%) in the upper layer but a negative priming effect (-12.4%) in the lower layer. The cumulative priming effect during the 30-day incubation was 3.24 mg C·g^-1 SOC for the upper layer soil and -1.24 mg C·g^-1 SOC for the lower layer soil. There was still a net SOC increase, even with positive priming effects in the upper layer soil. This was due to considerable amount of added glucose-C remained un-mineralized in the soil which would compensate the carbon loss from priming effects. Overall, our results demonstrated that the magnitude and direction of priming effects might differ between soil layers. Our findings contribute to a better understanding of the effects of conservation tillage practices (no-tillage and straw incorporation) on soil organic matter dynamics in agroecosystems.

With the aim to understand the response of different nitrogen forms in paddy soil under the conditions of urea combined with inhibitors and pig manure, and to explore the nitrogen retention and supply capacity of paddy soil under different management strategy, we conducted a pot experiment with ¹⁵N labeled urea. There were six treatments: no nitrogen fertilizer (CK), pig manure (M), urea (N), urea+pig manure (NM), urea+inhibitor (NI), urea+inhibitor+pig manure (NIM). Urease inhibitor (PPD+NBPT) and nitrification inhibitor (DMPP) were used as the inhibitor combination. Soil nitrogen pools, conservation of ¹⁵N labeled urea, and rice N adsorption were measured in rice seedling, tillering, and mature stages. Results showed that pig manure significantly increased soil ammonium concentration, soil microbial biomass nitrogen and fixed ammonium, as well as the storage of urea nitrogen in various pools at tillering stage, and significantly increased rice yield. Addition of the inhibitors increased NH₄⁺ fixation by clay minerals and nitrogen immobilization by microorganisms compared with treatment N, and increased urea-derived NH₄⁺ fixation by clay minerals compared with treatment NM. Pathway analysis showed that pig manure increased urea-N assimilation and yield of rice. The urea-derived ammonium fixed by clay minerals was temporarily stored after inhibitors application. NIM treatment stored more N in microbial biomass, and the released ammonium coupled the turnover and mineralization of microbial provided more available nitrogen for the later growth of rice. Both NM and NIM treatments are recommended in paddy fields of north China.

Deuterium (D) and oxygen-18 (¹⁸O) are common environmental tracers in water. Understanding the isotopic compositions of precipitation is necessary for further studies on local and global water cycling processes. To reveal the mechanism of isotopic compositions of precipitation in subtropical monsoon region in response to environmental changes, we collected 49 precipitation samples and recorded related environmental factors from May 2017 to August 2019 in Huitong field station of Chinese Academy of Sciences in Hunan Province. We analyzed the temporal variations in D and ¹⁸O values in precipitation and analyzed the influence of water vapor source and local environmental factor on stable isotopic compositions of precipitation. The local meteoric water line was established as δD=(7.45±0.17)δ¹⁸O+(10.10±1.25) (R²=0.93, P<0.01), the slope of which was slightly lower than China’s meteoric water line and the global meteoric line. The D and ¹⁸O values of precipitation samples were closely coupled with local meteorological conditions and dominant moisture sources. The ¹⁸O and D contents were depleted during summer monsoon season but enriched during winter monsoon season. During the summer monsoon and post-monsoon seasons, precipitation in this area came mainly from the Bay of Bengal, the South China Sea, and the West Pacific at three different types of air pressure, which lead to the similarity of the D-excess value of the precipitation to global average. The lower intercept of meteoric water line and the higher D-excess value for precipitation during the winter monsoon season resulted from moisture from remote westerly air masses, degenerated tropical marine air masses from the Bay of Bengal, and inland moisture in the pre-monsoon period, which were also affected by local environmental factors.

Analysis of plant photosynthesis and post-photosynthetic fractionation can improve our understanding of plant physiology and water management. By measuring δ¹³C in the atmosphere, and δ¹³C of soluble compounds in leaves and branch phloem of Platycladus orientalis, we examined discrimination pattern, including atmosphere-leaf discrimination during photosynthesis (ΔC_a-leaf) and leaf-twig discrimination during post-photosynthesis (ΔC_leaf-phlo), in response to changes of soil water content (SWC) and atmospheric CO₂ concentration (C_a). The results showed that ΔC_a-leaf reached a maximum of 13.06‰ at 95%-100% field water-holding capacity (FC) and C_a 400 μmol·mol^-1, and a minimum of 8.63‰ at 35%-45% FC and C_a 800 μmol·mol^-1. Both stomatal conductance and mesophyll cell conductance showed a significant linear positive correlation with ΔC_a-leaf, with a correlation coefficient of 0.43 and 0.44, respectively. ΔC_leaf-phlo was not affected by SWC and C_a. Our results provide mechanism of carbon isotopes fractionation and a theoretical basis for plant survival strategies in response to future climate change.

