腐殖酸-高岭石胶体对铀(Ⅵ)在饱和多孔介质中迁移过程的影响

为探讨可溶性有机质存在条件下黏土矿物胶体对铀(Ⅵ)在饱和多孔介质中迁移的影响,选取2种石英砂(细砂和粗砂)分别构建均质构型的石英砂柱,研究了饱和水流条件下腐殖酸存在时高岭石胶体对铀在2种不同粒径石英砂中迁移的影响。结果表明,示踪溶质在2种均质石英砂柱中的穿透曲线相似且分布对称。当在石英砂柱中只通入铀(Ⅵ)溶液时,铀(Ⅵ)在2种粒径石英砂柱中的穿透量均较小(25%),此时大量的铀(Ⅵ)通过络合作用吸附在石英砂表面上。当通入溶液中存在腐殖酸和高岭石胶体时,铀(Ⅵ)在2种石英砂柱中的穿透量增加,且在600～850μm石英砂中(58.36%)比250～425μm石英砂中(42.68%)穿透量更多。这充分表明腐殖酸—高岭石胶体和石英砂粒径是影响铀(Ⅵ)在地下环境中迁移的重要因素。     

非均质性对保守溶质及蒙脱石胶体在饱和多孔介质中运移的影响

选取2种粒径玻璃珠构建6种不同构型的柱子(2个均质和4个非均质),研究了饱和条件下多孔介质构型对保守溶质和蒙脱石胶体运移的影响。结果表明:在2种均质介质中,保守溶质的穿透曲线相似且对称,蒙脱石胶体在大粒径玻璃珠中的移动性更强。当介质中存在大孔隙通道时,优先流现象的出现会加速保守溶质和蒙脱石胶体的迁移,大孔隙通道与周围介质交界面越大保守溶质和蒙脱石胶体的拖尾现象越明显且穿透峰较低。在大、小粒径玻璃珠均匀混合填装的柱子中,保守溶质穿透曲线形状与均质柱子相似,蒙脱石胶体的移动性介于大、小粒径玻璃珠均匀填装的柱子之间。蒙脱石胶体在玻璃珠分层填装的柱子中,大粒径与小粒径交界面区域会有更多的蒙脱石胶体滞留。研究表明非均质性对保守溶质和蒙脱石胶体在土壤和地下环境中的运移过程有重要的影响。     

饱和多孔介质中高岭石胶体和SiO2胶体运移行为比较

胶体在多孔介质中的运移行为不仅取决于胶体本身的性质,同时还受到溶液离子强度、孔隙水流速等物理、化学因素的影响。本文通过室内饱和石英砂柱出流实验,探讨了高岭石胶体(Kaolinite胶体)和SiO2胶体在不同离子强度(0.001 5,0.01和0.03 mol L-1)和平均孔隙流速(约20和5 cm h-1)作用下的迁移行为。结果表明,随溶液离子强度增加,Kaolinite胶体和SiO2胶体的沉淀量增加;平均孔隙水流速的增加则会促进Kaolinite胶体和SiO2胶体的运移。同时还表明,离子强度和流速变化对粒径较大的Kaolinite胶体运移的影响要显著大于SiO2胶体。研究结果有助于加深胶体对污染物运移的促进机制的理解,同时对全面、客观评价地下环境的污染风险具有一定的指导意义。     

离子强度和pH对高岭石胶体运移影响的实验研究

离子强度和pH是控制胶体运移的两个重要化学影响因素。本文通过高岭石胶体（Kaolinite胶体）在饱和石英砂柱中的出流实验,分析了离子强度（0.0015～0.03mol L^-1）和pH值（5～9）两个因素的变化对胶体运移的影响。结果表明：随离子强度增加,Kaolinite胶体沉淀量增大,运移受阻增强。Kaolinite胶体在中性环境（pH值约为7.10）中沉淀量最小,在酸性环境（pH值约为4.98）中沉淀量最大。     

