Understanding the changes of natural abundance of stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N) along soil profile is of great importance in revealing the mechanisms of soil carbon and nitrogen cycling in terrestrial ecosystems. Based on a comprehensive review on the distribution of δ¹³C and δ¹⁵N along soil profile, the mechanisms underlying their vertical distribution were mainly introduced here. There were three mechanisms driving the δ¹³C vertical distribution in soil profile: 1) historical changes of vegetation δ¹³C value, 2) changes of C3-C4 species dominance in plant communities, 3) accumulation of ¹³C-enriched microbial-derived carbon during decomposition. The effects of ¹³C Suess effect on the vertical distribution of δ¹³C in soil profile were also discussed. There were four mechanisms underlying the vertical distribution of δ¹⁵N in soil profile: 1) ¹⁵N-depletion gas loss during denitrification, 2) accumulation of ¹⁵N-enriched microbial-derived nitrogen during decomposition, 3) accumulation of ¹⁵N-encriched mycorrhizal fungi residues in deep soil as a result of transferring ¹⁵N-depleted nitrogen compounds to plants by mycorrhizae, 4) intera-ction between soil organic matter and mineral substance. We proposed important concerning points for the future study on vertical distribution of natural abundance of stable carbon and nitrogen isotopes in soil profile.

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.

Mesophyll conductance (g_m), the total conductance of CO₂ diffusion from substomatal cavity to the site of carboxylation within chloroplast, is a major limiting factor for photosynthesis and a key parameter for improving photosynthetic resource use efficiency of crops. Online ¹³C discrimination method is an important method for plant eco-physiological studies and a well-established method for measuring g_m of C3 plants, although it has not been widely used due to challenges in methodology and high demands on experimental facilities. In this review, we summarized the characteristics of commonly used methods for g_m, introduced the basic theory of the online ¹³C discrimination method, namely Farquhar’s photosynthetic ¹³C discrimination model; systematically introduced the practical measurements, equations and the components of facilities; and reviewed the drivers for variation in g_m of C3 plants. At the last part, we discussed the outlook of the development of methodology, new experimental protocols, and applications in measurement scenarios.

The trophic dynamics of wetland organisms is the basis of assessing wetland structure and function. Stable isotopes of carbon and nitrogen have been widely applied to identify trophic relationships in food source, food composition and food web transport in wetland ecosystem studies. This paper provided an overall review about the current methodology of isotope mixing model and trophic level in wetland ecosystems, and discussed the standards of trophic fractionation and baseline. Moreover, we characterized the typical food sources and isotopic compositions of wetland ecosystems, summarized the food sources in different trophic levels of herbivores, omnivores and carnivores based on stable isotopic analyses. We also discussed the limitations of stable isotopes in tra-cing food sources and in constructing food webs. Based on the current results, development trends and upcoming requirements, future studies should focus on sample treatment, conservation and trophic enrichment measurement in the wetland food web, as well as on combing a variety of methodologies including traditional stomach stuffing, molecular markers, and multiple isotopes.

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.

We examined nitrogen use efficiency of ¹⁵N-urea of 15-year-old ‘Gala’ apple trees by broadcast fertilization (T₁), nitrogen fertilization with drip irrigation (T₂) and nitrogen fertilization with subsurface irrigation (T₃), to further improve the water and fertilizer integration technology for apple orchard and to improve nitrogen utilization efficiency. The results showed that leaf area, chlorophyll, and leaf nitrogen content of leaves were significantly higher in T₃ treatment than those in T₁ and T₂ treatments. Soil mineral nitrogen content (20-40 cm) in each period followed the order of T₃>T₂>T₁, while that in 0-20 cm followed an order of T₂>T₃>T₁. The Ndff value at organ level (the contribution rate of the ¹⁵N amount absorbed by various organs of the tree to the total nitrogen content of the organ) was the highest in T₃ treatment in each period, followed by T₂ and T₁ treatments. The utilization rate of ¹⁵N in the fruit ripening period followed an order of T₃>T₂>T₁. The ¹⁵N utilization rate of T₃ reached 24.2%, being 1.19 and 1.65 times of T₂ and T₁, respectively. The ¹⁵N distribution rate in the fruits during the fruit maturity stage was the highest in T₁ treatment, while that in the storage organs was the highest in T₂ treatment and that in the reproduction organs was the highest in T₃ treatment. The single fruit weight, yield, soluble solids, hardness, soluble sugar and sugar-acid ratio were the highest in T₃ treatment, followed by T₂ and T₁ treatments. In summary, nitrogen application by percolation irrigation (subsurface application) significantly promoted leaf growth and nitrogen utilization of apple tree, and improved fruit yield and quality.

