Tracing nitrate sources in one of the world's largest eutrophicated bays(Hangzhou Bay):insights from nitrogen and oxygen isotopes

Eutrophication caused by inputs of excess nitrogen(N) has become a serious environmental problem in Hangzhou Bay(China),but the sources of this nitrogen are not well understood.In this study,the August 2019 distributions of salinity,nutrients [nitrate(NO_(3)^(-)),nitrite,ammonium,and phosphate],and the stable isotopic composition of NO_(3)^(-)(δ^(15)N and δ^(18)O) were used to investigate sources of dissolved inorganic nitrogen(DIN) to Hangzhou B ay.Spatial distributions of nitrate,salinity,and nitrate δ^(18)O indicate that the Qiantang River,the Changjiang River,and nearshore coastal waters may all contribute nitrate to the bay.Based on the isotopic compositions of nitrate in these potential source waters and conservative mixing of nitrate in our study area,we suggest that the NO_(3)^(- )in Hangzhou B ay was likely derived mainly from soils,synthetic N fertilizer,and manure and sewage.End-member modeling indicates that in the upper half of the bay,the Qiantang River was a very important DIN source,possibly contributing more than 50% of DIN in the bay head area.In the lower half of the bay,DIN was sourced mainly from strongly intruding coastal water.DIN coming directly from the Changjiang River made a relatively small contribution to Hangzhou Bay DIN in August 2019.

Sources and transformations of nitrite in the Amundsen Sea in summer 2019 and 2020 as revealed by nitrogen and oxygen isotopes

In this study,the nitrogen and oxygen isotope compositions of nitrite in the upper 150 m water column of the Amundsen Sea in the summer of 2019 and 2020 were measured to reveal the distribution and transformation of nitrite in the euphotic zone of the Southern Ocean.We found that primary nitrite maxima(PNMs)are widely present in the Amundsen Sea,where the depth of occurrence deepens from east to west and nitrite concentrations increases.Evidence from dual isotopes suggests that the formation of PNMs in all regions of the Amundsen Sea is dominated by ammonia oxidation.More importantly,the nitrogen and oxygen isotope compositions of nitrite in the Amundsen Sea mixed layer are abnormal,and their depth profiles are mirror symmetrical.Isotopic anomalies exhibit spatial variations,with central surface water having the lowest nitrogen isotope composition(−89.9‰±0.2‰)and western surface water having the highest oxygen isotope composition(63.3‰±0.3‰).Isotopic exchange reaction between nitrate and nitrite is responsible for these isotope anomalies,as both nitrogen and oxygen isotopes have large isotopic fractionation and opposite enrichment effects.This proves that isotopic exchange reaction operates extensively in different regions of the Amundsen Sea.Our study highlights the unique role of dual isotopes of nitrite in deepening the understanding of nitrogen cycle.Further studies on ammonia oxidation and isotopic exchange between nitrate and nitrite are warranted in the future to understand their roles in the nitrogen cycle in the Southern Ocean.

Isotope constraints on seasonal dynamics of nitrogen in Zhanjiang Bay, a typical mariculture bay in South China

Eutrophication in coastal waters has been increasing remarkably,severely impacting the water quality in mariculture bays.In this study,we conducted multiple isotopic measurements on suspended particulate nitrogen(δ^(15)N-PN) and dissolved nitrate(δ^(15)N-NO_(3)^(-)and δ^(18)O-NO_(3)^(-)) in Zhanjiang Bay,a typical mariculture bay with a high level of eutrophication in South China,to investigate the changes in nitrogen sources and their cycling between the rainy and dry seasons.During the rainy season,the study found no significant relation between δ^(15)NPN and δ^(15)N-NO_(3)^(-)due to the impact of heavy rainfall and terrestrial erosion.In the upper bay,a slight nitrate loss and slightly higher δ_(15)N-NO_(3)^(-)and δ^(18)O-NO_(3)^(-)values were observed,attributed to intense physical sedimentwater interactions.Despite some fluctuations,nitrate concentrations in the lower bay mainly aligned with the theoretical mixing line during the rainy season,suggesting that nitrate was primarily influenced by terrestrial erosion and that nitrate isotopes resembled the source.Consequently,the isotopic values of nitrate can be used for source apportionment in the rainy season.The results indicated that soil nitrogen(36%) and manure and sewage(33%) were the predominant nitrogen sources contributing to nitrogen loads during this period.In contrast,the dry season saw a deficient ammonium concentration(<0.2 μmol/L) in the bay,due to nearly complete consumption by phytoplankton during the red tide period.Additionally,the significant loss of nitrate and simultaneous increase in the stable isotopes of dissolved and particulate nitrogen suggest a strong coupling of assimilation and mineralization during the dry season.More active biogeochemical processes during the dry season may be related to decreased runoff and increased water retention time.Overall,our study illustrated the major seasonal nitrogen sources and their dynamics in Zhanjiang B ay,providing valuable insights for formulating effective policies to mitigate eutrophication in mariculture bays.