胶体颗粒对不同粒径饱和多孔介质渗透性的影响

探讨胶体颗粒在多孔介质中迁移所发生的物理、化学及生物作用过程,在许多学科中具有重要的科学意义。采用室内石英砂柱实验,开展了定水头条件下不同浓度和粒径的胶体颗粒在饱和多孔介质中的运移行为研究。共使用了3种胶体粒径、3种浓度的胶体溶液和3种粒径范围的石英砂。实验表明:多孔介质的相对渗透系数K/K0(K为各时刻计算所得的渗透系数,K0为初始渗透系数)减小程度与颗粒浓度成正比;胶体颗粒越大,越易在表层沉积,而小颗粒易向下部迁移,但总体来看粒径效应没有浓度效应明显;当胶体颗粒在不同粒径的饱和多孔介质中迁移时,粒径大的多孔介质各段K/K0均有明显降低,而粒径小的只在表层变化明显。不同条件下总体相对渗透系数与时间之间呈二次方相关关系,但当多孔介质粒径较小时,相关性不显著。介质渗流流速及砂柱不同位置胶体颗粒浓度变化与介质渗透性变化相对应。用扫描电镜(Scanning Electron Microscope,SEM)进行定性表征,进一步说明胶体颗粒会堵塞多孔介质孔隙影响其渗透性。实验中发现当输入浓度C0小于0.5 g·L–1且dp/Dp>0.018(dp为胶体颗粒粒径,Dp为石英砂算数平均粒径)时,会出现多孔介质局部K/K0增大的现象。     

氧化石墨烯和五价砷在改性多孔介质中的共迁移特征

砷是农田土壤重金属污染的主要元素之一,在砷污染农田土壤的修复过程中往往忽视纳米颗粒能够使结合态的砷重新释放,导致有效态砷浓度升高,探究土壤中黏土矿物对氧化石墨烯(Graphene Oxide,GO)和五价砷(As(V))在多孔介质中迁移行为的影响,对进一步完善农田土壤砷修复理论以及提高农作物产量、保护人体健康具有重要意义。该研究利用蒙脱石和高岭石改性石英砂,通过砂柱迁移试验系统地研究了GO、As(V)和GO-As(V)在填加0%、10%、30%和50%的蒙脱石和高岭石改性石英砂柱中的迁移行为。研究结果表明,随着高岭石和蒙脱石改性石英砂填加比例的增加,GO和As(V)的迁移能力均呈降低趋势,且GO和As(V)在不同条件下的迁移曲线均存在显著差异(P0.05);GO在50%高岭石和蒙脱石改性石英砂柱中的回收率相对于石英砂柱分别下降了14%和17%,As(V)分别下降了15%和12%;在共迁移试验中,GO和As(V)在石英砂柱中回收率分别上升至99%和100%。分析表明,As(V)在蒙脱石改性石英砂柱中的迁移能力大于高岭石改性石英砂,而GO与之相反;当GO和与As(V)共迁移时,二者在介质中的迁移能力均大于其单独迁移。本研究表明GO、As(V)释放到土壤后,能够加速As(V)的迁移,造成土壤砷污染的扩大化。     

土壤胶体在不同饱和度土壤介质中的释放与淋溶行为研究

采用饱和与非饱和填充土柱纵向淋溶研究方法,结合对流弥散模型方程（CDE）对穿透曲线的拟合计算,全面考察了土壤介质水饱和度、土壤水pH/离子强度、土壤孔隙水流速和土壤胶体颗粒大小对天然土壤胶体在实际土壤介质中释放、沉积迁移行为的影响。分别获取胶体扩散系数和阻滞因子值,定量说明实验中水化学、水动力学等条件的作用影响力。结果显示,介质不饱和条件不利于胶体的释放和淋溶;高pH和低离子强度条件对土壤胶体释放与迁移有利;淋溶过程的间断干扰,可以促使土壤胶体的增量淋溶释放;淋溶强度及胶体颗粒粒径大小,能够影响胶体穿透时间和穿透浓度峰值大小。     

流动电位法研究高岭石胶体对包铝石英砂zeta电位的影响

为探讨采用流动电位法表征带电颗粒间相互作用的可行性,采用自制的流动电位测量装置测定了高岭石胶体流经包铝石英砂时zeta电位的变化(Δζ),研究高岭石与包铝石英砂之间的作用程度,考察了包铝程度、介质离子强度和离子种类对带电颗粒之间相互作用的影响。结果表明,可采用流动电位法表征不同尺寸的异电荷颗粒间的相互作用。随着石英砂表面包铝程度的增加和介质离子强度的降低,包铝石英砂的Δζ增加,说明其与高岭石之间的作用程度增加。含低价态离子的溶液中高岭石胶体与包铝石英砂的作用程度大于其在含有高价离子的溶液中。采用经典的DLVO理论计算得到的高岭石胶体与包铝石英砂间的作用能可以很好地解释高岭石对包铝石英砂表面电化学性质的影响,两种异电荷颗粒间的静电引力大小是决定高岭石与包铝石英砂间作用程度的关键因素。在静电力的作用下,高岭石通过电荷屏蔽作用和双电层重叠作用改变包铝石英砂的zeta电位。高岭石与包铝石英砂间的静电引力越大,高岭石对包铝石英砂表面电化学性质的影响越大。     

