玉米秸秆与油页岩混烧的热重试验

为了充分燃烧利用农业生物质与油页岩,采用热重-差示扫描量热(TG-DSC,thermogravimetric-differential scanning calorimetry)技术对玉米秸秆与油页岩混合燃料进行了热重试验,对其各燃烧特性与燃烧机理进行了分析并计算了燃烧动力学参数。结果表明,混烧试样的微分热重(DTG,differential thermogravimetric)曲线出现4个峰值,主要分别是半纤维素热解、纤维素热解、木质素和油页岩热解,以及油页岩焦炭燃烧和油页岩无机盐热解。在DTG曲线第1至第3峰之间出现DSC曲线放热峰,在DTG曲线第3峰至第4峰之间出现DSC曲线吸热峰,需要强化混合燃料的前期燃烧;玉米秸秆与油页岩的质量比例为4∶1时的混合燃料S2,其混烧的可燃特性、着火特性及综合燃烧特性指数均最大,燃尽时间最短,燃尽特性指数较大;在前3个温度区段,混合试样的活化能及频率因子均随温度区段的升高而降低,体现了活化能与频率因子变化的一致性;低温阶段与高温阶段反应级数分别约为1.5及0.7。研究结果可为生物质和油页岩的混烧利用提供参考。     

应用同步热分析仪确定小麦秸秆热解需热量

为了解决生物质热解过程需热量的定量问题，该文应用热重—差示扫描（TG/DSC）同步热分析仪对小麦秸秆进行了热解实验研究。将约5 mg的小麦秸秆粉样品装入带盖的铂铑坩锅中，放在热解炉中的DSC-cp高精度样品支架上，在流量为25 mL/min的高纯氮气吹扫下，以10 K/min的升温速率从常温升至973 K，记录生物质的热重(TG)曲线和差示扫描(DSC)曲线。通过对实验所得微分热重(DTG)曲线和DSC曲线对比分析，对小麦秸秆热解过程进行了详细的探讨。在DSC曲线上扣除水分的影响后对其积分得出热解过程需热量的规律。结果表明，要使1 kg干小麦秸秆完成从常温303 K到673 K，773 K，873 K的升温和热解，所需的总热量分别为523 kJ，558 kJ，592 kJ。     

农业生物质燃烧特性及燃烧动力学

为了充分燃烧利用农业生物质,采用 TG-DTG-DSC ( thermogravimetric-differential thermogravimetric-differential scanning calorimetry)联用技术对玉米杆、玉米芯、稻草、龙眼枝、荔枝条及其混合燃料进行了热重试验,考察了其可燃特性、着火特性、燃尽特性及综合燃烧特性,计算了燃烧动力学参数.结果表明,玉米杆及玉米芯燃烧前期DTG曲线分别出现2、3个峰值,而稻草、龙眼枝及荔枝条燃烧前期DTG曲线均只出现一个峰值;玉米杆及稻草燃烧中后期出现DSC曲线的吸热峰;玉米杆的可燃特性指数及着火特性指数均最大,且着火温度最低,荔枝条的燃尽特性指数最大,玉米芯的综合燃烧特性最好;低温阶段反应级数约为1.0~1.2,高温阶段反应级数约为0.5~0.8,低温阶段活化能大于高温阶段的活化能;生物质燃烧前期属于均相着火,后期属于多相着火.秸秆类生物质纯烧的后期稳定性较差,在木质类生物质中适当加入秸秆类生物质有利于混合燃料的前期燃烧,研究结果可为农业生物质的燃烧利用提供指导.     

食用菌菌糠的热解特性及动力学分析

为考察食用菌菌糠的热解特性和机理,该研究采用热重和热重-红外联用对香菇菌糠在氮气气氛下的热解特性进行研究,考察不同升温速率下菌糠的热重(TG)曲线和微分热重(DTG)曲线的变化规律,并对DTG曲线进行分峰分析,通过计算得到香菇菌糠热分解的反应活化能E、反应级数n及频率因子A,以及热解产物析出特性;在固定床上开展了香菇菌糠的定温热解试验,试验温度分别为500、550、600℃,并对三相产物产率和成分组成等进行了分析。结果表明:香菇菌糠热解可以分为3个阶段,水分析出段,热解段和炭化段,其中主要热解阶段为250～550℃,失质量率达到58%;升温速率对香菇菌糠热解影响不明显,但是随着升温速率的增大,试样的TG和DTG曲线向高温区移动;菌糠热解的表观活化能为66.33 kJ/mol,较低的表观活化能表明菌糠更容易发生热解。固定床定温热解结果表明菌糠热解气的主要成分为CO_2、CO、CH_4、H_2,4种气体成分含量由高到低为:CO_2、CO、CH_4、H_2;随着热解温度的升高,菌糠热解所得生物油成分以CxHyOz为主,表明菌糠热解生成的生物油具有一定的潜在利用价值;香菇菌糠热解半焦的主要成分是固定碳,相较于原料,挥发分的占比大幅度减少。     

