Natural Abundance of ^(15)N in Main N-Containing Chemical Fertilizers of China

As early as the 1950s, there had already been reports on the δ 15N value of some chemical fertilizers (Hoering, 1955). Since Kohl and his co-workers (1971) published the report that attempt was made to distinguish the respective contribution of soil nitrogen and fertilizer nitrogen to the source of nitrates in surface water according to the differ-

Study on Variation in ^(15)N Natural Abundance. Ⅰ. Characteristics of Variation in ^(25)N Natural Abundance of Forest Soils

INTRODUCTION A lot 0fwork has been done on the variation of natural ^(15)N abundance in soils andtheir N components (Chen et al., 1964). However, these reports mostly concerned thegeochemical mean of natural ^(15)N abundance, and only a little work has been done on thecharacteristics of variation of the natural ^(15)N abundance in different ecosystems and itsrelationship with the properties and conditions of soil, which is the aim of our recent re-

Enhanced Loading of ^(40)K from Natural Abundance Potassium Source with a High Performance 2D^+ MOT

40K is one of the most important atomic species for ultra-cold atomic physics. Due to the extremely low con- centration （0.012%） of 40K in natural abundance of potassium, most experiments use 4-10% enriched potassium source, which have greatly suffered from the extremely low annual production and significant price hikes in recent years. Using naturally abundant potassium source, we capture 5.4 × 10 6 cold 40K atoms with the help of a high performance of two-dimensional magneto-optical trap （2D＋ MOT）, which is almost three orders of magnitude greater than previous results without the 2D＋ MOT. The number of the 40K atoms is sufficient for most ultra-cold 40K experiments, and our approach provides an ideal alternative for the field.

Nitrogen uptake strategies of mature conifers in Northeastern China, illustrated by the ^(15)N natural abundance method

Background:Conifers partition different N forms from soil,including ammonium,nitrate,and dissolved organic N(DON),to sustain plant growth.Previous studies focused on inorganic N sources and specific amino acid forms using ^(15)N labelling,but knowledge of the contribution of DON to mature conifers’N uptake is still scarce.Here,we quantified the contribution of different N forms(DON vs.NH_(4)^(+)vs.NO_(3)^(−))to total N uptake,based on ^(15)N natural abundance of plant and soil available N,in four mature conifers(Pinus koraiensis,Pinus sylvestris,Picea koraiensis,and Larix olgensis).Results:DON contributed 31%,29%,28%,and 24%to total N uptake by Larix olgensis,Picea koraiensis,Pinus koraiensis,and Pinus sylvestris,respectively,whereas nitrate contributed 42 to 52%and ammonium contributed 19 to 29%of total N uptake for these four coniferous species.Conclusions:Our results suggested that all four conifers could take up a relatively large proportion of nitrate,while DON was also an important N source for the four conifers.Given that DON was the dominant N form in study soil,such uptake pattern of conifers could be an adaptive strategy for plants to compete for the limited available N sources from soil so as to promote conifer growth and maintain species coexistence.

Responses of plant^(15)N natural abundance and isotopic fractionation to N addition reflect the N status of a temperate steppe in China

Aims Elevated anthropogenic nitrogen(N)deposition could alter N status in temperate steppe.However,threshold observations of N status change from N limit to N saturation by far are not conclusive in these ecosystems.Research on the natural abundance of ^(15)N( δ^(15)N)could greatly help provide integrated information about ecosystem N status.The goal of this study was to investigate the suitability of measurements of δ^(15)N of major ecosystem N pools and several key species,plant ^(15)N fractionation,together with key vegetation and soil indicators in response to N fertilization as a tool to identify the N status in a temperate steppe in Inner Mongolia.Methods We carried out a N addition experiment during 2011-14 on a Stipa krylovii steppe in Inner Mongolia,Northern China.We investigated the response of several key N transformation processes,vegetation and soil properties to N addition.Aboveground biomass and below-ground biomass(BGB) δ^(15)N,root and foliar δ^(15)N of three dominant species(Artemisia frigida,S.krylovii and Leymus chinensis), δ^(15)N of soil total N and soil KCl-extractable NO_(3)^(−)-N were determined.The responses of isotope fractionation during plant N uptake and reallo-cation to N addition were also determined.Important Findings Our results suggest that the N addition rate of 5g N m^(−2) yr^(−1) could be regarded as threshold of early N saturation in this S.krylovii steppe as indicated by an increase in plant fraction-ation and a decrease in plant δ^(15)N.When N input rate is>10 g N m^(-2) yr^(-1),increased N deposition can lead to an apparent reduction in species richness and BGB as well as an increase in NO_(3)^(−)in extractable soil pools<30-cm soil profile.With N addition,S.krylovii and A.frigida undergo earlier N status shift from N limitation toward N excess compared with L.chinen-sis,contributing to L.chinensis out-competing other species.Overall,this study provides a better understanding of N status change in temperate steppe based on isotope evidence and several other functional variables and contributes to predicting the responses of temperate steppe to future global N deposition scenario.

