大棚蔬菜种植年限、种植茬口对土壤微生物的影响

利用土壤微生物平板计数方法,对寿光市、苍山县(现兰陵县)两个蔬菜产区的土壤微生物区系进行了研究。获得如下结果:(1)蔬菜种植年限对大棚土壤微生物的区系变化有显著的影响。苍山以种植4年、8年的土壤细菌最多,寿光以种植2年、6年的细菌数量最大,两蔬菜产区土壤细菌数量都以种植12年和7年最低;寿光市土壤真菌数量以种植3年、2年最高,苍山以8年、10年最高,两个区都以种植12年和7年的真菌量降低最多;寿光市土壤放线菌都以种植10年、5年最高,苍山县以2年、10年最高,超过6年下降加快。蔬菜大棚连续种植12年时,两地蔬菜大棚土壤中的细菌、真菌、放线菌的数量都急剧减少。(2)蔬菜轮作有利于各种微生物数量维持较高的水平;单一蔬菜品种种植中,西红柿、大蒜对土壤细菌有不良影响,辣椒、大蒜能够抑制和减少土壤真菌数量,大蒜对放线菌有很强的抑制作用。(3)寿光市大棚蔬菜土壤微生物总量以黄瓜-架豆轮作最高,茄子的土壤微生物总量最少。苍山县土壤微生物总量以茄子-辣椒轮作种植最高,茄子、辣椒单一品种能够维持较高的土壤微生物水平,蔬菜轮作和茄子、辣椒单一种植均有利于提高土壤微生物总量。     

河北粮田和菜地土壤大、中、微量元素肥力研究

采用小麦—玉米轮作、大白菜—辣椒轮作的4年肥料定位试验和土壤养分系统研究法相结合的方法,探讨了河北粮田和菜地土壤供氮、磷、钾、硫、锌、硼的能力,土壤氮、磷、钾的收支平衡及土壤养分的消长,以确定氮、磷、钾的合理用量。结果表明:小麦—玉米轮作中:全肥处理(NPKZnS)小麦的年均产量7278～7475kghm-2,-N、-P、-K、-S的相对产量分别是全肥的50%～52%、84%～87%、91%～95%、92%～93%;全肥处理玉米年均产量8396～9041kghm-2,-N、-P、-K、-Zn的相对产量分别为全肥的55%～59%、83%～86%、79%～91%、89%～94%,差异均达到显著或极显著水平。土壤养分限制因子排序为N>P>K>Zn>S,施用1/2N、1/2P、1/2K基本可维持土壤氮、磷、钾表观收支平衡。小麦—玉米轮作中推荐N、P2O5、K2O的用量为450～500、135～150、90～135kghm-2。磷肥、硫肥施在小麦茬口、钾肥和锌肥施在玉米茬口。大白菜—辣椒轮作中:全肥处理(NPKZnB)白菜的年均产量为113.9thm-2,-N、-P、-K的相对产量分别为全肥的57%、64%、87%;辣椒的年均产量为23.7thm-2,-N、-P、-K、-Zn、-B的相对产量分别为全肥的46%、46%、68%、88%、85%,差异均达到显著或极显著水平。土壤养分限制因子的排序依次为N>P>K>Zn>B。大白菜—辣椒轮作中,大白菜茬口N、P2O5、K2O推荐用量200、90、150kghm-2,辣椒茬口分别为300、170～180、400kghm-2,Zn和B在辣椒上有显著增产效应。     

不同轮作体系土壤残留硒锌对小麦产量与营养品质的影响

采用盆栽试验,通过玉米-小麦和大豆-小麦轮作,探讨了不同种植模式中,土壤残留硒锌对小麦产量与营养品质的影响。结果表明,土壤中残留硒锌对小麦生物量和产量的影响与作物轮作体系有关。玉米-小麦轮作,土壤残留硒锌并没有表现出增加小麦产量的趋势,而大豆-小麦轮作,土壤残留硒锌对小麦生物量和产量呈增加趋势。土壤残留硒锌可改变作物体内元素组成。玉米-小麦轮作,土壤残留硒锌有利于小麦籽粒对氮、钾、硫、钙、铁、锌、硒、铜元素的吸收累积。大豆-小麦轮作,土壤残留硒锌促进小麦对钾、硫、镁、铁、锌、硒、铜、锰、硼的吸收,而氮、磷、钙吸收减少。     