马铃薯糊化过程中介电特性和热特性的分析(简报)

该文采用介电特性测定仪(DES100)和示差扫描量热仪(DSC120)对马铃薯片(含水率80.7%,淀粉含量16.5%)介电特性和热特性进行了比较研究,以确定马铃薯淀粉糊化过程中各特征参数值之间的关系。由介电特性曲线得出:在1～100kHz频率范围内的,65～80℃温度范围内,介电常数e′随温度的升高出现跃变型变化,67℃时变化速率最大,之后逐渐下降;由热吸收曲线得出热焓值的变化与e′变化规律相同,其糊化的3个特征温度(糊化起始、峰值、结束)分别是65、67和80℃,从两曲线得到的各特征值基本一致,说明利用马铃薯的介电特性和热特性的综合分析可以更准确地掌握其糊化过程,为进一步利用和控制马铃薯的糊化作用提供参考。     

流化床藻类生物质快速热裂解试验

为实现藻类生物质资源的综合利用,该文选取藻类生物质中的马尾藻进行热重分析,并在自行设计的小型流化床上进行快速热裂解试验,分别研究了马尾藻热解过程及热解产物的产率随温度的变化规律。结果表明,随着温度的升高,热解经历了3个阶段：预热解、快速热解、慢速热解,并且由于多糖、蛋白质等物质热稳定性的不同引起马尾藻在快速热解阶段出现两个失重峰。在快速热裂解试验中,选择450、500、550、600℃ 4个反应温度对马尾藻的热裂解规律进行了研究,主要考察了不同反应温度对热裂解产物收集率的影响。研究发现残炭的产率随着温度的升高而降低,而热解气的产率则随着温度的升高而升高,生物油的产率随温度变化先升高后降低,在550℃左右时产率最高,约为30.5%,这为马尾藻快速热裂解制油的推广与工业利用提供参考。     

亚麻籽粉含水率对其蛋白质变性温度的影响

本文研究亚麻籽粉含水率对其蛋白质变性温度的影响,通过对亚麻籽热变性的研究,为下一步亚麻籽的脱毒试验研究奠定一定的理论基础.应用差示扫描量热(DSC)技术测定不同的亚麻籽含水率下的亚麻籽粉蛋白质变性温度,分析在不同的条件下亚麻籽粉蛋白质变性温度特性.试验表明,亚麻籽粉蛋白质的变性温度较高,在129～143℃之间;随着升温速率的提高,DSC曲线的峰幅增大,峰形尖锐,变性温度升高,吸热焓值变化不大;亚麻籽蛋白质的变性温度与含水率有密切的关系,随含水率升高而降低.研究为亚麻耔深加工和利用提供了一定的理论依据.     