Variation of soil-plant-atmosphere continuum stable isotope and water source in Qinghai spruce forest of the eastern Qilian Mountains

Understanding the hydrogen and oxygen stable isotope composition and characteristics of different water bodies in soil-plant-atmosphere continuum is of significance for revealing regional hydrological processes and water cycle mechanisms.In this study,we analyzed the stable isotopic composition,relationship and indicative significance of precipitation,soil water(0~100 cm depth)and xylem water of Qinghai spruce(Picea crassifolia)forest in the eastern Qilian Mountains,and explored the circulation process among different water bodies.The results show that the stable isotopes of precipitation vary greatly during the entire observation period.The values ofδ2H andδ^(18)O in the precipitation in the warm season are richer than those in the cold season,and the slope and intercept of local meteoric water line(LMWL,δ2H=6.79δ18O+7.13)are both smaller than global meteoric water line(GMWL,δ2H=8.17δ18O+10.56).The stable isotopes of soil water at different depths underwent different degrees of evaporative fractionation,and theδ18O andδ2H of shallow soil water varied greatly,while the deep soil water tended to be similar.The topsoil(0~10 cm)can respond quickly to precipitation,and the response of the deep soil has a time lag.In the whole growing season,0~30 cm and 60~100 cm soil water are the main water sources of Qinghai spruce.The water source of Qinghai spruce was from all soil layers in May and September,mainly from the shallow soil layer(0~30 cm)in August and October,and mainly from the deep soil layer(60~100 cm)in June and July.

Estimating distribution of water uptake with depth of winter wheat by hydrogen and oxygen stable isotopes under different irrigation depths

Crop root system plays an important role in the water cycle of the soil-plant-atmosphere continuum. In this study, com- bined isotope techniques, root length density and root cell activity analysis were used to investigate the root water uptake mechanisms of winter wheat （Triticum aesfivum L.） under different irrigation depths in the North China Plain. Both direct inference approach and multisource linear mixing model were applied to estimate the distribution of water uptake with depth in six growing stages. Results showed that winter wheat under land surface irrigation treatment （Ts） mainly absorbed water from 10-20 cm soil layers in the wintering and green stages （66.9 and 72.0%, respectively）; 0-20 cm （57.0%） in the jointing stage; 0-40 （15.3%） and 80-180 cm （58.1%） in the heading stage; 60-80 （13.2%） and 180-220 cm （35.5%） in the filling stage; and 0-40 （46.8%） and 80-100 cm （31.0%） in the ripening stage. Winter wheat under whole soil layers irrigation treatment （Tw） absorbed more water from deep soil layer than Ts in heading, filling and ripening stages. Moreover, root cell activity and root length density of winter wheat under TW were significantly greater than that of Ts in the three stages. We concluded that distribution of water uptake with depth was affected by the availability of water sources, the root length density and root cell activity. Implementation of the whole soil layers irrigation method can affect root system distribution and thereby increase water use from deeper soil and enhance water use efficiency.