三峡水库消落带土壤胶体释放与迁移特征

三峡水库消落带是库区陆源污染物进入水库的最后屏障。高强度、周期性的干湿交替对消落带土壤理化性质、结构和可蚀性产生潜在影响,进而影响胶体颗粒的释放和迁移特征。探究消落带土壤胶体的释放与迁移行为是衡量胶体促进污染物迁移入库的重要前提。该研究对比消落带与非消落带土壤,通过原状土柱淋洗试验,研究饱和流中土壤胶体释放及迁移特征。结果表明,消落带原状土柱饱和淋洗液中胶体颗粒浓度先总体快速降低(184.58～28.04 mg/L)再缓慢增加(21.18～97.58 mg/L),存在较大的时间变化(变异系数为0.46)。胶体颗粒累计释放量为714.43 mg,比非消落带土柱高34.4%,而淋洗液的峰值粒径(13.25～19.90μm)和中值粒径(14.98～22.90μm)均远远小于非消落带土柱的相应值,表明反复淹水-排干作用导致消落带土壤中胶体及细颗粒的释放和迁移潜力增大。溶解性有机碳(DOC,Dissolved Organic Carbon)是影响消落带饱和土壤中胶体释放的关键因子,对胶体浓度动态变化的解释率高达42.3%,而水化学因素(EC、Ca~(2+)及Mg~(2+))对非消落带土壤中胶体颗粒的释放影响相对更大。在消落带管理中,应注意减控DOC的流失,以减少消落带土壤胶体颗粒的释放,同时建议加强消落带土壤DOC来源及其与胶体偶合并促进污染物如农化物质迁移进入库区水体的研究。     

孔隙水流速对胶体在饱和多孔介质中运移的影响

采用室内石英砂柱出流实验和数学模型,研究了流速对自然土壤胶体、蒙脱石胶体和腐殖酸钠胶体在饱和多孔介质中运移的影响。结果表明:孔隙水流速影响着胶体在多孔介质中的运移过程,流速的提高可加速土壤胶体的出流过程,减小蒙脱石胶体和腐殖酸钠胶体的沉积率。胶体的运移过程除受孔隙水流速的影响外,胶体特性的不同亦决定着胶体运移过程的差异,由于胶体表面性质不同,与介质之间的作用力不同,决定了胶体运移过程中的沉积和释放。     

Soil-profile distribution of primary and secondary plant-available nutrients under conventional and no tillage

Nutrient distributions under no tillage (NT) compared with conventional disk-and-bed tillage (CT) management in the warm, humid region of the southeastern USA need to be assessed so that future placement, quantity, and type of fertilizers can be altered, if necessary, to efficiently match crop demands. We determined soil-profile distributions of pH, N, P, S, K, Ca, Mg, Na, Zn, Fe, Mn, and Cu to a depth of 0.9 m at the end of 8.5 years of continuous CT and NT management on a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) in southcentral Texas. Most dramatic changes occurred within the 0–0.05 m depth, where soil under NT had lower pH, Fe, and Cu than under CT, but greater P, K, Zn, and Mn. Greater P and K under NT than under CT also occurred below the till-zone (0.15–0.3 m). At a depth of 0–0.3 m, soil under NT contained greater amounts of extractable P, K, Zn, Fe, Mn, and Cu than under CT. Nitrogen fertilization had little effect on nutrient distributions, except resulting in greater extractable K at 0–0.05 m and greater nitrate at 0–0.15 m. Few changes in soil-profile distributions were observed for extractable S, Ca, Mg, and Na. Long-term continuous use of NT on this fine-textured, high-fertility (except for N) soil had no apparent adverse effects on nutrient distributions relative to CT, but enhanced conservation and availability of P, K, Zn, Fe, Mn, and Cu near the soil surface where crop roots proliferate.     