Natural Nitrogen-15 Abundance of Ammonium Nitrogen and Fixed Ammonium in Soils

The present article deals with the natural nitrogen-15 abundance of ammonium nitrogen and fixed ammonium in different soils. Variations in the natural 15N abundance of ammonium nitrogen mineralized in soils under anaerobic incubation condition were related to soil pH. The δ 15N of mineralizable N in acid soils was lower but that in neutral and calcareous soils was higher compared with the δ 15N of total N in the soils. A variation tendency was also found in the δ 15N of amino-acid N in the hydrolysates of soils. The natural 15N abundance of fixed ammonium was higher than that of total N in most surface soils and other soil horizons, indicating that the increase of δ 15N in the soil horizons beneath subsurface horizon of some forest soils and acid paddy soils was related to the higher δ 15N value of fixed ammonium in the soil.

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.

Reduced turnover rate of topsoil organic carbon in old-growth forests:a case study in subtropical China

Background:Old-growth forests are irreplaceable with respect to climate change mitigation and have considerable carbon(C)sink potential in soils.However,the relationship between the soil organic carbon(SOC)turnover rate and forest development is poorly understood,which hinders our ability to assess the C sequestration capacity of soil in old-growth forests.Methods:In this study,we evaluated the SOC turnover rate by calculating the isotopic enrichment factor β(defined as the slope of the regression between ^(13)C natural abundance and log-transformed C concentrations)along 0-30 cm soil profiles in three successional forests in subtropical China.A lower β(steeper slope)is associated with a higher turnover rate.The three forests were a 60-year-old P.massoniana forest(PF),a 100-year-old coniferous and broadleaved mixed forest(MF),and a 400-year-old monsoon evergreen broadleaved forest(BF).We also analyzed the soil physicochemical properties in these forests to examine the dynamics of SOC turnover during forest succession and the main regulators.Results:The β value for the upper 30-cm soils in the BF was significantly(p<0.05)higher than that in the PF,in addition to the SOC stock,although there were nonsignificant differences between the BF and MF.The β value was significantly(p<0.05)positively correlated with the soil recalcitrance index,total nitrogen,and available nitrogen contents but was significantly(p<0.01)negatively correlated with soil pH.Conclusions:Our results demonstrate that SOC has lower turnover rates in old-growth forests,accompanied by higher soil chemical recalcitrance,nitrogen status,and lower soil pH.This finding helps to elucidate the mechanism underlying C sequestration in old-growth forest soils,and emphasizes the important value of old-growth forests among global C sinks.

Temporal changes in nitrogen acquisition of Japanese black pine(Pinus thunbergii) associated with black locust(Robinia pseudoacacia)

The alien woody legume, black locust （Robinia pseudoaca-cia）, has invaded Japanese black pine （Pinus thunbergii） forests located in Japan’s coastal plain and hill regions where gaps are formed in pine forests after nematode infestation. Nitrogen fixation by legumes acceler-ates N cycling in forest ecosystems. We studied temporal change in the annual tree-ring resolution N stable isotope composition （δ15N, a per mil deviation of δ15N/14N ratio, relative to atmospheric N2δ15N=0‰） at two natural locations of Japanese black pine forest with black locust that differed in the time since black locust establishment （Shohnai in north-east and Kita-Kyushu in southwest Japan）. Analyzed tree-rings covered the period from 1990/1992 to 2009. N acquisition by Japanese black pine from black locust N input to the soil was evidenced by temporal shifting of N stable isotope composition on the annual pine tree rings. With pro-gressive development of the forest stand,δ15N values of earlier tree-ringsδ15N of -5‰） from black pine associated with black locust shifted to-wards values similar to those of black locustδ15N values nearly to-1‰）, which suggests acquisition of N by N2 fixation （Shohnai site）. In con-trast, in a forest where black locust had settled for two or three genera-tions, in a black pine stand （Kita-Kyushu site）, longer periods of N en-richment in the soil were reflected in the elevated tree-ringδ15N values of newly established black pine trees. Based on tree-ringδ15N data from the Shohnai site, we determined that about 10 years after black locust establishment, soil N had already been enriched by black locust N, this, in turn, contributed to N fertilization of surrounding trees in mixed stands.