To investigate the differences of soil carbon, nitrogen, and carbon stable isotopes in Zoige wetland, we analyzed the abundance of carbon stable isotopes and the contents of carbon and nitrogen in soils of three kinds of retrogressive successional grasslands (swampy meadow, steppe meadow, and degraded meadow). The results showed that the δ¹³C values in the meadow soil of Zoige alpine wetland ranged from -26.21‰ to -24.72‰. The abundance of δ¹³C increased with the depth of soil. There was a significantly negative relationship between δ¹³C values and logarithm of soil organic carbon content across the soil profile. The δ¹³C values in the surface soil (0-10 cm) followed the order of grassland meadow>degraded meadow>swampy meadow, and the β values followed the order of grassland meadow>swampy meadow>degraded meadow. Soil organic carbon content was 105.32, 42.11 and 31.12 g·kg^-1, while nitrogen content was 8.74, 3.41 and 2.81 g·kg^-1, and C/N was 11.26, 11.23 and 10.89, in the swampy meadow, grassland meadow and degraded meadow, respectively. The soil C/N was lower in degraded meadow than that in swampy meadow and grassland meadow. The total organic carbon and nitrogen content decreased obviously with increasing soil depth. The variations of soil δ¹³C among meadows with different degradation stages were mainly restricted to the 0-10 cm soil layer. The β values and C/N were the lowest in degraded meadow among three stages, suggesting that the mineralization rate of degraded meadow soil organic matter was relatively fast.

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.

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.

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.

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.

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.

Soil-vegetation-atmosphere continuum (SPAC) is one of the important research objects in the field of terrestrial hydrology, ecology and global change. The process of water and carbon cycling, and their coupling mechanism are frontier issues. With characteristics of tracing, integration and indication, stable isotope techniques contribute to the estimation of the relationship between carbon sequestration and water consumption in ecosystems. In this review, based on a brief introduction of stable isotope principles and techniques, the applications of stable isotope techniques to water and carbon exchange in SPAC using optical stable isotope techniques were mainly explained, including: partitioning of net carbon exchange into photosynthesis and respiration; partitioning of evapotranspiration into transpiration and evaporation; coupling of water and carbon cycle at the ecosystem scale. Advanced techniques and methods provided long-term and high frequency measurements for isotope signals at the ecosystem scale, but the issues about the precision and accuracy for measurements, partitioning of ecosystem respiration, adaptability for models under non-steady state, scaling up, coupling mechanism of water and carbon cycles, were challenging. The main existing research findings, limitations and future research prospects were discussed, which might help new research and technology development in the field of stable isotope ecology.

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.

In the past several decades, a variety of methods have been developed for measuring the isotopic composition of ammonium (δ¹⁵N) and nitrate (δ¹⁵N and δ¹⁸O). This review summarized the advantages and disadvantages of these methods. Nowadays, the most popular method for measu-ring δ¹⁵N of ammonium is the combined hypobromite (BrO^-) and hydroxylamine (NH₂OH) me-thod, while for δ¹⁵N and δ¹⁸O of nitrate is the denitrifier method and the sodium azide (NaN₃) me-thod. These methods convert NH₄⁺ or NO₃^- into nitrous oxide (N₂O) and measure its isotopic compositions, with higher analytical precision because of the lower background concentration of atmospheric N₂O. Accordingly, these methods are suitable for the samples with lower N concentration, and normally require 10-60 nmol N. The development of new methods for measuring N isotopic composition has greatly stimulated the studies in nitrogen cycling worldwide.

Organic nitrogen can play an important role in plant growth, and soil pH changed greatly due to the over-use of chemical fertilizers, but the effects of soil pH on the competitive uptake of amino acids by plants and rhizosphere microorganisms are lack of detailed research. To study the effects of soil pH on the uptake of amino acids by maize and soil microorganisms, two soils from Hangzhou and Tieling were selected, and the soil pH was changed by the electrokinesis, then the ¹⁵N-labeled glycine was injected to the centrifuge tube with a short-term uptake of 4 h. Soil pH had a significant effect on the shoot and root biomass, and the optimal pH for maize shoot growth was 6.48 for Hangzhou red soil, while it was 7.65 for Tieling brown soil. For Hangzhou soil, the ¹⁵N abundance of maize shoots under pH=6.48 was significantly higher than under other treatments, and the uptake amount of ¹⁵N-glycine was also much higher. However, the ¹⁵N abundance of maize shoots and roots under pH=7.65 Tieling soil was significantly lower than it under pH=5.78, but the uptake amount of ¹⁵N-glycine under pH=7.65 was much higher. The microbial biomass C was much higher in pH=6.48 Hangzhou soil, while it was much lower in pH=7.65 Tieling soil. According to the results of root uptake, root to shoot transportation, and the competition with microorganisms, we suggested that although facing the fierce competition with microorganisms, the maize grown in pH=6.48 Hangzhou soil increased the uptake of glycine by increasing its root uptake and root to shoot transportation. While in pH=7.65 Tieling soil, the activity of microorganisms was decreased, which decreased the competition with maize for glycine, and increased the uptake of glycine by maize.

