Differential response of radial growth and δ^(13)C in Qinghai spruce(Picea crassifolia) to climate change on the southern and northern slopes of the Qilian Mountains in Northwest China

Tree radial growth can have significantly differ-ent responses to climate change depending on the environ-ment.To elucidate the effects of climate on radial growth and stable carbon isotope(δ^(13)C)fractionation of Qing-hai spruce(Picea crassifolia),a widely distributed native conifer in northwestern China in different environments,we developed chronologies for tree-ring widths and δ^(13)C in trees on the southern and northern slopes of the Qilian Mountains,and analysed the relationship between these tree-ring variables and major climatic factors.Tree-ring widths were strongly influenced by climatic factors early in the growing season,and the radial growth in trees on the northern slopes was more sensitive to climate than in trees on the southern.Tree-ring δ^(13)C was more sensitive to climate than radial growth.δ^(13)C fractionation was mainly influenced by summer temperature and precipitation early in the growing season.Stomatal conductance more strongly limited stable carbon isotope fractionation in tree rings than photosynthetic rate did.The response between tree rings and climate in mountains gradually weakened as climate warmed.Changes in radial growth and stable carbon isotope fractionation of P.crassifolia in response to climate in the Qilian Mountains may be further complicated by continued climate change.

土壤呼吸及其^(13)C同位素测定方法的对比研究

为比较不同方法在土壤呼吸及其^(13)C同位素测定中的差异,应用气相色谱仪法(GC-TCD)、稳定同位素比值质谱仪气体进样法(Gasbench-IRMS)、甲酚红显色法(MicroResp)、碱液吸收法4种方法测定土壤呼吸速率,并采用Gasbench-IRMS法和碱液吸收法两种方式检测土壤呼吸的^(13)C同位素丰度,以期准确评估土壤呼吸及碳排放,并为相关研究提供参考。结果表明:(1)两种仪器法(GC、IRMS)测定土壤呼吸速率的数值结果相近(基础呼吸)或趋势一致(诱导呼吸),且重复性好(标准偏差分别为0.011、0.010mg/(kg·h)),准确度高;MicroResp法的测定结果与仪器测量值较为相近,但分辨率较低;碱液吸收法的测定结果较真实值偏高(当土壤有机质含量低时)或偏低(当土壤有机质含量高时)。(2)在测定CO_(2)中的^(13)C含量上,Gasbench-IRMS法直接测定的结果误差小(δ^(13)C值的标准偏差为0.137‰),接近实际值,可以准确地反映出土壤微生物呼吸对底物的利用状况。综上,仪器法较化学分析法(MicroResp、碱液吸收)更能准确测定土壤呼吸及其^(13)C同位素。

一种基于高分辨液质联用仪的^(13)C标记葡萄糖同位素丰度检测方法

[U-^(13)C_(6)]葡萄糖是一种重要的同位素试剂,在代谢组学等生物医药领域具有重要应用。开发了一种基于超高效液相色谱-高分辨质谱联用仪的[U-^(13)C6]葡萄糖同位素丰度检测方法。在确定检测[U-^(13)C_(6)]葡萄糖同位素丰度的适用条件后,进行液质联用上机检测,根据不同同位素标记模式峰的强度值进行丰度计算;考察了样品浓度、质谱分析图数、流动相背景干扰对同位素丰度计算的影响,在最优条件下取出峰时间窗口内所有质谱图并扣除流动相背景干扰后,重复7次进样测得浓度为10μg/mL的[U-^(13)C_(6)]葡萄糖试剂^(13)C丰度为99.05%(理论值99.2%)且RSD为0.15%。该方法无需复杂的前处理过程,具有较好的准确性和稳定性,是一种具有潜力的同位素试剂质量控制方法。

酶催化法测定^(13)C标记尿素同位素丰度

建立酶催化法测定^(13)C标记尿素同位素丰度的快速检测方法。在特殊制作的三球反应管中经过2步反应:(1)利用脲酶的专一性将尿素-^(13)C催化转化生成碳酸铵-^(13)C,在40℃下,反应5 min;(2)经过酸化生成^(13)C标记CO_(2),将气体引入MAT-271气体同位素质谱仪中,检测质荷比为44和45离子流强度,并计算^(13)C同位素丰度。结果表明,测定值与标样的^(13)C标示值误差在0.1%以内,精密度RSD<0.1%,建立的检测方法与现行行业标准方法测试结果基本一致,该方法操作相对简便快速、专一性强,适用于呼气试验诊断试剂及原料药尿素-^(13)C中同位素丰度的快速检测。