Application of Stable Isotope Techniques in Studies of Carbon and Nitrogen Biogeochemical Cycles of Ecosystem

Stable isotope techniques have been proved useful as tools for studying the carbon (C) and nitrogen (N) biogeochemical cycles of ecosystem. This paper firstly introduced the basic principles and the distribution characteristics of stable isotope, then reviewed the recent advances and applications of stable isotope in the C and N biogeochemical cycles of ecosystem. By applying the 13 C natural abundance technique, ecologists are able to understand the photosynthetic path and CO 2 fixation of plants, the CO 2 exchange and C balance status of ecosystem, the composition, distribution and turnover of soil organic C and the sources of organic matter in food webs, while by using the 13 C labeled technique, the effects of elevated CO 2 on the C processes of ecosystem and the sources and fate of organic matter in ecosystem can be revealed in detail. Differently, by applying the 15 N natural abundance technique, ecologists are able to analyze the biological N 2 -fixation, the N sources of ecosystem, the N transformation processes of ecosystem and the N trophic status in food webs, while by using the 15 N labeled technique, the sources, transformation and fate of N in ecosystem and the effects of N input on the ecosystem can be investigated in depth. The applications of both C and N isotope natural abundance and labeled techniques, combined with the elemental, other isotope ( 34 S) and molecular biomarker information, will be more propitious to the investigation of C and N cycle mechanisms. Finally, this paper concluded the problems existed in current researches, and put forward the perspective of stable isotope techniques in the studies on C and N biogeochemical cycles of ecosystem in the future.

Analysis of carbon and oxygen stable isotopes in carbonate rocks by the laser micro-sampling technique

The analysis of stable isotopes of carbon and oxygen in different carbonate rocks by the phosphoric acid method is not easier than that by the laser sampling method developed in recent years, which optically focuses laser beams with sufficient energy on a micro area of a thin section in a vacuum sample box via microscope. CO 2 produced by heating decomposition of carbonate was purified by the vacuum system, and the stable isotopic values of carbon and oxygen were calculated and analyzed on a mass spectrometer. This paper adopted the laser micro-sampling technique to analyze the stable isotopes of carbon and oxygen in dolomite, carbonate cement, stromatolite and different forms of dawsonite (donbassite). Results indicated that the laser micro-sampling method is effective in analyzing carbonate composition and could be a convincing proof for justification on carbonate composition analysis.

Carbon and nitrogen isotopes analysis and sources of organic matter in surface sediments from the Sanggou Bay and its adjacent areas, China

Naturally existing stable carbon and nitrogen isotopes are important in the study of sedimentary organic matter sources. To identify the sources of sedimentary organic matter in Sanggou Bay and its adjacent areas, which is characterized by high-density shellfish and seaweed aquaculture, the grain size, organic carbon （OC）, total nitrogen （TN）, carbon and nitrogen isotopic composition （δ13C andδ15N） of organic matter in the surface sediment were determined. The results showed that, in August, sedimentary OC and TN ranged from 0.17% to 0.76% and 0.04% to 0.14%, respectively. In November, OC and TN ranged from 0.23% to 0.87% and 0.05% to 0.14%, respectively. There was a significant positive correlation between OC and TN （R=0.98, P＆lt;0.0001）, indicating that OC and TN were homologous. In August, theδ13C andδ15N of organic matter varied from -23.06‰ to -21.59‰ and 5.10‰ to 6.31‰, respectively. In November,δ13C andδ15N ranged from -22.87‰ to -21.34‰ and 5.13‰ to 7.31‰, respectively. This study found that the major sources of sedimentary organic matter were marine shellfish biodeposition, seaweed farming, and soil organic matter. Using a three-end-member mixed model, we estimated that the dominant source of sedimentary organic matter was shellfish biodeposition, with an average contribution rate of 65.53% in August and 43.00% in November. Thus, shellfish farming had a significant influence on the coastal carbon cycle.

Application of ^15N Stable Isotope Labeling Technology in Sugarcane Nitrogen Research

Nitrogen is one of the essential nutrient elements for plant growth,which plays an important role in the growth and development of sugarcane. The whole growth cycle of sugarcane needs a large amount of nitrogen. Increasing the application of nitrogen can improve the yield of sugarcane,but it will also cause environmental pollution. Therefore,how to control or reduce the application of nitrogen fertilizer while continuously increasing sugarcane yield,reduce the increase of sugarcane production cost and environmental pollution caused by excessive application of nitrogen fertilizer has become an important scientific problem faced by sugarcane industry in China.^15N stable isotope labeling technology has been applied to many crops as a nitrogen research tool. In order to better understand the demand of nitrogen fertilizer in soil-cane system,this paper reviewed nitrogen allocation in plants,nitrogen loss,nitrogen recycling and endogenous nitrogen fixation of sugarcane based on^15N stable isotope labeling technology used in the nitrogen uptake and utilization,providing a theoretical basis for the improvement of sugarcane nitrogen use efficiency and the efficient nitrogen fertilizer management of sugarcane.