Remediation of preservative ethylparaben in water using natural sphalerite:Kinetics and mechanisms

As a typical class of emerging organic contaminants(EOCs), the environmental transformation and abatement of preservative parabens have raised certain environmental concerns. However, the remediation of parabens-contaminated water using natural matrixes(such as, naturally abundant minerals) is not reported extensively in literature. In this study, the transformation kinetics and the mechanism of ethylparaben using natural sphalerite(NS) were investigated. The results show that around 63% of ethylparaben could be absorbed onto NS within 38 hr, whereas the maximum adsorption capacity was 0.45 mg/g under room temperature. High temperature could improve the adsorption performance of ethylparaben using NS. In particular, for the temperature of 313 K, the adsorption turned spontaneous. The well-fitted adsorption kinetics indicated that both the surface adsorption and intra-particle diffusion contribute to the overall adsorption process. The monolayer adsorption on the surface of NS was primarily responsible for the elimination of ethylparaben. The adsorption mechanism showed that hydrophobic partitioning into organic matter could largely govern the adsorption process, rather than the Zn S that was the main component of NS. Furthermore, the ethylparaben adsorbed on the surface of NS was stable, as only less than 2% was desorbed and photochemically degraded under irradiation of simulated sunlight for 5 days. This study revealed that NS might serve as a potential natural remediation agent for some hydrophobic EOCs including parabens, and emphasized the significant role of naturally abundant minerals on the remediation of EOCs-contaminated water bodies.

Discrepant responses of soil organic carbon dynamics to nitrogen addition in different layers: a case study in an agroecosystem

Empirical research indicates that heightened soil nitrogen availability can potentially diminish microbial decomposition of soil organic carbon(SOC).Nevertheless, the relationship between SOC turnover response to N addition and soil depth remains unclear. In this study, soils under varying N fertilizer application rates were sampled up to 100 cm deep to examine the contribution of both new and old carbon to SOC across different soil depths,using a coupled carbon and nitrogen isotopic approach. The SOC turnover time for the plot receiving low N addition(250 kg·ha^(-1)·yr^(-1) N) was about 20-40 years. Conversely, the plot receiving high N(450 kg·ha^(-1)·yr^(-1) N) had a longer SOC turnover time than the low N plot, reaching about 100 years in the upper 10-20 cm layer. The rise in SOC over the entire profile with low N addition primarily resulted from an increase in the upper soil(0-40 cm)whereas with high N addition, the increase was mainly from greater SOC in the deeper soil(40-100 cm). Throughout the entire soil layer, the proportion of new organic carbon derived from maize C_4 plant sources was higher in plots treated with a low N rate than those treated with a high N rate. This implies that, in contrast to low N addition agricultural practices, high N addition predominantly enhances the soil potential for fixing SOC by transporting organic matter from surface soils to deeper layers characterized by more stable properties. This research offers a unique insight into the dynamics of deep carbon under increased N deposition, thereby aiding in the formulation of policies for soil carbon management.

Impact of Cutting a Clover Crop on the Nitrogen Supplied to Winter Wheat in an Intercropping System

The major function of clover in a winter wheat–white clover intercropping system is to supply nitrogen （N） for the wheat. A field experiment was conducted at Yucheng Comprehensive Station of the Chinese Academy of Sciences, to evaluate the effect of cutting white clover on N fixation and the transfer of fixed N to the associated winter wheat. A method of 15N natural abundance was used to determine the nitrogen dynamics in the intercropping system. The results showed that the amount of N transferred from the clover to the wheat, throughout the growing season, varied between 34.4 and 57.5 kg ha-1. Compared to leaving the clover standing, cutting the clover increased the amount of N that accumulated in the soil and also resulted in reduced N concentrations in the leaves and stems of the wheat. Using the cut clover as mulch between wheat rows led to decreased N concentrations in the wheat plants’ leaves and stems. The present study provides preliminary information on the amount of N transferred from clover to wheat in an intercropping system.