Trace element geochemistry and stable isotopic(δ^(13)C andδ^(15)N)records of the Paleocene coals,Salt Range,Punjab,Pakistan

The Paleocene coals of the Salt Range in the Punjab Province of Pakistan have great economic potential;however,their trace element and stable isotopic characteristics have not been studied in detail except for a few sporadic samples.In this study,a total of 59 coal samples of which 14 are obtained from open cast mines have been investigated for elemental composition andδ^(13)C-δ^(15)N isotopic signatures.Average contents of trace elements such as Co,Cr,Cu,Pb,Sr,Th,U,V,and Zn are 7.4,41.7,11.2,12.5,90.2,4.0,1.9,128,and 31.1 mg/kg,respectively.These values,when compared with the World Coal Clarke values,were relatively higher in low-rank coals in comparison with Clarke values for brown coals.Likewise,As(20.4 mg/kg),Co(6.6 mg/kg),Cr(22.4 mg/kg),Cu(^(13).3 mg/kg),Pb(19.2 mg/kg),Sr(^(15)4.7 mg/kg),Th(2.5 mg/kg),V(47.8 mg/kg),and Zn(75.1 mg/kg)were significantly higher in the sub-bituminous to bituminous coals of the Salt Range.Mineralogical analysis,based on X-ray diffraction and energy dispersive X-ray spectroscopy,revealed that the studied samples contain illite,kaolinite calcite,gypsum,pyrite,and quartz.Elemental affinity with organic and inorganic phases of coals calculated by an indirect statistical approach indicated a positive association of ash content with Ag,Al,Co,Cr,Cs,Cu,Mn,P,Rb,Pb,Th,U,and V,suggesting the presence of inorganic components in studied coals.However,As,Fe,Sr,and Zn exhibit negative correlations that imply their association with the organic fraction.Theδ^(13)C andδ^(15)N isotopic range and average−24.94‰to−25.86‰(−25.41‰)and−2.77‰to 3.22‰(0.96‰),respectively,reflecting 3C type modern terrestrial vegetation were common in the palaeomires of studied coal seams.In addition,the trivial variations of 0.92‰and 0.45‰among^(13)C and^(15)N values can be attributed to water level fluctuations and plant assemblies.

气相色谱-质谱联用选择离子监测方法定量分析低浓度胞内游离氨基酸的^(13)C同位素丰度

胞内游离氨基酸具有周转快的特点,其13C同位素丰度能快速反映胞内代谢状态的变化。但胞内游离氨基酸的浓度很低,现有的基于气相色谱-质谱联用全扫描模式的13C同位素丰度检测方法不能满足要求。本研究考察理论上检测精度更高的选择离子监测方法在胞内游离氨基酸13C同位素丰度分析中应用的可能性。首先在全扫描模式下分析了不同氨基酸的断裂规律,找出与每种氨基酸对应的特征碎片,建立起包含有16种胞内游离氨基酸的特征碎片库。利用此特征碎片库,在样品分析时只需检测特定m/z处的信号,从而实现选择离子监测,提高信号质量。对标准品的检测结果表明,与全扫描模式相比,本方法的信噪比、测量精度和准确性分别提高了17倍、2倍和3.8倍。在对辅酶Q10生产菌株样品的分析中,本方法成功检测出8种胞内游离氨基酸的同位素丰度。

美沙西汀-甲氧基-^(13)C的同位素丰度和化学纯度表征

本研究建立了美沙西汀-甲氧基-13C高效液相色谱-质谱联用检测方法,成功的对该化合物的同位素丰度和化学纯度进行了同时表征。通常传统的13C同位素丰度表征是气体同位素质谱法,但其存在着不能检测13C原子部分标记化合物同位素丰度检测的缺陷。本研究首次提出"质量簇"分类方法,结合高效液相色谱-质谱联用技术,成功的解决了单13C标记的美沙西汀-甲氧基-13C同位素丰度分析难题。

MAT-251质谱计在^(13)CO_(2)呼气检查法中的应用

本文简要介绍了ＭＡＴ－２５１气体质谱计的结构特点、测试方法及其主要性能指标，并从１３ＣＯ２呼气检查法的试验方法以及对质谱仪器的性能指标要求两个方面阐述了ＭＡＴ－２５１气体质谱计在１３ＣＯ２呼气检查法中的应用。