The flux and form of dissolved inorganic carbon (DIC), an important part of carbon budget, play a key role in the biogeochemistry of aquatic ecosystem. By analyzing physicochemical parameters and water DIC and δ¹³C_DIC characteristics in Wanfenghu Reservoir, we examined the behavior and source of DIC. In the epilimnion, water pH in the entire reservoir was conservative, being weakly alkaline. Nitrate (NO₃^--N) had the maximum coefficient of variation and a high spatio-temporal variation. Due to the dilution effect, the lowest values of electrical conductivity (EC), partial pressure of carbon dioxide (pCO₂) and DIC appeared during the summer high flow phase. On the water column in summer, redox potential (Eh) and NO₃^--N did not change with water depth, while other indicators changed significantly, with greatest variation in the thermocline. Water temperature (T), pH and Eh all decreased with increasing water depth in both seasons, while pCO₂ showed an opposite trend. Water EC, total alkalinity (TA), and DIC decreased with increa-sing water depth in summer, but with a smaller gradient of change in winter. The DIC in water was negatively correlated with water pH and Eh, while positively correlated with EC and pCO₂ in both seasons. 2) The concentration of DIC was 2.66-4.9 mmol·L^-1 in summer and 3.38-4.52 mmol·L^-1 in winter. During the period of thermal stratification, the variation gradients of DIC and δ¹³C_DIC in the thermocline were most significant. DIC was positively correlated with δ¹³C_DIC of epilimnion in summer. DIC was negatively correlated with δ¹³C_DIC in epilimnion in winter and on water column in both summer and winter. However, the variation of DIC and δ¹³C_DIC with water depth was not obvious in winter. 3) In summer, δ¹³C_DIC was -7.71‰- -1.38‰, indicating that the dissolution of carbonate minerals was dominant. In winter, δ¹³C_DIC was -16.93‰- -9.44‰, signifi-cantly lower than that in summer but with a wider range, indicating biological input of CO₂ and mineralization of organic matter were the main sources. The δ¹³C_DIC varied significantly in different seasons and water depths because of differences in carbon sources and changes in the relative contribution proportion of carbon sources.

Water availability is the key factor limiting plant growth in arid regions. Populus simonii is a typical shelterbelt tree species in Zhangbei County, Hebei Province, with an important role in constructing ecological barrier. With stable isotope technique, graphical method, and multiple linear mixing model, we analyzed water sources and water use strategies of P. simonii in different growth periods with four different degrees of degradation (non-degraded, slightly degraded, modera-tely degraded and severely degraded) in Zhangbei County. Results would help improve our understanding on the cause and mechanism of the large-scale degradation of P. simonii in this area. The results showed that water sources of P. simonii in the early growth stage (May-June) from all four degradation degrees were relatively simple. P. simonii mainly used soil water in 0-40 cm, with the utilization rates being 34.2%, 50.1%, 41.6%, and 55.7% for the four degradation degrees, respectively. At the middle growth stage (July-August), non-degraded P. simonii utilized soil water from layers of 200-280 cm and 280-400 cm, with utilization rates of 20.2% and 30.9%, respectively. Soil water at 200-280 cm and 280-400 cm layers was utilized by slightly degraded poplar, with the contribution rates of each layer being 33.2% and 27.9%, respectively. Moderately degraded P. simonii utilized soil water from the depths of 0-40 cm and 40-120 cm, with the rates of 30% and 26.9%, respectively. Water utilization rate of severely degraded P. simonii to 0-40 cm depth was 55.4%. At the late growth stage (September-October), water sources of non-degraded P. simonii transferred to the upper-middle soil layers, with the utilization rate of 0-40 cm, 40-80 cm, and 80-120 cm being 23.3%, 17.2%, and 16.5%, respectively. The utilization rate of the slightly degraded P. simonii was 35.7% at 0-40 cm and 20.6% at 80-200 cm. The moderately and severely degraded P. simonii mainly utilized soil water at 0-40 cm layer, with the contribution rates of soil water being 43.7% and 51.8%, respectively. With the exacerbation of degradation, the main water source of P. simonii gradually transferred from deep to surface soil water.

