土地利用变化对艾比湖流域生态系统服务价值的影响

基于遥感与GIS技术,运用生态系统服务价值系数分析了1970-2009年艾比湖流域生态系统服务价值的变化,探讨了干旱区典型内陆湖流域生态系统服务变化对土地利用/覆被变化(LUCC)的响应.结果表明,近40 a艾比湖流域土地利用状况变化显著,耕地和建设用地的面积增加,其他土地利用类型的面积减少;流域生态系统服务价值整体变化较小,总价值先增后减,林地、水域和草地面积的减少是总服务价值减少的主要原因,耕地面积的增加补偿了总价值的部分损失,但损失略大于收益;各项生态功能的服务价值对总生态系统服务价值贡献大小的等级基本稳定,其中,水源涵养、土壤形成保护和废物处理能力的贡献率最大;敏感性分析表明,研究区生态系统服务价值对于生态价值系数是缺乏弹性的,各土地利用类型生态系统服务价值敏感性指数都小于1,研究结果是可信的.     

抚仙湖流域土地利用变化及其生态系统生产总值影响

为了研究抚仙湖流域土地利用变化对其生态系统生产总值产生的影响。基于2005年、2010年、2015年抚仙湖流域3期遥感影像数据,采用ArcGIS技术,生态价值评估方法,分析了2005年、2010年、2015年抚仙湖流域土地利用变化对生态系统生产总值时空变化的影响。结果表明:(1)近10年流域土地利用结构以水域、耕地和林地为主。建设用地和林地面积呈持续增加的趋势,耕地、草地持续减少,水域面积基本保持不变,耕地向建设用地和林地转移;(2)时间上,土地利用变化使抚仙湖流域生态系统生产总值呈逐年上升趋势。2005年、2010年、2015年生态系统生产总值分别为193 252.63万元、283 174.04万元和513 769.28万元。空间上,近10年单位面积生态系统生产总值在空间上由湖泊向外围区域逐渐增加,湖泊北部、南部供给服务价值增加最多,右所镇北部、湖泊西部、路居镇南部调节服务价值增加较为明显,而文化服务价值沿湖泊周边逐渐增加;(3)流域生态系统生产总值以生态系统供给服务价值为主,分别占生态系统生产总值的47.61%,55.22%,53.55%,调节服务价值分别占生态系统生产总值的33.49%,21.76%,12.24%,而文化服务价值最少,分别占生态系统生产总值的18.90%,23.02%,34.21%。其中,生态系统供给服务价值以2005年和2010年的贡献最大,到2015年逐渐减少。生态系统的调节服务价值则逐渐降低。生态系统的文化服务价值比例较小,但逐年增加;(4)从不同的生态系统类型来看,各价值增量体现为林地耕地草地水域建设用地,其中林地、耕地的价值增量最大,而草地、水域、建设用地价值增量较少。     

重庆市武陵山区土地利用转型的生态系统服务价值效应

[目的] 分析重庆市武隆区土地利用转型的生态系统服务价值（ESV）效应,为武陵山区土地合理利用,资源优化配置,实现保护中开发和生态服务价值最大化提供决策依据。[方法] 以武隆区2010,2015,2020年3期的土地利用数据为数据源,测算武隆区生态服务价值效应,运用土地利用转移矩阵、变化贡献率模型及交叉敏感性系数,分析武隆区土地利用转型对生态系统服务价值损益的时空演变。[结果] ①2020年武隆区ESV空间上呈现高值区集中分布在武隆区中部的仙女山街道和白马镇,低值区集中分布在武隆区东西两侧和中部的土坎镇;②2010—2020年武隆区ESV总量呈增加趋势;③2010—2020年武隆区各用地类型ESV变化和变化贡献率排序一致,即：林地>水域>耕地>草地>荒漠;④草地转换为林地的敏感性系数最高,其他土地利用类型转换为草地时都缺乏敏感性。[结论] 林地和水域面积的增加可大幅提高ESV总量,建议农业结构调整时,增加园地和林地面积,严格控制耕地撂荒。     

三峡库区（重庆段）土地利用变化对生态系统服务价值的影响分析

在分析1995-2000年三峡库区(重庆段)土地利用变化的基础上,计算了由此导致的生态系统服务价值变化.采用Costanza的生态系统服务价值计算公式和谢高地等人的中国陆地生态系统单位面积生态服务价值表进行计算,结果发现,1995-2000年研究区林地、草地、水域和难利用地面积在减少,其中草地减少最多,其次为林地;耕地和建设用地在增加,且增加面积都较大;生态系统服务价值(ESV)由1995年的549.387 9亿元减少到2000年的546.706 5亿元,5年减少了2.681 4亿元;敏感性分析表明,ESV对生态价值系数(VC)缺乏弹性,研究结果可信.     

尉犁县土地利用/覆被变化的区域生态系统服务价值变化

基于尉犁县1993年、2001年、2009年和2017年的遥感影像数据,采用中国陆地生态系统服务价值测算方法,结合研究区实际情况,对生态系统服务价值系数进行了修正,分析了尉犁县土地利用/覆被变化导致的生态系统服务价值变化。结果表明:1993—2017年,尉犁县不同土地利用类型面积总体呈“六增三减”态势;即耕地、建设用地、水域、湿地、盐渍化地与沙漠面积有所增加,林地、草地与未利用地面积有所减少;尉犁县土地利用生态系统服务总价值总体呈增加趋势,从1993年的173.61亿元增加到2017年的206.30亿元,增加了32.69亿元。在整个研究时段内,研究区各土地利用类型对生态系统服务总价值的贡献大小从高到低依次为湿地 > 草地 > 未利用地 > 沙漠 > 水域 > 林地 > 耕地 > 盐渍化地 > 建设用地;敏感性分析结果表明各土地利用类型敏感性指数均<1,表明各土地利用类型生态系统服务价值对生态系统服务总价值的影响较小,缺乏弹性,研究结果可信。     