Stable Oxygen and Deuterium Isotope Techniques to Identify Plant Water Sources

There is still very little information on the sources of water absorbed by oil palm plant. This information is very important for water management system in oil palm plantation. Thus, this study was carried out to determine current water sources absorbed by the oil palm roots using oxygen (δ18O) and deuterium isotopes (δD) techniques. Sketches of oxygen and deuterium isotope were total rainfall, throughfall, runoff, measurement at 5 soil depths (namely: 20 cm, 50 cm, 100 cm, 150 cm, and 200 cm), and oil palm stem. Results of this study showed huge variance in the values of oxygen and deuterium isotope. Based on Least Significant Difference (LSD) test, there was no significant value in the oxygen and deuterium isotope of stem water and others;however, a similar value was obtained at the depths of 0 - 20 cm and 20 - 50 cm with the stem water. This indicated that oil palm absorbed water from 0 - 50 cm depth. This result agreed with the oil palm rooting system, which has verified that the root quarter is the most active root of oil palm.

Stable isotopes of water as a tracer for revealing spatial and temporal characteristics of groundwater recharge surrounding Qinghai Lake,China

Studying spatial and temporal characteristics of regional groundwater recharge will guide the scientific management and sustainable development of regional water resources.This study investigated stable isotopes(δ^(18)O and δ^(2) H)of precipitation,groundwater,river water and lake water during 2019-2020 in Qinghai Lake Basin to reveal the spatial and temporal characteristics of groundwater recharge.The local meteoric water line was simulated using ordinary least squares regression(δ^(2) H=7.80δ^(18)O+10.60).The local evaporation lines of the river water,lake water and groundwater were simulated asδ^(2) H=6.21δ^(18)O-0.72,δ^(2) H=5.73δ0-3.60 and δ^(2) H=6.59δ0+1.76,respectively.The δ^(2) H and δ^(18)O of river water and groundwater were in more depleted values due to the recharge by precipitation at high altitudes or precipitation effects,and theδ^(2) H andδ^(18)O of the lake water were in more enriched values because of evaporation.The relationship between the δ^(2) H and δ^(18)O of groundwater and river water was not significantly different,indicating a strong hydrological connection between the groundwater and river water surrounding Qinghai Lake.Additionally,the maximum values of δ^(18)O and the minimum values of lc-excess of groundwater in most regions were both in August,and the minimum values of δ^(18)O and the maximum values of lc-excess of groundwater in most regions were both in October.Therefore,the groundwater was recharged by soil water with strong evaporation in August and recharged by precipitation at high altitudes in October.The recharge rate of groundwater was relatively fast in areas with large slopes and large hydraulic gradients(e.g.,south of Qinghai Lake),and in areas with strong hydrological connections between the groundwater and river water(e.g.,the Buha River Valley).Those results can provide data support for protection and utilization of water resources in Qinghai Lake Basin,and provide reference for groundwater research in closed lake basins on the Qinghai-Tibet Plateau.

Effect of dietary stable isotopic ratios of carbon and nitrogen on the extent of their incorporation into tissues of rats