高含量乳清粉的仔猪配合饲料热特性及调质温度控制

为探究热敏性饲料原料乳清粉及不同含量乳清粉的仔猪配合饲料的热物理特性,该文以仔猪料配方中的4种主要饲料原料玉米、豆粕、乳清粉和鱼粉为研究对象,采用混料设计的方法得到33种不同含量(0～30％)乳清粉的仔猪配合饲料,并利用差示扫描量热法(differential scanning calorimetry,DSC)测定了4种单一原料在25～120℃范围内以及33种仔猪配合饲料在25～110℃范围内的比热,分析了乳清粉及高含量乳清粉(质量分数≥14.548％)的仔猪配合饲料的热变性过程.结果显示:玉米、豆粕和鱼粉的比热分别与温度(25～120℃)呈线性、对数和二次关系,而乳清粉的比热与温度(25～110℃)遵循三次多项式的关系;当配合饲料中含有≥6.25％的乳清粉时,其比热与温度遵循三次多项式的关系;配合饲料的比热显著受温度、原料配比以及二者交互作用的影响(P＜0.001),其中,温度的影响最为显著,而乳清粉含量的影响次之.DSC热焓曲线上,乳清粉在109.79℃会出现吸热峰,为乳清蛋白的热变性导致;而随着温度由20℃升高到110℃,乳清粉颗粒由存在许多凸起与微孔的粗糙表面结构逐渐过渡为光滑、粘结的状态.与乳清粉相似,高含量乳清粉的配合饲料也会在77.95～87.69℃出现吸热峰.在仔猪配合颗粒饲料的加工过程中,为降低乳清蛋白的变性程度、减少环模制粒机的堵机现象,应将调质温度降低至70℃以下为宜.研究结果为高含量乳清粉的仔猪配合饲料的调质、制粒等热处理过程的工艺优化提供理论指导.     

莲子热风干燥过程对其淀粉热特性及凝胶化的影响

为解决莲子干燥过程中淀粉形态结构变化造成莲子结壳、硬化,不利于干燥以及复水难、易返生问题,该文利用差示扫描量热技术(differential scanning calorimetry,DSC)对新鲜莲子以及不同热风干燥(70、80、90℃)莲子的淀粉热特性与凝胶化过程进行了研究.研究发现,莲子淀粉在低水分环境(42.2%,以质量比计)时存在2个明显的吸热峰,高水分环境(71.1%,以质量比计)时存在1个明显的吸热峰;莲子在干燥过程中不断失水,并伴随着淀粉凝胶化.方差分析(analysis of variance,ANOVA)表明,高温干燥显著影响莲子淀粉的热特性,其淀粉凝胶化温度(峰起温度To、峰顶温度Tp以及峰止温度Te)部分显著升高.相同干燥条件下,莲子淀粉糊化焓ΔH受水分显著影响,但干燥温度、升温速率对其影响不显著(P>0.01).采用Kissinger、Crane方程获得了淀粉凝胶化动力学参数(活化能Ea、指前因子Z以及反应级数n).莲子淀粉的非等温凝胶化反应可近似为一级反应,高温干燥后其Ea值出现增加,并随着水分增加呈现降低趋势.研究结果可为确定莲子高品质干燥工艺以及干莲子、莲子淀粉后续加工过程提供技术支持.     

食盐添加量对预制鲈鱼冷藏保鲜及热加工特性的影响

为研究不同食盐添加量(0~2.0%)对鲈鱼冷藏保鲜及热加工特性的影响,采用差示扫描量热法,研究鲈鱼鱼肉的含水率、冰点、变性温度、热焓和比热容等热特性并进行分析。研究发现,预制鲈鱼的冰点和含水率随食盐添加量的增加而逐渐降低;加热温度50℃附近吸热峰的起始温度、终止温度及变性热焓随食盐添加量的增加均逐渐降低;70℃附近吸热峰仅明显存在于新鲜鲈鱼样品中,其他食盐添加量条件下70℃附近吸热峰消失;鲈鱼鱼肉的比热容随食盐添加量的增加未呈现明显的变化趋势(P0.05),当鱼肉处于冻结状态、肌球蛋白及肌动蛋白处于变性状态时,2.0%食盐添加量鲈鱼的比热容最高,当鱼肉处于未冻结状态时,1.0%食盐添加量鲈鱼的比热容最高。研究表明:在不同食盐添加量(0~2.0%)条件下,随着食盐添加量的增多,鲈鱼冰点和含水率逐渐下降,肌球蛋白变性温度向低温方向移动,峰高逐渐降低,变性热焓逐渐下降,肌动蛋白峰消失;添加食盐的鲈鱼比热容变化皆小于新鲜鲈鱼(除1.0%食盐添加量)(P0.05);在不同食盐添加量(0~2.0%)条件下,未冻结鱼肉的比热容高于冻结鱼肉(P0.05),加工后的鱼肉的比热容高于未加工的鱼肉(P0.05)。研究结果为预制鲈鱼热加工和低温贮藏的品质控制提供参考。     

Genetic Diversity among Syrian Cultivated and Landraces Wheat Revealed by AFLP Markers