四川几种轮套作体系的产量及小麦玉米磷素利用效率研究

通过2011和2012年2年田间试验研究了"蚕豆-玉米"、"小麦-甘薯"、"小麦-大豆"3种两熟轮作体系和"小麦/玉米/甘薯"、"小麦/玉米/大豆"2种三熟套作体系的产量、小麦玉米磷素积累分配及磷利用效率的变化。结果表明:三熟套作具有明显的产量优势,小麦/玉米、玉米/大豆和玉米/甘薯的土地当量比(LER)分别为1.79、1.51、1.62,小麦/玉米/大豆、小麦/玉米/甘薯周年体系的LER平均分别为2.13、2.32,与同面积单作相比,两体系周年作物产量分别提高了12.5%和31.5%。小麦、玉米产量套作比单作分别高30.3%、12.3%,而大豆和甘薯产量在套作与单作间无显著差异。套作较单作显著提高了小麦和玉米的地上部磷素吸收总量、籽粒磷积累量、磷肥偏生产力,小麦分别提高34.3%、39.3%、29.1%,玉米分别提高11.1%、19.8%、12.4%。玉米磷收获指数套作比轮作平均高7.8%。套作促进了小麦、玉米磷营养从营养器官向籽粒的转运分配。表明三熟套作相对两熟轮作(单作)能显著提高周年产量,促进小麦、玉米对磷素的吸收利用,一定程度提高了磷肥利用效率。     

成都平原不同种植模式下重金属镉污染风险和经济效益评价

为了评估成都平原常规粮油种植模式下的重金属镉污染风险和经济效益,寻求稳定、高产且利于推广的安全种植模式。选取成都平原最常见的6种粮油作物,设置了小麦-玉米轮作、油菜-玉米轮作、小麦-红苕轮作、油菜-红苕轮作、小麦-大豆轮作、油菜-大豆轮作、小麦-水稻轮作、油菜-水稻轮作8种种植模式,在轻中度污染农田开展了两年田间试验,对各个种植模式的产量、经济产出,以及收获物、秸秆和土壤的重金属镉含量进行测定。结果表明,当前试验区最广泛使用的小麦-水稻种植模式下的小麦和水稻籽粒镉含量均超标,存在极大的生产风险。玉米-油菜种植模式的籽粒镉含量最低,均未超标,能够实现安全生产,但经济产出较小麦-水稻轮作模式显著降低。油菜-水稻轮作模式的农田产出效益最大。相比种植小麦,种植油菜后,后茬水稻对应的重金属镉含量降低70.2%。综合生产和环境效益,将小麦-水稻种植模式更换为油菜-水稻模式是目前既能保证农民收益又能有效降低重金属镉污染风险的最佳措施。     

长期氮磷化肥配施对不同种植体系土壤水分的影响

利用长期定位实验,在黄土高原旱地研究了22a氮磷化肥配施下不同种植体系土壤水分的变化状况,为合理利用水肥资源提供参考。在小麦连作、豌豆-小麦（2a）＋糜子轮作、红豆草-小麦（2a）轮作、豌豆-小麦（2a）＋玉米轮作、玉米-小麦（2a）＋糜子轮作、小麦（2a）＋糜子-玉米轮作6个不同种植体系中,测定土壤剖面（0-300cm）含水量、储水量。结果表明：不同种植体系土壤含水量在土壤剖面上的分布不同,所有处理在土壤剖面140-300cm土层含水量均低于0-140cm土层含水量。不同种植体系在土壤剖面不同土层的储水量不同,在0-300cm土壤储水量各处理表现为豌豆-小麦（2a）＋玉米轮作（644.8mm）〉小麦（2a）＋糜子-玉米轮作（599.6mm）〉豌豆-小麦（2a）＋糜子轮作（582.3mm）〉红豆草-小麦（2a）轮作（574.0mm）〉玉米-小麦（2a）＋糜子轮作（550.8mm）〉小麦连作（536.9mm）。轮作体系0-300cm土壤储水量均高于小麦连作。粮豆轮作土壤储水量高于小麦连作,是黄土高原地区较好的种植方式。     