Effects of long-term nitrogen addition on theδ^(15)N andδ^(13)C of Larix gmelinii and soil in a boreal forest

Background:Natural abundance of carbon(C)and nitrogen(N)stable isotope ratios(δ^(13)C andδ^(15)N)has been used to indicate the state and cycle of ecosystem C and N.However,it is still unclear how C and N cycle of boreal forests respond to the N deposition.Results:We conducted an 8-year continuous N addition field experiment in a Larix gmelinii forest in Greater Khingan Mountains,Northeast China.Four N treatments(0,25,50,75 kg N ha^(−1)year^(−1))were built.The effects of N addition on theδ^(13)C andδ^(15)N of needle,branch,bark,and fine root of Larix gmelinii and soil were studied.The result of the balance between the N input and output flux showed that N addition significantly increased theδ^(15)N in each organ of Larix gmelinii,but did not change theδ^(15)N of soil.We also found that the N absorption by needles of Larix gmelinii could increase the needle photosynthesis rate andδ^(13)C by increasing carboxylation,but N addition had no significant effect on theδ^(13)C of soil and other organs.In addition,both the soilδ^(15)N andδ^(13)C increased with the soil depth.Conclusions:Long-term N addition may lead to more open C and N cycles and further affect plant nutrient acquisition strategies in boreal forest ecosystems.

Use of ^(15)N stable isotope to quantify nitrogen transfer between mycorrhizal plants

Aims Mycorrhizas(fungal roots)play vital roles in plant nutrient acquisition,performance and productivity in terrestrial ecosystems.Arbuscular mycorrhizas(AM)and ectomycorrhizas(EM)are mostly important since soil nutrients,including NH+4,NO
3 and phosphorus,are translocated from mycorrhizal fungi to plants.Individual species,genera and even families of plants could be interconnected by mycorrhizal mycelia to form common mycorrhizal networks(CMNs).The function of CMNs is to provide pathways for movement or transfer of nutrients from one plant to another.In the past four decades,both ^(15)N external labeling or enrichment(usually expressed as atom%)and ^(15)N naturally occurring abundance(d^(15)N,&)techniques have been employed to trace the direction and magnitude of N transfer between plants,with their own advantages and limitations.Important Findings The heavier stable isotope ^(15)N is discriminated against 14N during biochemical,biogeochemical and physiological processes,due to a greater atomic mass.In general,non-N2-fixing plants had greater d^(15)N values than N2-fixing(;0&)ones.Foliar d^(15)N often varied by 5 to 10&in the order:non-mycorrhizas/AMs>EMs>ericoid mycorrhizas.Differences in d^(15)N(&)or ^(15)N(atom%)values could thus provide N transfer information between plants.A range of between 0 to 80%of one-way N transfer had been observed from N2-fixing mycorrhizal to non-N2-fixing mycorrhizal plants,but generally less than or around 10%in the reverse direction.Plant-to-plant N transfer may provide practical implications for plant performance in N-limited habitats.Considering that N translocation or cycling is crucial,and the potential benefits of N transfer are great in both agricultural and natural ecosystems,more research is warranted on either oneway or two-way N transfers mediated by CMNs with different species and under field conditions.

Agronomic evaluation of Herbaspirillum seropedicae strain ZAE94 as an inoculant to improve maize yield in Brazil

Diazotrophic bacteria applied as a seed inoculant can improve the grain yield of several crops including maize. The current study aimed to test the agronomic efficiency and contribution of biological nitrogen fixation(BNF) of the endophytic diazotroph Herbaspirillum seropedicae strain ZAE94 to maize under field conditions. Eighteen field assays were conducted in four different locations during consecutive years on two hybrids and two varieties of maize in a random block design with four replicates using a peat-based inoculant. The inoculant containing the ZAE94 strain was applied without nitrogen(N)fertilization or with 40 kg N ha^(-1) and was compared to the application of 40 and 80 kg N ha^(-1) without inoculation. Crop productivity and N accumulation in the grain were evaluated in addition to ^(15)N natural abundance(δ^(15)N) to evaluate BNF in the treatments without N fertilization. Fertilization at 40 kg N ha^(-1) plus bacterial inoculation produced crop yields similar to the treatment with 80 kg N ha^(-1) and increased grain N content, especially in the off-season with 40 kg N ha^(-1). The inoculation treatments showed lower δ^(15)N values than the non-inoculated treatments, which was most evident in the off-season. The BNF contributed about 30% of N accumulated in plants inoculated with ZAE94. On average, 64% of the N fertilized plots showed an increase of the parameters evaluated in the inoculated treatments, compared with the control. Inoculation also increased root length, root volume, and leaf area, and these parameters were positively correlated with plant weight using a hydroponic assay. This study revealed that the application of H. seropedicae inoculant increased the amount of N in plants owing to BNF, and there is a better chance of yield response to inoculation under low N fertilizer application in the off-season.