As one of three major greenhouse gases, nitrous oxide (N₂O) has solicited substantial attention. Stable isotope has been widely used to explore the sources of N₂O emissions. Here, we briefly introduced the microbial processes involved in N₂O emissions, and the main influencing factors. We further summarized the development of N₂O isotope signature of δ¹⁵N, δ¹⁸O and SP (site preference of ¹⁵N in different positions of N₂O molecule) in exploring the N₂O formation mechanism. The application of these techniques, especially the SP values, is still at the primary stage in China. Therefore, this paper focused on the development of the isotope ratios analysis in partitioning N₂O sources from foreign countries, and put forward suggestions on the future research in China.

To examine the allocation of rice photosynthates and its response to the elevated CO₂ (800 μL·L^-1) and N fertilization (100 mg·kg^-1) at both tillering stage and booting stage in plant-soil system, rice was continually labelled with ¹³CO₂. The results showed that the rice root biomass at the tillering stage and the shoot biomass at the booting stage were significantly increased under elevated CO₂. Elevated CO₂ increased the rice biomass and root-shoot ratio at tillering stage, but reduced it at booting stage. Under elevated CO₂, N fertilization promoted shoot biomass during rice growth, but significantly decreased the root biomass at booting stage. Elevated CO₂ significantly increased the allocation of assimilated ¹³C to the soil at the booting stage. N fertilization did not promote the elevated CO₂-induced stimulation of assimilated ¹³C allocated to the soil, and it even decreased the proportion of assimilated ¹³C in the soil. In summary, elevated CO₂ increased the photosynthetic C allocation into soil and promoted the turnover of soil organic carbon in paddy soil. N fertilization enhanced rice shoot biomass but decreased the belowground allocation of photosynthetic C.

Engraulis japonicus, an important fishery resource, is a key species in ecosystem trophodynamics studies. In this study, we examined stomach content of E. japonicusby stable isotope analyses, with samples collected from the East China Sea in 2008-2009 and 2020. The aim of this study was to demonstrate their diet composition, diel and ontogenetic changes in feeding habits and trophic level. Results of the stomach content analysis showed that E. japonicus mainly fed on planktonic crustaceans and small fish. The main prey species were Euphausia pacifica [index of relative importance (IRI)=87.6%; frequency(F)=57.6%], Paracalanus parvus (IRI=3.2%, F=15.3%), and Themisto gracilipes (IRI=2.1%, F=13.1%). Results of the stable isotope analysis showed that Copepoda were the main food source of E. japonicus, followed by Euphausiacea, and the contribution rate of Amphipoda was the least, which was less than 1%. There was significant diel change in diet composition. Feeding intensity was higher in the daytime than at night, with the highest in the dusk and the lowest at midnight. Ontogenetic change in feeding habit occurred when fork length reached 90 mm, over which the fish fed both zooplankton and small fishes. The δ¹³C of E. japonicus was between -21.66‰ and -18.14‰, with an average of (-19.92±0.86)‰. The δ¹⁵N of E. japonicus ranged from 4.07‰ to 10.78‰, with an average of (8.14±2.48)‰. Both δ¹³C and δ¹⁵N values were positively correlated with fork length. Trophic level of the fish was 3.4 with stomach content analysis and 2.7 with stable isotope analysis. The results would provide important reference for understanding nutritional status of pelagic small fish, and offer some basic data to establish ecopath model.

Elevated atmospheric CO₂ affects plant photosynthesis process and biomass accumulation, furthermore alters the distribution of photosynthetic carbon (C) above- and below-ground. The formation and turnover of soil organic carbon (SOC) depends on the input of photosynthetic C, so the change of plant physiology and metabolism caused by increasing CO₂ concentration will further affect the balance of SOC pool. Therefore, stable isotope ¹³C technique is powerful for clarifying the influence of elevated atmospheric CO₂ on C cycling in plant-soil system, including the distribution of photosynthetic C among plant organs, and the transformation and accumulation of photosynthetic C in soil. This review summarized research focused on the effects of elevated atmospheric CO₂ on C cycling in terrestrial ecosystems based on ¹³C natural abundance or ¹³C tracing technique, including: 1) isotopic fractionation effect in plant photosynthesis; 2) the distribution of photosynthetic C in plant organs; 3) the transformation and stabilization of photosynthetic C in SOC driven by microbial process. Clarifying the above processes and controlling mechanisms is essential to predict long-term influence of elevated CO₂ on C cycling and evaluate the source-sink function of SOC in terrestrial ecosystems.