Genetic diversity among some important Syrian wheat cultivars was estimated using Amplified Fragment Length Polymorphism (AFLP) markers. Five Triticum aestivum L. and 10 Triticum turgidum ssp. durum were analyzed with 11 EcoRI–MseI primer pair combinations. Of the approximately 525 detected AFLP markers, only 46.67% were polymorphic. Cluster analysis with the entire AFLP data divided all cultivars into two major groups reflecting their origins. The first one contained T. aestivum L. cultivars, and the T. turgidum ssp. durum cultivars and landraces were grouped in the second. Narrow genetic diversity among all cultivars was detected with an average genetic similarity of 0.884. The lowest similarity index (0.9) was found between Cham5 and Hamary (durum wheat), whereas this value was 0.93 between Salamony and Bouhouth 4 (T. aestivum L.). The narrow genetic diversity level indicates that these genotypes could be originated from the same source. AFLP analysis provides crucial information for studying genetic variation among wheat cultivars and provides important information for plant improvement.     

Trace gas emissions from soil of the central highlands of Mexico as affected by natural vegetation: a laboratory study

In the central highlands of Mexico, mesquite (Prosopis laevigata) and huisache (Acacia schaffneri), N₂-fixing trees or shrubs, dominate the vegetation and are currently used in a reforestation program to prevent erosion. We investigated how natural vegetation or cultivation of soil affected oxidation of CH₄, and production of N₂O. Soil was sampled under the canopy of mesquite (MES treatment) and huisache trees (HUI treatment), outside their canopy (OUT treatment) and from fields cultivated with maize (ARA treatment) at three different sites while production of CO₂, and dynamics of CH₄, N₂O and inorganic N (NH₄⁺, and NO₃^–) were monitored in an aerobic incubation. The production of CO₂ was 2.3 times higher and significantly greater in the OUT treatment, 3.0 times higher in the MES treatment and 4.0 times higher in the HUI treatment compared to the ARA treatment. There was no significant difference in oxidation of CH₄ between the treatments, which ranged from 0.019 g CH₄–C kg^–1 day^–1 for the HUI treatment to 0.033 CH₄–C kg^–1 day^–1 for the MES treatment. The production of N₂O was 30 g N₂O–N kg^–1 day^–1 in the MES treatment and >8 times higher compared to the other treatments. The average concentration of NO₃^– was 2 times higher and significantly greater in the MES treatment than in the HUI treatment, 3 times greater than in the OUT treatment and 10 times greater than in the ARA treatment. It was found that cultivation of soil decreased soil organic matter content, C and N mineralization, but not oxidation of CH₄ or production of N₂O.     

厌氧发酵牛粪对浮萍生长和养分吸收的影响

养分从牛粪到农田的循环利用是养牛场可持续发展、资源利用和环境保护的重点研究内容。利用奶牛场废水种植水生植物被认为是一种有效的废水处理及养分循环的方法。本文研究了3种浮萍[少根紫萍0128（Landoltia punctata 0128）、膨胀浮萍7589（Lemna gibba 7589）和小浮萍9517（Lemna minuta 9517）]在厌氧发酵过的奶牛场废水中种植时的养分吸收和生物质变化。在28 d的测试期间,种植在稀释比例为1：18的厌氧发酵过的牛奶场废水中的少根紫萍01283获得最高的总氮吸收率（11.6%±1.6%）,种植在稀释比例为1：27厌氧发酵过的牛奶场废水中的少根紫萍0128获得最高的总磷吸收率（15.4%±4.4%）;相应地少根紫萍鲜重的增长率分别为0.11 g·d^-1和0.17 g·d^-1。3种浮萍中,少根紫萍最具有吸收牛奶场废水氮、磷并获得较高生物质的潜力。     