^(13)C同位素示踪法研究半纤维素与木素之间的连接方式

为了阐明植物纤维原料中木素与半纤维素之间的相互关系,本文从甘露糖单元的角度分析糖与木素苯丙烷结构单元之间的连接方式。为了使生长中的植物的细胞壁甘露糖单元上的碳原子被碳-13同位素示踪并抑制甘露糖向木素转化,将带D-13C6-甘露糖、外源性木素前驱物松伯醇β-D-葡萄糖苷以及苯丙氨酸解氨酶的抑制剂一起投入到银杏植株中。通过碳-13丰度检测得知银杏中半纤维素被碳-13同位素成功标记,外源性的D-13C6-甘露糖主要在次生壁中发生聚合沉积。FT-IR谱图分析发现投入的外源性糖等没有影响银杏植株的正常生长,高分辨率CP/MAS13C-NMR证实了甘露糖单元的C6位置与木素苯丙烷结构上的α-C存在连接键,主要为醚键。

Use of δ^(18)O, δ^(13)C and NO_(3)^(-) to identify hydrogeochemical processes related to contamination in an aquifer located in central Mexico

In this work,an isotopic analysis of δ^(18)O,δ^(13)C,and NO_(3)^(-) concentrations was carried out to identify the origin and the processes related to the contamination of an aquifer located in the state of Guanajuato,Mexico.The research identified the possible sources of δ^(13)C in groundwater.During groundwater flow,CO_(2) participates in different hydrogeochemical reactions in which the dissolution of carbonates or biochemical processes related to biodegradation stand out.Isotopic data of δ^(13)C,δ^(18)O,and the hydrogeochemical behavior of NO_(3)^(-) and HCO_(3)^(-) in water,in addition to isotopic data and the chemical composition of limestones in the study area,were determined to establish the isotopic signature and the processes undergone by the rocks.The isotopic signature of rock and water samples indicated that metamorphic limestones contributed with carbon dioxide to deep groundwater,while in the upper aquifer,bacterial metabolic reactions during nitrification–denitrification could modify the isotopic signature of δ^(13)C in some wells,although atmospheric contribution also plays a role.The modification of the carbon isotopic component is related to the precipitation of calcite in specific regions of the study area,input of atmospheric CO_(2),and soil(e.g.the possible participation of C4-type plants in the assimilation-release of carbon).This process is not confirmed or completely ruled out in this study since agriculture is excessively developed throughout the region.The joint interpretation of isotopic values and the hydrogeochemical behavior of major and conservative elements help in identifying possible pollution processes in which different carbon sources are related.

Temporal and spatial variations of carbon isotope signature reveal substantial contribution of bracts and internode assimilates to grain filling of japonica rice

Carbon isotope composition(δ^(13)C)of a plant organ is an inherent signature reflecting its physiological property,and thus is used as an integrative index in crop breeding.It is also a non-intrusive method for quantifying the relative contribution of different source organs to grain filling in cereals.Using the samples collected from two-year field and pot experiments with two nitrogen(N)fertilization treatments,we investigated the temporal and spatial variations of δ^(13)C in source organs of leaf,sheath,internode,and bracts,and in sink organ grain.Constitutive nature of δ^(13)C was uncovered,with an order of leaf(−27.84‰)<grain(−27.82‰)<sheath(−27.24‰)<bracts(−26.81‰)<internode(−25.67‰).For different positions of individual organs within the plant,δ^(13)C of the leaf and sheath presented a diminishing trend from the top(flag leaf and its sheath)to the bottom(the last leaf in reverse order and its sheath).No obvious pattern was found for the internode.For temporal variations, δ^(13)C of the leaf and sheath had a peak(the most negative)at 10 days after anthesis(DAA),whereas that of the bracts showed a marked increase at the time point of anthesis,implying a transformation from sink to source organ.By comparing the δ^(13)C in its natural abundance in the water-soluble fractions of the sheath,internode,and bracts with the δ^(13)C in mature grains,the relative contribution of these organs to grain filling was assessed.With reference to the leaf,the internode accounted for as high as 32.64%and 42.56%at 10 DAA and 20 DAA,respectively.Meanwhile,bracts presented a larger contribution than the internode,with superior bracts being higher than inferior bracts.In addition,N topdressing reduced the contribution of the internode and bracts.Our findings clearly proved the actual significance of non-foliar organs of the internode and bracts for rice yield formation,thus extending our basic knowledge of source and sink relations.