Plants could maintain growth by foliar water uptake and reverse sap flow under certain conditions, particularly in regions with seasonal drought. This physiological activity is often overlooked, however, leaving a gap in quantitatively understanding the processes and mechanisms underlying water utilization of forest vegetation under drought stress. In this study, with both field comparison experiments and pot experiments, we used heat ratio method with stable isotope technique to monitor a typical plantation tree species, Platycladus orientalis, in the Beijing mountainous area. We aimed to analyze the patterns and the influencing factors of the reverse sap flow occurrence in P. orientalis, to quantify the amount and the replenishment rate of reverse sap flow, and to examine the characteristics and processes of reverse sap flow at different parts of plants. In the field comparison experiment, reverse sap flow was detected at the breast height of stem and in the root in the controlled plot (drought plot) after rainfall. The reverse sap flow of root system was detected later than that in the stem. By contrast, no reverse sap flow was observed in the natural plot. In the pot experiments, the recharge rate of all the groups reached the peak value two hours after the rainfall treatment. Except for the groups of severe and moderate drought, recovery of δD to the original level was observed eight hours after rainfall, and the reverse sap flow on plants generally lasted no more than 24 h. The amount of foliar water uptake and the reverse sap flow to the branches and rhizosphere soil had a negative relationship with the initial soil moisture. The maximum recharge rates for leaves, branches, and rhizosphere soil were (9.5±0.1)%, (5.9±0.3)% and (5.7%±0.6)%, respectively. Different rates and timing of the reverse sap flow were observed at different parts of P. orientalis. Under complex and variable conditions of water supply, it is of great significance to examine the process and mechanism of reverse water movement of plants to better understand its survival and competitive strategies.

As plant species for riparian ecological restoration in northern China, Tamarix ramosissima and Salix matsudana play an important role in river protection, flood control, regional climate regulation, and landscape construction of riparian vegetation. Two sampling sites were selected in the riparian zones along the Lanzhou section of Yellow River, where plant xylems and potential water sources were collected. The direct comparison method, Bayesian mixture model MixSIAR and the proportional similarity index (PS index) were used to determine the proportions of water utilization for each potential water source and the relationship of two species in water utilization. The results showed that shallow soil (0-30 cm) was the main water source during growing season, with utilization ratio being 28.3% for T. ramosissima and 24.4% for S. matsudana. For T. ramosissima, river water had the lowest contribution (16.6%), and for S. matsudana, groundwater contributed the least (17.9%). In the months with low soil moisture, plants increased the utilization ratios of river water and groundwater. The PS index at the sampling site S₁ and S₂ was 91.0% and 87.7%, respectively. On a monthly basis, the index in May was the highest, indicating an inter-month divergence in water use relationship. At the floodplain, there were even utilization ratios for each potential water source, which is an optimal strategy to obtain water from each potential source to the maximum extent. Our results provided theoretical basis for riparian tourism development along the Lanzhou section of the Yellow River and plant water management in environment protection in the Yellow River Basin.

Clarifying ¹⁸O isotope composition of leaf water (δ_L,b) would provide theoretical refe-rence for the study of leaf physiology and forest hydrology. We continuously monitored the concentration of atmospheric water vapor (W_a) and ¹⁸O isotope composition of atmospheric water vapor (δ_v) at the canopy of Platycladus lateralis plantation in the mountain area of Beijing. We analyzed the effects of kinetic fractionation coefficients ε_k1(32%) and ε_k2(28%) on the prediction of δ_L,b by combining the measured leaf water ¹⁸O isotope (δ_x) and δ_L,b of P. lateralis. The results showed that the diurnal variation of W_a was irregular. Atmospheric relative humidity (RH) showed a “V” shape of diurnal variation, and stomatal conductance (g_s) increased first and then decreased at the diurnal scale. W_a, RH, and g_s showed a significant negative correlation with δ_L,b when isotopes approached a steady-state equilibrium around noon. The kinetic fractionation coefficient ε_k1 and ε_k2 were applied to the Craig-Gordon model to predict δ_L,b under the isotopic quasi-steady-state condition. The results showed that the predicted values of ε_k2 approached the observed values of δ_L,b. This result indicated that the application of ε_k2 to the model was more consistent with the change of water isotope concentration in the leaves of P. lateralis in the mountain area of Beijing. These results would improve our understanding of water isotope enrichment model and evapotranspiration resolution model in leaves.

Based on stable carbon isotope, we quantitatively partitioned ecosystem respiration in a Platycladus orientalis forest in the west mountainous area of Beijing. Results from this study could lay the foundation for carbon exchange research in forest ecosystems of this region. The spectroscopy technique was used to continuously measure CO₂ concentrations and δ¹³C values at different height of the forest. Soil and branch chambers were used for measuring nighttime δ¹³C values in underground and aboveground respiration, and then the proportions of respiration components were calculated. Combined with soil respiration efflux measurement, ecosystem respiration was then quantitatively partitioned. The results showed that δ¹³C values of respiratory components fluctuated, which ranged from -31.74‰ to -23.33‰ in aboveground respiration of plants and from -32.11‰ to -27.74‰ in soil respiration. The δ¹³C values of ecosystem respiration was at the middle of those ranges. Soil respiration averaged 1.70 μmol·m^-2·s^-1 at night, accounting for 47%-91% of ecosystem respiration. Aboveground respiration averaged 0.72 μmol·m^-2·s^-1, contributing less to ecosystem respiration. Daytime respiration based on isotope mixing model calculation had greater variability than that based on temperature response model, with a mean value of 2.31 μmol·m^-2·s^-1 and 2.28 μmol·m^-2·s^-1, respectively.