三种不同绿肥的腐解和养分释放特征研究

利用网袋法模拟研究旱地条件下箭筈豌豆（Vicia sativas L.）、苕子（Vicia villosa Roth. Var.）、山黧豆（Lathyrus palustris L. Var. Pilosus ledeb） 三种绿肥的腐解和养分释放特征。结果表明,三种绿肥均在翻压15 d内腐解较快,腐解率均在50%以上,之后腐解速率逐渐减慢,翻压70 d时,箭筈豌豆、苕子和山黧豆的累积腐解率分别达71.7%、67.3%和74.1%。氮和钾在翻压10 d内释放较快,碳和磷在翻压15 d内释放较快,之后释放速率均减慢。箭筈豌豆、苕子和山黧豆在翻压70 d时的碳累积释放率分别为71.3%、67.0%和74.1%。三种绿肥的养分累积释放率均是K>P>N,在翻压70 d时钾的累积释放率均在90%以上,磷的累积释放率为73.3%～78.7%,氮的累积释放率为59.9%～71.2%,其中山黧豆的氮和磷累积释放率高于箭筈豌豆和苕子,而三种绿肥钾的累积释放率无显著差异。养分释放量结果表明,箭筈豌豆和苕子的养分累积释放量表现为K>N>P,而山黧豆表现为N>K>P,不同绿肥的养分累积释放量不同,山黧豆的氮累积释放量最高,箭筈豌豆的磷和钾累积释放量最高,苕子各养分的累积释放量都最低。     

Effect of N-(n-butyl) thiophosphoric triamide on hydrolysis of urea by plant,microbial, and soil urease

Summary Comparison of the effects of N-(n-butyl) thiophosphoric triamide (NBPT) and phenylphosphorodiamidate (PPD) on hydrolysis of urea by plant (jackbean), microbial (Bacillus pasteurii), and soil urease showed that whereas NBPT was considerably more effective than PPD for inhibiting hydrolysis of urea added to soil, it was much less effective than PPD for inhibiting hydrolysis of urea by plant or microbial urease. Studies to account for this observation indicated that NBPT is rapidly decomposed in soil to a compound that is much more effective than NBPT for inhibition of urease activity and that this compound is N-(n-butyl) phosphoric triamide.     

赤子爱胜蚓谷胱甘肽和丙二醛含量变化指示重金属污染土壤的生态毒性

为探讨赤子爱胜蚓谷胱甘肽(GSH)和丙二醛(MDA)含量变化对重金属污染土壤生态毒性的指示作用,在试验室模拟条件下,利用人工土壤和自然土壤,研究了重金属含量与蚯蚓GSH和MDA含量变化的剂量效应关系。结果显示,在人工土壤试验中,Cu可诱导蚯蚓GSH和MDA含量上升,暴露2 d、7 d、14 d后,Cu浓度与蚯蚓GSH含量的相关系数分别为0.556、0.807和0.775,与MDA含量的相关系数分别为0.760、0.672和0.544,均存在显著的剂量效应关系(p<0.05)。自然土壤试验结果显示,赤子爱胜蚓的GSH和MDA含量与土壤Cu全量的偏相关系数分别为-0.830(p<0.001)和-0.599(p<0.05),与Cd全量的偏相关系数分别为-0.697(p<0.05)和-0.690(p<0.05)。同时发现,赤子爱胜蚓GSH含量变化对土壤重金属响应的敏感性要高于MDA。     

Contribution of fine roots to ecosystem biomass and net primary production in black spruce, aspen, and jack pine forests in Saskatchewan

Fine root (<2 mm) processes contribute to and exhibit control over a large pool of labile carbon (C) in boreal forest ecosystems because of the high proportion of C allocated to fine root net primary production (NPP), and the rapid decomposition of fine roots relative to aboveground counterparts. The objective of this study was to determine the contribution of fine roots to ecosystem biomass and NPP in a mature black spruce (Picea mariana Mill.) (OBS), aspen (Populus tremuloides Michx.) (OA), and jack pine (Pinus banksiana Lamb.) (OJP) stand, and an 11-year-old harvested jack pine (HJP) stand in Saskatchewan. Estimates of fine root biomass and NPP were obtained from nine minirhizotron (MR) tubes at each of the four Boreal Ecosystem Research and Monitoring Sites (BERMS). Fine root data were collected once a month for May–September in 2003 and 2004. Additional C biomass and NPP data for various components of the forest stands were obtained from Gower et al. (1997) and Howard et al. (2004). Annual fine root biomass averaged 3.10 ± 0.89, 1.71 ± 0.49, 1.62 ± 0.32, and 2.96 ± 0.67 Mg C ha⁻¹ (means ± S.D.) at OBS, OA, OJP, and HJP, respectively, comprising between 1 and 6% of total stand biomass. Annual fine root NPP averaged 2.66 ± 0.97, 2.03 ± 0.43, 1.44 ± 0.43, and 2.16 ± 0.81 Mg C ha⁻¹ year⁻¹ (means ± S.D.) at OBS, OA, OJP, and HJP, respectively, constituting between 41 and 71% of total stand NPP. Results of this study indicate that fine roots produce a large amount of C in boreal forests. It is speculated that fine root NPP may control a large amount of labile C-cycling in boreal forests and that fine root responses to environmental and anthropogenic stress may be an early indicator of impaired ecosystem functioning.     