基于高分辨液质联用的双标记氨基酸同位素丰度测定方法研究

近年来,稳定同位素标记化合物,尤其是同位素标记氨基酸在合成生物学、蛋白质组学、代谢组学等前沿科学研究中发挥着越来越重要的作用。建立了一种基于高分辨液质联用的双标记氨基酸同位素丰度检测方法,以^(13)C_(2),^(15)N-甘氨酸为例,考察了样品浓度、质谱扫描张数等影响因素对同位素丰度计算结果的影响,结果表明该方法具有良好的准确性和稳定性,所需样品用量少,操作简单,有望成为一种简便、通用的同位素试剂质控方法;也将为代谢组学、海洋学等同位素示踪技术应用研究提供数据支持。

Pyrolysis mechanism of glucose and mannose: The formation of 5-hydroxymethyl furfural and furfural

Fast pyrolysis of biomass will produce various furan derivatives, among which 5-hydroxymethyl furfural（5-HMF） and furfural（FF） are usually the two most important compounds derived from holocellulose. In this study, density functional theory（DFT） calculations are utilized to reveal the formation mechanisms and pathways of 5-HMF and FF from two hexose units of holocellulose, i.e., glucose and mannose. In addition, fast pyrolysis experiments of glucose and mannose are conducted to substantiate the computational results, and the orientation of 5-HMF and FF is determined by 13C-labeled glucoses. Experimental results indicate that C1 provides the aldehyde group in both 5-HMF and FF, and FF is mainly derived from C1 to C5 segment. According to the computational results, glucose and mannose have similar reaction pathways to form 5-HMF and FF with d-fructose（DF） and 3-deoxy-glucosone（3-DG） as the key intermediates. 5-HMF and FF are formed via competing pathways. The formation of 5-HMF is more competitive than that of FF, leading to higher yield of 5-HMF than FF from both hexoses. In addition, compared with glucose,mannose can form 5-HMF and FF via extra pathways because of the epimerization at C2 position. Therefore, mannose pyrolysis results in higher yields of 5-HMF and FF than glucose pyrolysis.

A quantum cascade laser infrared spectrometer for CO_2 stable isotope analysis：Field implementation at a hydrocarbon contaminated site under bio-remediation

Real-time methods to monitor stable isotope ratios of CO_2 are needed to identify biogeochemical origins of CO_2 emissions from the soil–air interface. An isotope ratio infra-red spectrometer（IRIS） has been developed to measure CO_2 mixing ratio with δ~13C isotopic signature, in addition to mixing ratios of other greenhouse gases（CH_4, N2_O）. The original aspects of the instrument as well as its precision and accuracy for the determination of the isotopic signature δ~13C of CO_2 are discussed. A first application to biodegradation of hydrocarbons is presented, tested on a hydrocarbon contaminated site under aerobic bio-treatment. CO_2 flux measurements using closed chamber method is combined with the determination of the isotopic signature δ~13C of the CO_2 emission to propose a non-intrusive method to monitor in situ biodegradation of hydrocarbons. In the contaminated area, high CO_2 emissions have been measured with an isotopic signature δ~13C suggesting that CO_2 comes from petroleum hydrocarbon biodegradation.This first field implementation shows that rapid and accurate measurement of isotopic signature of CO_2 emissions is particularly useful in assessing the contribution of contaminant degradation to the measured CO_2 efflux and is promising as a monitoring tool for aerobic bio-treatment.

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.

Developing an authentication approach using SPME-GC-IRMS based on compound-specific δ^(13)C analysis of six typical volatiles in wine

An analytical method using gas chromatography isotope ratio mass spectrometry(GC-IRMS)combined with solid phase micro-extraction(SPME)was developed to measure the 613C values of six typical volatiles commonly occurring in wine(isoamyl acetate,2-octanone,limonene,2-phenylethanol,ethyl octanoate and ethyl decanoate)for the first time.SPME selected with a divinylbenzene/carboxen/polydimethylsiloxane fiber was combined with the GC-IRMS for pretreatment optimization.The optimized SPME parameters of extraction time,extraction temperature and salt concentration were 40 min,40℃ and 10%,respectively.The 613C values measured by SPME-GC-IRMS were in good agreement with those measured via elemental analyzer(EA)-IRMS and GC-IRMS.The differences range from 0.02‰to 0.44‰ with EA-IRMS and from 0 to 0.28‰ with GC-IRMS,indicating the high accuracy of the method.This newly established method measured the precision within 0.30‰ and was successfully validated to discriminate imported real wine samples with identical label but amazing price differences from different importers.