轮作对作物生物量影响的研究

美国中西部地区的农作物以玉米为主,轮作是提高作物产量和保护环境的重要耕作措施之一。研究单作玉米(CC),玉米—大豆轮作(CS),玉米—大豆—小麦轮作(CSW)对作物产量和碳(C)和氮(N)质量分数及果实、秸秆和玉米芯中C、N所占比例。在威斯康星州3个试验点,连续4 a(2011—2014年)测定作物生物量。方差分析表明,轮作对作物总生物量、秸秆中C质量分数及试验点与轮作交互作用对大豆产量影响差异显著。轮作可提高玉米和大豆产量,CSW玉米的产量比CC和CS提高了15%和8%。CS或CSW玉米产量比CC提高了540 kg/hm^2,轮作玉米的果实、秸秆、玉米芯生物量占总生物量比例分别为52%、41%和7%。轮作对玉米果实、秸秆中N质量分数、C/N比影响差异不显著。     

控释氮肥在我国小麦－玉米轮作体系的应用研究进展

氮素在小麦－玉米轮作体系中占据重要地位。施用与小麦、玉米各生育时期需氮规律相吻合的控释氮肥,可改善作物生长特性及提高氮素利用效率,保障其高质稳产,并降低氮素淋溶污染风险。概述了控释氮肥性质和施用控释氮肥对小麦-玉米轮作体系作物产量、氮素利用效率、农艺学性状和土壤氮素含量及酸碱性等方面的影响,探讨了控释氮肥应用研究中存在的问题,并对未来控释氮肥的研发提出了建议和展望,以期为控释氮肥在我国小麦－玉米轮作体系中的进一步推广应用提供理论依据。     

长期施用氮磷和有机肥对不同种植体系土壤有效硫累积的影响

以22年定位试验为基础,研究了长期施用氮磷和有机肥（NPM）对不同种植体系土壤有效硫在剖面上分布与累积状况。结果表明,在60–80 cm土层各处理出现第一个累积峰,累积峰值为:粮饲轮作31.3、玉米连作29.2、小麦连作27.9、粮豆轮作25.6、苜蓿连作24.0 mg/kg;在140–180 cm 土层各处理又出现有效硫的第二个累积峰,累积峰值为:粮饲轮作44.7、粮豆轮作43.1、小麦连作41.0、玉米连作39.7、苜蓿连作36.5 mg/kg。第二累积峰值均大于第一累积峰值。0–200 cm土层有效硫总累积量粮饲轮作高达746.3 kg/hm2,其次为玉米连作640.6、粮豆轮作为638.3、小麦连作为622.4、苜蓿连作最小为557.3 kg/hm2。长期施用磷肥和有机肥是有效硫在土壤中累积的主要因素,有效硫在土壤剖面上有向深层迁移的趋势。不同作物对硫的吸收利用差异和不同种植方式对有效硫的累积与分布产生影响。     

超量覆盖物对土壤氮矿化作用和玉米产量的影响

１９９３年，在美国的密执安州建立了长期作物栽培系统试验，以测量轮作和作物覆盖在玉米产量方面的影响。轮作和作物覆盖目的都是为了作物在生育期内得到足够的氮。土壤氮含量用ＰＳＮＴ得到。试验包括玉米—玉米—大豆—小麦轮作系统和玉米连作两种种植系统。实验地设１个裂区，每个裂区有４次重复，设有两种肥料管理：有机肥及化肥。撒播的覆盖作物有黑麦草和红苜蓿。研究结果表明：有覆盖作物的玉米比无覆盖的玉米产量明显提高，轮作的玉米比连作的玉米产量高，轮作有覆盖的玉米产量明显高于无覆盖玉米。     

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