<Emphasis Type="Italic">Rhizobium</Emphasis>,<Emphasis Type="Italic"> Bradyrhizobium</Emphasis> and<Emphasis Type="Italic"> Agrobacterium</Emphasis> strains isolated from cultivated legumes

The present study was conducted to isolate and characterize rhizobial strains from root nodules of cultivated legumes, i.e. chickpea, mungbean, pea and siratro. Preliminary characterization of these isolates was done on the basis of plant infectivity test, acetylene reduction assay, C-source utilization, phosphate solubilization, phytohormones and polysaccharide production. The plant infectivity test and acetylene reduction assay showed effective root nodule formation by all the isolates on their respective hosts, except for chickpea isolate Ca-18 that failed to infect its original host. All strains showed homology to a typical Rhizobium strain on the basis of growth pattern, C-source utilization and polysaccharide production. The strain Ca-18 was characterized by its phosphate solubilization and indole acetic acid (IAA) production. The genetic relationship of the six rhizobial strains was carried out by random amplified polymorphic DNA (RAPD) including a reference strain of Bradyrhizobium japonicum TAL-102. Analysis conducted with 60 primers discriminated between the strains of Rhizobium and Bradyrhizobium in two different clusters. One of the primers, OPB-5, yielded a unique RAPD pattern for the six strains and well discriminated the non-nodulating chickpea isolate Ca-18 from all the other nodulating rhizobial strains. Isolate Ca-18 showed the least homology of 15% and 18% with Rhizobium and Bradyrhizobium, respectively, and was probably not a (Brady)rhizobium strain. Partial 16S rRNA gene sequence analysis for MN-S, TAL-102 and Ca-18 strains showed 97% homology between MN-S and TAL-102 strains, supporting the view that they were strains of B. japonicum species. The non-infective isolate Ca-18 was 67% different from the other two strains and probably was an Agrobacterium strain.     

Revegetation of copper mine tailings with ryegrass and willow

To restore vegetation on metal mine tailings is very difficult because they often contain high concentrations of heavy metals, low nutrient content and low water retention capacity. This study involved 3 experiments that evaluated the effects of 4 treatment amendments: montmorillonite, rice straw, organic manure and chemical fertilizer on the growth of ryegrass (Loliurn perenne L.) and willow (Salix virninalis L.) with Cu and Zn mine tailings from two mining areas. The results showed that ryegrass was the most tolerant of 4 crops to Cu toxicity. Also when organic manure, which contained high concentrations of inorganic salts, was added to the mine tailings, it significantly hindered ryegrass growth (P=0.05).Meanwhile, with ryegrass organic manure significantly increased (P=0.05) the extractable Cu concentration in both mine tallings. When montmorillonite was used as a mine tailings amendment with willow, the height and tress number at the 1st cut were significantly greater (P = 0.05) than a control without montmorillonite. However there was no significant difference for height, tress number, dry weight or root dry weight at the 2nd cut. So, amendment applications to reduce metal toxicity and increase nutrients retention in mine tailings were essential during revegetation of mine tailings.     

Direct monitoring of Rhizobium sp. (Cicer) in nodules and soil using lac Z fusions

We constructed lacZ fusions in Rhizobium sp. (Cicer) by random Tn5-lacZ mutagenesis. The lacZ⁺ fusants formed blue colonies on a Rhizobial minimal medium containing 5-bromo-4-chloro-3-indolyl--D-galactopyranoside (X-gal). Rhizobium sp. (Cicer) fusant HSL-2 was identified in nodules and soil in a mixed population on the basis of the lacZ⁺ phenotype. Nodule occupancy of inoculated Rhizobium sp. (Cicer) HSL-2 (lacZ⁺) was assessed by directly streaking the nodule sap on X-gal plates. This method revealed differences between rhizobia carrying identical antibiotic markers. The rhizobial population in soil was estimated by direct plate counts using a medium containing X-gal. Introduction of lacZ into the Rhizobium sp. thus provided a simple and direct method for identifying strains from nodules and soil.