Influencing factors and partitioning methods of carbonate contribution to CO_(2)emissions from calcareous soils

In calcareous soils,recent studies have shown that soil-derived CO_(2)originates from both soil organic carbon(SOC)decomposition and soil inorganic carbon(SIC)dissolution,a fact often ignored in earlier studies.This may lead to overestimation of the CO_(2)emissions from SOC decomposition.In calcareous soils,there is a chemical balance between precipitation and dissolution of CaCO_(3)-CO_(2)-HCO_(3),which is affected by soil environmental factors(moisture,temperature,pH and depth),root growth(rhizosphere effect)and agricultural measures(organic materials input,nitrogen fertilization and straw removal).In this paper,we first introduced the contribution of SIC dissolution to CO_(2)emissions from calcareous soils and their driving factors.Second,we reviewed the methods to distinguish two CO_(2)sources released from calcareous soils and quantify the 13C fractionation coefficient between SIC and SIC-derived CO_(2)and between SOC and SOC-derived CO_(2),and to partition three CO_(2)sources released from soils with plants and organic materials input.Finally,we proposed methods for accurately distinguishing three CO_(2)sources released from calcareous soils.This review helps to improve the accuracy of soil C balance assessment in calcareous soils,and also proposes the direction of further investigations on SIC-derived CO_(2)emissions responses to abiotic factors and agricultural measures.

^(13)C analysis of cow tail hair and farm slurry can be used to implicitly distinguish between different dairy production systems

Background Public interest in the way food is produced on the farm and processed along the food-production chain is increasing.The analysis of isotopic signatures(^(13)C)in cow tail hair provides a method to reconstruct the dietary proportion of maize in cow diets.Based on this,we further investigated whether there is a relationship between isotopic signatures in cow tail hair or farm slurry and the proportion of maize of the total utilized agricultural area per farm[%].We did an on-farm survey on 17 dairy farms in coastal Northwest Germany and collected cow tail hair from dairy cows and slurry samples on each farm.The farms differed in their feeding regime(C_(3) vs.C_(4) plants),their site conditions(sandy soil=‘Geestland’;organic soil=‘Peatland’;clayey soil=‘Marshland’),and in the area cultivated with maize as a proportion of the total utilized agricultural area per farm.Results We found a positive relationship between δ^(13)C values in both cow tail hair and slurry and the annual dietary proportion of maize(R^(2)=0.67;and R^(2)=0.63).Furthermore,we confirmed that there was a relationship betweenδ^(13)C values in cow tail hair and area of maize as a proportion of the total utilized agricultural area per farm(R^(2)=0.69).Conclusion Our findings suggest a general applicability of using isotopic signatures(^(13)C)along a wide gradient of site conditions and productions systems in practice.

Combined application of biochar with fertilizer promotes nitrogen uptake in maize by increasing nitrogen retention in soil

Combined application of biochar with fertilizers has been used to increase soil fertility and crop yield.However,the coupling mechanisms through which biochar improves crop yield at field scale and the time span over which biochar affects carbon and nitrogen transformation and crop yield are still little known.In this study,a long-term field trial(2013-2019)was performed in brown soil planting maize.Six treatments were designed:CK-control;NPK-application of chemical fertilizers;C1PK-low biochar without nitrogen fertilizer;C1NPK,C_(2)NPK and C_(3)NPK-biochar at 1.5,3 and 6 t ha^(−1),respectively,combined with chemical fertilizers.Results showed that theδ^(15)N value in the topsoil of 0-20 cm layer in the C_(3)NPK treat-ment reached a peak of 291‰at the third year(2018),and demonstrated a peak of 402‰in the NPK treatment in the initial isotope trial in 2016.Synchronously,SOC was not affected until the third to fourth year after biochar addition,and resulted in a significant increase in total N of 2.4 kg N ha^(−1) in 2019 in C_(3)NPK treatment.During the entire experiment,the ^(15)N recovery rates of 74-80%were observed highest in the C_(2)NPK and C_(3)NPK treatments,resulting in an annual increase in yields significantly.The lowest subsoilδ^(15)N values ranged from 66‰to 107‰,and the ^(15)N residual rate would take 70 years for a complete decay to 0.001%in the C_(3)NPK.Our findings suggest that biochar compound fertilizers can increase C stability and N retention in soil and improve N uptake by maize,while the loss of N was minimized.Biochars,therefore,may have an important potential for improving the agroecosystem and ecological balance.

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.