Assessing the effectiveness of conservation measures: Resolving the “wicked” problem of the Steller sea lion

“Wicked” problems are those that are complex and that change when solutions are applied. Many conflicts in conservation fall into this category. The study approached the problem of how to constrain the apparent wickedness of a problem in the conservation management of a species by using simple empirical indicators to carry out iterative assessment of the risk to a population and to document how this risk evolves in relation to the addition of new data and the implementation of management actions. Effects of high levels of uncertainty within data and also concerning population structure were examined through stochastic simulation and by exploration of scenarios. Historical trends in the example used, the Steller sea lion, showed rapid declines in abundance in some regions during the 1980s. The current total population is 130,000-150,000 Steller sea lions through Alaska and British Columbia and this number has been stable since about 1990 in spite of regional differences in population dynamics. Regional differences in the sequence of changes in the number of pups and non-pups, suggested that an internal re-distribution of juveniles could have happened between 1980 and 1990. Current productivity also appears close to the long-term mean. Stochastic population projection using various scenarios showed that, based upon this history, the risk of extinction for the population has declined and is below reasonable thresholds for considering the population to be endangered. The trends in risk suggest that management actions taken since 1990 have probably been effective. Consequently, the conservation management objectives for the Steller sea lion are probably being met. The approach provides a mechanism, based upon experience and scenario analysis, for exploring future policy options and may help to constrain the debate amongst stakeholders about the cost-benefit trade-offs associated with different options.     

Enhanced establishment and growth of giant cardon cactus in an eroded field in the Sonoran Desert using native legume trees as nurse plants aided by plant growth-promoting microorganisms and compost

To evaluate the feasibility of long-term desert reforestation technology of mixed vegetation, cardon cactus (Pachycereus pringlei) seedlings from indoor and outdoor nurseries were planted in the field adjacent to one seedling of potential legume nurse trees: mesquite amargo (Prosopis articulata), yellow palo verde (Parkinsonia microphylla), and blue palo verde (Parkinsonia florida). Some of the planting holes were also supplemented with common dairy compost. Additionally, the combinations of legume tree–cactus were inoculated with either a consortium of desert arbuscular mycorrhizal (AM) fungi, plant growth promoting bacteria (PGPB; the diazotroph Azospirillum brasilense Cd, and the phosphate solubilizer Paenibacillus sp.), or a mixture of all. The field experiments were evaluated periodically during 30 months for survival and growth. Cardons reared in an outdoor screen house survived better in the field than those reared in a controlled growth chamber and hardened later outdoors. Association with any legume nurse tree increased survival and enhanced growth of untreated cardons. For cardons growing alone, application of either compost, AM fungi, and all the treatments combined increased survival. For these plants, no treatment affected plant growth during the first 3 months after transplanting. Later, all treatments, except for AM fungi, enhanced plant growth. However, only 2 years after transplanting the enhanced growth effect of AM fungi was also significant. In the presence of the legume nurse trees, transient positive effects on cardon growth were recorded. General evaluation after 30 months of cultivation showed that the treatments positively affected cardon growth when growing alone or in combination only with mesquite amargo but not with the other two legume trees. This study proposes that young legume trees have the capacity to enhance survival and growth of cardon cactus, depending on the legume cactus combination. Additional treatments such as compost or PGPB can either amplify the effect or else attenuate it.     

Karyotypic and genome size variation in Cajanus cajan (L.) Millsp. (pigeonpea) and some wild relatives

Detailed karyotypes and 4C DNA amounts have been studied in five cultivars of Cajanus cajan and 20 species belonging to Cajanus, Rhynchosia, Dunbaria, Flemingia and Paracalyx. C. cajan shows intraspecific variability and its karyotype is most similar to that of C. cajanifolius (sect. Cajanus) in the morphology and number of satellite chromosomes and the lack of any correlation between chromosome size within the complement and asymmetry. Karyotypes of C. lineatus and C. sericeus belonging to sect. Atylia are similar with respect to maximum r-index and the ratio of longest and shortest chromosomes in their respective complements. C. acutifolius (sect. Frutocosa) is distinct in having a very low ratio (1.44) between the longest and the shortest chromosomes, while in C. albicans, C. goensis, C. scarabaeoides (sect. Cantharospermum) there is no chromosome pair with r-index >2.0. C. mollis and C. volubilis (sect. Volubilis) show similarity with regard to ratio between the longest and the shortest chromosomes and a distinct chromosome pair with an arm ratio of 2.7 and C. platycarpus is distinct (sect. Rhynchosoides) in having the smallest ratio (1.36) between the longest and the shortest chromosomes. 4C DNA amounts in Cajanus and Rhynchosia vary between 3.28 pg to 11.69 pg and 4.92 pg to 11.13 pg respectively, while in Dunbaria, Flemingia and Paracalyx these vary between 4.70 and 7:06 pg. In the genus Cajanus karyotypic features and 4C DNA amounts agree with the sectional classification and bring out a close relationship between C. cajan and C. cajanifolius. This is supported by studies based on seed protein patterns, isozyme analysis, trypsin and chymotrypsin inhibitor patterns, RFLP and RAPD data and crossability relationships. A clear relation between DNA amount and abnormalities effecting the extent of pairing and recombination in interspecific hybrids shows the importance of this study in developing future breeding programme involving C. cajan and its wild relatives that are of potential value.     