This study was conducted to investigate the effect of different dietary ratios of 13C to 12C or 15 N to 14N on their relative incorporation into tissues. Eighty male rats were used in two 21-day feeding trials in which they were fed diets with either high δ13C levels （δ13C=-13.89%o and δ15N =2.37%o in experiment 1 and δ13C=-19.34%o and δ15N = 4.73%o in experiment 2） or low 613C levels （δ13C =-17.90%o and δ15N = 3.08%o in experiment 1 and δ13C =-21.76%o and δ15N = 0.53‰ in experiment 2）, meanwhile, the dietary δ15N levels were designed to two ranks Blood, liver, adipose and muscle tissues were collected on day 0, 3, 7, 14, and 21 for determination of 13C, 12C, 15N and 14 N isotopes. Rat growth rate, antioxidant capacity and metabolic parameters were also assessed. The results indicate that adipose tissue tend to deplete 13C before the stable isotopic ratios achieved final equilibrium. Therefore, feeds with different isotopic signatures had different incorporation rates into tissues. Low dietary 13C levels decreased tissue δ13C values whereas high dietary 13C levels did not alter tissue δ13C values during the 21-d experiment. Blood δ15N values were a reliable parameter in assessing the relative contribution of dietary nitrogen to tissues. This study revealed a relationship between dietary tissues. However, more studies are needed to illustrate the sotopic signatures and their incorporation rates into rat mechanism through which dietary isotopic ratios nfiuence the extent of isotopic incorporation into the tissues

Carbon and Oxygen Isotopic Composition of Surface-Sediment Carbonate in Bosten Lake (Xinjiang,China) and its Controlling Factors

Bosten Lake is a mid-latitude lake with water mainly supplied by melting ice and snow in the Tianshan Mountains. The depositional environment of the lake is spatially not uniform due to the proximity of the major inlet and the single outlet in the western part of the lake. The analytical results show that the carbon and oxygen isotopic composition of recent lake sediments is related to this specific lacustrine depositional environment and to the resulting carbonate mineralogy. In the southwestern lake region between the Kaidu River inlet and the Kongqi River outlet, carbon isotope composition （δ^13C） values of the carbonate sediment （-1‰ to -2‰） have no relation to the oxygen isotope composition of the carbonate （δ^18O） values （-7‰ to -8‰）, with both isotopes showing a low variability. The carbonate content is low （〈20%）. Carbonate minerals analyzed by X-ray diffraction are mainly composed of calcite, while aragonite was not recorded. The salinity of the lake water is low in the estuary region as a result of the Kaidu River inflow. In comparison, the carbon and oxygen isotope values are higher in the middle and eastern parts of the lake, with δ^13C values between approximately ＋0.5‰ and ＋3‰, and δ^18O values between -1‰ and -5‰. There is a moderate correlation between the stable oxygen and carbon isotopes, with a coefficient of correlation r of approximately 0.63. This implies that the lake water has a relatively short residence time. Carbonate minerals constitute calcite and aragonite in the middle and eastern region of the lake. Aragonite and Mg-calcite are formed at higher lake water salinity and temperatures, and larger evaporation effects. More saline lake water in the middle and eastern region of the lake and the enhanced isotopic equilibrium between water and atmospheric CO2 cause the correlating carbon and oxygen isotope values determined for aragonite and Mg-calcite. Evaporation and biological processes are the main reasons for the salinity and carbonate mineralogy influence of the surface-sediment carbonate in Bosten Lake. The lake water residence time and the CO2 exchange between the atmosphere and the water body control the carbon and oxygen isotope composition of the carbonate sediment. In addition, organic matter pollution and decomposition result in the abnormally low carbon isotope values of the lake surface-sediment carbonate.

Spatial distributions of organic carbon and nitrogen and their isotopic compositions in sediments of the Changjiang Estuary and its adjacent sea area

The spatial distribution patterns of total organic carbon and total nitrogen show significant correlations with currents of the East China Sea Shelf. Corresponding to distributions of these currents, the study area could be divided into four different parts. Total organic carbon, total nitrogen, and organic carbon and nitrogen stable isotopes in sediments show linear correlations with mean grain size, respectively, thus ＂grain size effect＂ is an important factor that influences their distributions. C/N ratios can reflect source information of organic matter to a certain degree. In contrast, nitrogen stable isotope shows different spatial distribution patterns with C/N and organic carbon stable isotope, according to their relationships and regional distributions. The highest contribution （up to 50%） of terrestrial organic carbon appears near the Changjiang Estuary with isolines projecting towards northeast, indicating the influence of the Changjiang dilution water. Terrestrial particulate organic matter suffers from effects of diagenesis, benthos and incessant inputting of dead organic matter of plankton, after depositing in seabed. Therefore, the contribution of terrestrial organic carbon to particulate organic matter is obviously greater than that to organic matter in sediments in the same place.