Net ecosystem carbon exchange for Bermuda grass growing in mesocosms as affected by irrigation frequency

Intensification of grazed grasslands following conversion from dryland to irrigated farming has the potential to alter ecosystem carbon(C)cycling and affect components of carbon dioxide(CO_(2))exchange that could lead to either net accumulation or loss of soil C.While there are many studies on the effect of water availability on biomass production and soil C stocks,much less is known about the effect of the frequency of water inputs on the components of CO_(2)exchange.We grew Bermuda grass(Cynodon dactylon L.)in mesocosms under irrigation frequencies of every day(I_(1) treatment,30 d),every two days(I_(2) treatment,12 d),every three days(I_(3) treatment,30 d),and every six days(I_(6) treatment,18 d,after I_(2) treatment).Rates of CO_(2)exchange for estimating net ecosystem CO_(2)exchange(F_(N)),ecosystem respiration(R_(E)),and soil respiration(R_(S))were measured,and gross C uptake by plants(F_(G))and respiration from leaves(R_(L))were calculated during two periods,1–12 and 13–30 d,of the 30-d experiment.During the first 12 d,there were no significant differences in cumulative F_(N)(mean±standard deviation,61±30 g C m^(-2),n=4).During the subsequent 18 d,cumulative F_(N) decreased with decreasing irrigation frequency and increasing cumulative soil water deficit(W),with values of 70±22,60±16,and 18±12 g C m^(-2) for the I_(1),I_(3),and I_(6) treatments,respectively.There were similar decreases in F_(G),R_(E),and R_(L) with increasing W,but differences in R_(S) were not significant.Use of the C_(4) grass growing in a C_(3)-derived soil enabled partitioning of R_(S) into its autotrophic(R_(A))and heterotrophic(R_(H))components using a^(13)C natural abundance isotopic technique at the end of the experiment when differences in cumulative W between the treatments were the greatest.The values of R_(H) and its percentage contributions to R_(S)(43%±8%,42%±8%,and 8%±5%for the I_(1),I_(3),and I_(6) treatments,respectively)suggested that R_(H) remained unaffected across a wide range of W and then decreased under extreme W.There were no significant differences in aboveground biomass between the treatments.Nitrous oxide(N_(2)O)emission was measured to determine if there was a trade-off effect between irrigation frequency and increasing W on net greenhouse gas emission,but no significant differences were found between the treatments.These findings suggest that over short periods in well-drained soil,irrigation frequency could be managed to manipulate soil water deficit in order to reduce net belowground respiratory C losses,particularly those from the microbial decomposition of soil organic matter,with no significant effect on biomass production and N_(2)O emission.

comCompetition for resources is ameliorated by niche differentiation between Solidago virgaurea life-history stages in the Arctic

Aims competition has been shown to modify the niche breadth of coex-isting species,but within-species interactions have received little attention.Establishing small juvenile individuals and established,larger,sexually reproducing adult individuals represent two life-his-tory stages within species.We investigated the nitrogen and carbon resource use of adult and juvenile individuals and similarity of sym-biotic fungal community composition in these two plant life stages.We used the plant Solidago virgaurea growing in a simplified system in the low Arctic as model species.Methods Isotopic signatures(foliarδ15N and foliarδ13c)were analysed to characterize nitrogen acquisition and water-use efficiency of the plants.Symbiotic root fungal community composition was esti-mated by cloning and sequencing small subunit ribosomal RNA gene.Important Findings The isotopic signatures differed significantly between the life stages,indicating that the establishing juvenile cohort used relatively more amino acids or gained N through mycorrhizal symbiosis in com-parison to the established adult plants.Symbiotic fungal commu-nities did not differ between the two plant cohorts suggesting a possibility that the plants shared the same mycorrhizal network.We conclude that competition-mediated differences in plant resource use may create niche differentiation between the two life-history stages and enable them to coexist.