Effect of some fungicide seed treatments on the survival of Bradyrhizobium japonicum and on the nodulation and yield of soybean [Glycine max. (L) Merr.]

Several commercial fungicide seed treatments were evaluated for their possible effect on the survival of Bradyrhizobium japonicum on seeds and on the nodulation and yield of soybeans in a greenhouse and a field experiment. quinolate Pro (carbendazim and oxine copper), Vitavax 200FF (carboxin and thiram), and Monceren (pencycuron) had a small effect or no effect on the survival of B. japonicum and on the nodulation and yield of soybeans. They can thus be considered compatible with soybean seed inoculation. Germipro UFB (carbendazim and iprodione), Apron 35J (metalaxyl), and Tachigaren (hymexazol) decreased B. japonicum survival and the nodulation and yield of soybeans and thus cannot be considered compatible with soybean seed inoculation.     

Einfluss der düngungsmethoden auf die entwicklung von zuckerrübenkrankheiten

Während neunjähriger (1994-2002) Untersuchungen in der Versuchsanstalt Ba?cyny (Fahlerde, Fall-Gleyboden, ausgebildet aus leichtem Lehmboden) wurde der Befall mit Krankheiten an der Zuckerrübe in Abhängigkeit von der Düngungsmethode bewertet. Folgende Methoden wurden angewendet: A - Mineraldünger, B - Stalldung und Mineraldünger, C - Stroh-, Gründung und Mineraldünger, D - Kompostdünger und Biopräparate (ökologische Düngung). Auf den Objekten A, B, C wurden bei konstanter Phosphor-Kalium-Düngung die Stickstoffgaben von 0 bis 180 kg.ha^?1 differenziert. Auf dem ökologischen Objekt D ohne Mineraldünger wurden statt Stickstoff die Biopräparate P₅₀₀ und P₅₀₁ verabreicht. Der geringste Befall von Krankheiten wurde an der Zuckerrübe auf jenem Objekt festgestellt, das mit Kompost und den Biopräparaten gedüngt worden war. Im Rahmen der konventionellen Düngungsmethoden wurde allgemein die höchste Intensität der Krankheiten an der mit Stroh, Gründung und Mineraldünger gedüngten Zuckerrübe beobachtet. Ausnahmen von dieser Regel bildete das Vorkommen von echtem Mehltau und Keimlingsfäule. Auf den Objekten A, B, C nahm die Intensität der Krankheiten mit der Steigerung der Stickstoffgabe zu. Auch die auf Objekt D verabreichten Biopräparate stimulierten die Entwicklung der Krankheiten. The effect of various fertilization methods on the diseases of sugar beet was estimated on the basis of nine-year studies (1994-2002) conducted in the Experimental Production Farm in Ba?cyny on loess-based, sedimentary-gley, light-clay soil. The following fertilization methods were applied: A - mineral fertilization, B - mineral and manure fertilization, C - straw, green manure and mineral fertilization, D - ecological compost and biodynamic preparations fertilization. In the objects A, B and C fertilized with phosphorus and potassium, various doses of nitrogen ranging from 0 to 180 kg/ha^?1 were used. In the ecological object D, where mineral fertilization was not used, biodynamic preparations P₅₀₀ and P₅₀₁ were applied instead of nitrogen. The lowest ratio of the diseases of sugar beet was observed in the object fertilized with the compost and biodynamic preparations. In view of the conventional fertilization methods, generally the highest incidence of diseases was noted in the sugar beet fertilized with straw, green manure and mineral fertilizers. The exception was the occurrence of powdery mildew and the black leg. In objects A, B and C the incidence of the diseases increased with the increase in nitrogen doses. The applied biodynamical preparations in the object D stimulated the development of diseases too.     