Stable isotope analysis of water sources for Tamarix laxa in the mega-dunes of the Badain Jaran Desert, China

The complex interactions in desert ecosystems between functional types and environmental conditions could be reflected by plant water use patterns. However, the mechanisms underlying the water use patterns as well as the water sources of Tamarix laxa in the mega-dunes of the Badain Jaran Desert, China, remain unclear. This study investigated the water sources and water use patterns of T. laxa using the stable oxygen isotope method. The δ18O values of xylem water, soil water in different layers(0–200 cm), rainwater, snow water, lake water, atmospheric water vapor, condensate water, and groundwater were measured. The sources of water used by T. laxa were determined using the IsoSource model. The results indicate that T. laxa mainly relies on soil water. At the beginning of the growing season(in May), the species is primarily dependent on water from the middle soil layer(60–120 cm) and deep soil layer(120–200 cm). However, it mainly absorbs water from the shallow soil layer(0–60 cm) as the rainy season commences. In September, water use of T. laxa reverts to the deep soil layer(120–200 cm). The water use patterns of T. laxa are closely linked with heavy precipitation events and soil water content. These findings reveal the drought resistance mechanisms of T. laxa and are of significance for screening species for ecological restoration.

Application of Stable Isotope Tracing Technologies in Identification of Transformation among Waters in Sanjiang Plain,Northeast China

In order to investigate the transformation among the precipitation,groundwater,and surface water in the Sanjiang Plain,Northeast China,precipitation and groundwater samples which were collected at the meteorological station of the Sanjiang Mire Wetland Experimental Station,Chinese Academy of Sciences and the surface water which collected from the Wolulan River were used to identify the transformation of three types of water.The isotope composition of different kinds of water sources were analyzed via stable isotope(deuterium and oxygen-18) investigation of natural water.The results show a clear seasonal difference in the stable isotopes in precipitation.During the cold half-year,the mean stable isotope in precipitation in the Sanjiang Plain reaches its minimum with the minimum temperature.The δ18O and δD values are high in the rainy season.In the Wolulan River,the evaporation is the highest in August and September.The volume of evaporation and the replenishment to the river is mostly same.The groundwater is recharged more by the direct infiltration of precipitation than by the river flow.The results of this study indicate that the water bodies in the Sanjiang Plain have close hydrologic relationships,and that the transformation among each water system frequently occurs.

Late Holocene glacier variations indicated by theδ18O of ice core enclosed gaseous oxygen in the central Tibetan Plateau

Theδ18O of ice core enclosed gaseous oxygen(δ18Obub)has been widely used for climate reconstruction in polar regions.Yet,less is known about its climatic implication in the mountainous glaciers as the lack of continuous record.Here,we present a long-term,continuousδ18Obub record from the Tanggula glacier in the central Tibetan Plateau(TP).Based on comparisons of its variation with regional climate and glacier changes,we found that there was a good correlation between the variation of theδ18Obub in this alpine ice core and the accumulation and melting of this glacier.The more developed the firn layer on glacier surface,the more positive theδ18Obub.Conversely,the more intense the glacier melting,the more negative theδ18Obub.Combined with the chronology of ice core enclosed gases,the glacier variations since the late Holocene in the central TP were reconstructed.The result showed that there were four accumulation and three deficit periods of glaciers in this region.The strongest glacier accumulation period was 1610-300 B.C.,which corresponds to the Neoglaciation.The most significant melting period was the last 100 years,which corresponds to the recent global warming.The Medieval Warm Period was relatively significant in the central TP.However,during the Little Ice Age,there was no significant glacier accumulation in the central TP,and even short deficit events occurred.Comparisons of the late Holocene glacier variation in the central TP with glacier and climate variations in the TP and the Northern Hemisphere showed that it was closely related to the North Atlantic Oscillation.