Prospects for control of Pythium damping-off of lettuce with Trichoderma,Gliocladium and Enterobacter spp.

Summary Damping-off of lettuce (Lactuca sativa) caused by Pythium ultimum was studied in pots containing a non-sterile potting mix in the glasshouse. Fifty P. ultimum sporangia g^-1 compost reduced the plant stand to 15% and shoot dry weight to 18%, but this reduction was totally prevented by applying Trichoderma harzianum at 2×10⁵ viable propagules g^-1 potting mix. Gliocladium virens and Enterobacter cloacae also alleviated the damping-off. E. cloacae did not adversely affect the action of the fungal antagonists. The co-existence of the bacterial and fungal antagonists was revealed on the root surface and inner surface of the testa by scanning electron microscopy, indicating the compatibility of the biocontrol agents.     

细菌、真菌及植物氮营养信号研究进展

氮素吸收代谢是所有生物生命活动的核心部分,因此,各种生物对氮的吸收与同化过程都有精细的调节。这种调节依赖于生物对体内氮素状况信号及体外（生长介质中）氮素信号的感受。过去几十年中,在细菌、真菌中已经对氮素营养信号有了较好的研究,而植物中则相对缓慢,但也有了一定的认识。本文重点比较细菌、真菌及植物中氮信号系统的组成,以期为进一步认识植物中的氮信号接受与转导系统提供启示。     

Aircraft operations near concentrations of birds in Antarctica: The development of practical guidelines

Aircraft operations have the potential to disturb and to impact negatively on bird life. A gradient of increasing behavioural response is evident in birds when exposed to increasing aircraft stimulus. The most major disturbance is likely to lead to impacts on the health, breeding performance and survival of individual birds, and perhaps bird colonies. A process of revision to policies on aircraft operations contained in management plans for a number of specially protected areas in Antarctica by the United Kingdom, accompanied by consultations made within the scientific community through the Scientific Committee on Antarctic Research (SCAR) and with operational interests through the Council of Managers of National Antarctic Programmes (COMNAP) resulted in new guidelines being adopted by the Antarctic Treaty Consultative Parties in June 2004. The principal recommendations of the guidelines are that bird colonies should not be overflown below 2000 ft (∼610 m) above ground level and landings within 1/2 nautical mile (∼930 m) of bird colonies should be avoided wherever possible. These guidelines are less stringent and less specific than those that were recommended by the SCAR specialist group on birds, and represent a compromise to accommodate operational needs. While the adoption of clear and consistent guidelines for the operation of aircraft in Antarctica is welcome in that this provides practical advice that is likely to reduce incidences of close aircraft/bird encounters, there remains insufficient knowledge of the interactions between aircraft and birds in Antarctica, and the consequent impacts on individual birds and on bird populations. It is important, therefore, that the guidelines adopted are considered interim, and should be kept under scrutiny with revisions made as new and improved research results appear.     

The abundance of heterocystous blue-green algae in rice soils and inocula used for application in rice fields

Summary Algal populations were quantified (as colony-forming units [CFU] per square centimetre) in 102 samples of rice soils from the Philippines, India, Malaysia and Portugal, and in 22 samples of soil-based inocula from four countries. Heterocystous blue-green algae (BGA) were present in all samples. Nostoc was the dominant genus in most samples, followed by Anabaena and Calothrix. In soils, heterocystous BGA occurred at densities ranging from 1.0 × 10² to 8.0 × 10⁶ CFU/cm² (median 6.4 × 10⁴) and comprised, on average, 9% of the total CFU of algae. Their abundance was positively correlated with the pH and the available P content of the soils. In soil-based inocula, heterocystous BGA occurred at densities ranging from 4.6 × 10⁴ to 2.8 × 10⁷ CFU/g dw (dry weight), comprising only a moderate fraction (average 13%) of the total algae. In most soils, the density of indigenous N₂-fixing BGA was usually higher than that attained by applying recommended rates of soil-based inoculum. Whereas research on the practical utilization of BGA has been mostly directed towards inoculation with foreign strains, our results suggest that attention should also be given to agricultural practices that enhance the growth of indigenous strains already adapted to local environmental conditions.Maître de Recherches ORSTOM (France), Visiting Scientist at IRRI