Stable isotopes reveal varying water sources of Caragana microphylla in a desert-oasis ecotone near the Badain Jaran Desert

Understanding the variation in a plant’s water sources is critical to understanding hydrological processes in water-limited environments. Here, we measured the stable-isotope ratios(δ18 O) of xylem water of Caragana microphylla, precipitation,soil water from different depths, and groundwater to quantitatively analyze the proportion of water sources for the shrub.We found that the water sources of C. microphylla differed with the plant’s ages and the seasons. The main water source for young shrubs was upper-soil water, and it showed significant changes with seasonal precipitation inputs. In summer,the proportion contributed by shallow water was significantly increased with increased precipitation inputs. Then, the contribution from shallow-soil water decreased with the decline in precipitation input in spring and autumn. However, the adult shrubs resorted to deep-soil layers and groundwater as the main water sources during the whole growing season and showed much less seasonal variation. We conclude that the main water source of the young shrubs was upper-soil water and was controlled by precipitation inputs. However, once the shrub gradually grew up and the roots reached sufficient depth, the main water sources change from the upper-soil layer recharged by precipitation to deep-soil water and groundwater, which were relatively stable and abundant in the desert ecosystem. These results also suggest that desert shrubs may be able to switch their main water sources to deep and reliable water sources as their age increases, and this adjustment to water availability carries significant importance for their acclimation to the desert habitat.

Analyzing biases of nitrogen contents and δ^(15)N values arising from acidified marine sediments with different CaCO_3 concentrations

In some studies, the researchers pretreated and measured organic carbon, nitrogen and their isotopes（δ^（13）C and δ^（15）N） of marine sediment together, to save costs and resources of analysis. However, the procedure of acidification to remove inorganic carbon for analysing δ^（13）C can affect the values of nitrogen and δ^（15）N, and the biases vary a lot depending on the CaCO_3 contents of sediments. In this study, the biases of total nitrogen（TN）and δ^（15）N values arising from acidified sediments were compared between the CaCO_3-poor（1%–16%） and CaCO_3-rich（20%–40%） samples. TN and δ^（15）N values were altered during acid treatment（without centrifugation） that possibly led to N-containing compounds volatilization. For CaCO_3-poor samples, acidification led to a range of 0%–40% TN losses and 0‰–2‰ shift in δ^（15）N values; and 10%–60% TN losses and 1‰–14‰ shift in δ^（15）N values for CaCO_3-rich samples. The biases from most samples exceeded the precision of the instrument（0.002% for TN and 0.08‰ for δ^（15）N）, and high biases could mislead our judgment for the environmental implication of the data. Thus,avoiding co-analysis of organic carbon, nitrogen and their isotopes（δ^（13）C and δ^（15）N） in sediments, even for CaCO_3-poor marine sediments, is necessary.

In-situ nitrogen fate in the vadose zone of different soil types and its implications for groundwater quality in the Huaihe River Basin,China

This paper focused on nitrate fate in the vadose zone(VZ)and its implications for groundwater vulnerability under different soil types in the agricultural area of Huaihe River Basin,China.Isotopic compositions of nitrate(δ15N andδ18O)along with NO3-and Cl-concentrations were determined in the VZ-shallow groundwater continuum beneath silty-loam and silty-clay-loam,which are distinctive in texture and organic carbon(OC).In the soil zone(<1 m in depth),measuredδ18O-NO3-suggested the ubiquitous of nitrification regardless of soil types.In the subsoil zone(>1 m in depth),however,the concurrent enrichment ofδ15N-NO3-andδ18O-NO3-indicated the occurrence of denitrification,which showed a dependence on subsoil properties.Specifically,during wheat and maize land uses,denitrification removed as much as 76%-88%of the total nitrate where the subsoil was dominated by stratified OC-rich silty-clay-loam.In contrast,only 0%-28%of the nitrate was degraded via denitrification where the subsoil was composed of uniform,OC-depleted silty-loam.Furthermore,inactive denitrification and higher permeability in the silty-loam VZ implied higher groundwater vulnerability.This observation was consistent with the fact that groundwater NO3--N concentration beneath silty-loam(11.24 mg L-1)was over two times higher than that of the silty-clay-loam(5.32 mg L-1),where stricter fertilization management and conservation strategies should be applied to protect groundwater quality.