近30年乌伦古河流域土地利用与生态系统服务价值变化研究

[目的]识别新疆乌伦古河流域近30 a土地利用/覆被变化及生态系统服务价值响应,为指导该区水土资源优化配置提供科学依据。[方法]选取研究区1989,1998,2006和2017年4期Landsat系列遥感影像,综合利用3S技术,监测并分析研究区近30 a LUCC时空演变特征,采用修正后的生态系统服务价值当量因子,计算并分析生态系统服务价值响应。[结果]①近30 a乌伦古河流域耕地和建设用地面积呈持续增加扩张趋势,水体和湿地面积呈微增加趋势,未利用地呈显著减少趋势,林地面积呈波动增加趋势,草地面积1989—2006年呈减少趋势,2006—2017年呈微弱增加趋势。其中,耕地净增加量最大(847 km~2),未利用地净减少量最大(744 km~2),各土地利用类型转移显著区域主要位于福海县、杜热乡和齐干吉迭乡;②乌伦古河流域土地利用综合指数由1989年的146.75增加至2017年的168.29,生态系统服务价值(ESV)由1989年的2.635亿元增加至2017年的2.915亿元,净增加0.28亿元。[结论]近30 a乌伦古河流域各土地利用类型面积均发生不同程度变化及相互转换,研究区土地资源总体处于高强度发展期,生态系统服务价值做出相应响应,总生态系统服务价值增加。     

“十二五”期间重庆市土地利用变化及生态效应

[目的]揭示"十二五"期间重庆市土地利用及生态效应变化规律,为该市用地结构调整和生态可持续发展提供理论支持。[方法]基于重庆市2011—2015年5期TM遥感影像,采用土地利用动态度、土地利用程度指数、生态系统服务价值评价法、敏感性分析等方法开展研究。[结果]"十二五"期间,重庆市耕地、林地、草地、水域湿地、建设用地和未利用地年变化率分别为-0.37%,-0.01%,-1.98%,0.67%,6.77%,-2.54%;土地利用程度综合指数由248.89提高至250.11;生态系统服务价值共减少了2.73亿元。[结论]"十二五"期间,重庆市各土地利用类型动态变化存在差异性,呈现出耕地、林地、草地、未利用地面积减少,水域湿地和建设用地面积增加的趋势,土地利用在这阶段总体处于发展期;生态系统服务总价值整体减少,生态环境轻微朝着不健康的方向发展。     

基于土地利用的祊河流域生态系统服务价值动态变化

为了保护生态环境和合理利用土地资源,以1978年、1985年、1995年、2005年、2015年5个时点的土地利用数据为基础,采用生态系统服务价值评估法、空间自相关分析、热点分析等方法分析了1978—2015年祊河流域生态系统服务价值时空演变规律。结果表明:(1)流域土地利用变化趋势为耕地和草地减少,建设用地、水域、林地、未利用地增加。各地类间的转移以耕地向建设用地的转移为主,其次是耕地向林地以及耕地和草地之间的转移。(2)流域生态系统服务总价值变化呈"先减少后增加再减少"的波动减少趋势。耕地和草地的服务价值减小,林地、水域和未利用地的服务价值增加;土壤形成与保护、食物生产、生物多样性保护、气体调节、原材料和气候调节的服务价值减小;水源涵养、娱乐文化和废物处理的服务价值增加。(3)流域生态系统服务价值呈空间集聚分布格局且集聚程度略有减弱。冷点区范围呈明显的扩张趋势,且集中分布于平邑县、费县和兰山区的城区内;热点区范围基本保持不变,主要分布在蒙山山脉、水库、河流和低山丘陵。     

滨海城市土地利用变化对陆地生态系统服务价值与储碳的影响——以辽宁省大连市为例

[目的] 生态系统服务价值(ESV)是评估区域土地利用变化是否科学的重要手段之一,关系地区可持续发展。评估滨海城市ESV和陆地碳储存(TCS)变化,为辽宁省大连市经济发展与生态系统服务价值调控提供依据。[方法] 利用2000—2020年5期土地利用数据和相关数据,基于土地动态度、当量因子法、土地转移矩阵、ESV贡献率、TCS和主成分分析法,计算大连市土地利用变化动态及其对ESV,TCS和陆地碳储存价值(TCSV)的影响。[结果] ①在空间分布上,大连市建设用地主要侵占西南部沿海地区的耕地和林地。②2000—2020年的4个时间阶段,大连市生态系统服务价值呈现先降后升,持续加剧下降的发展趋势;耕地面积减少,建设用地面积增长对生态系统服务功能保持造成直接影响。③2000—2020年的4个时间阶段,大连市TCS和TCSV呈现先升后降,持续加剧下降的发展趋势;林地面积减少直接影响TCS功能的保持。④2000—2020年大连市土地利用转移矩阵中,以耕地、林地、草地转出,建设用地转入为主;造成不同地类的生态系统服务贡献差异,贡献度大小顺序为耕地、林地、湿地、水域、草地和未利用地。⑤2000—2020年大连市的ESV,TCS变化主要受经济增长,三大产业结构调整和镇化水平发展影响。[结论] 从大连市经济发展,ESV提升,TCS能力提升和可持续发展角度出发,需要着重关注滨海岸线建设用地扩张与生态用地和农业用地的冲突问题,加强对北部林草生态用地的保护,加快滨海地带生态环境修复工作,提升生态用地的碳封存能力,避免建设用地无限制盲目扩张。     

河北省生态系统服务价值变化研究

本研究分析了1993～2000年河北省土地利用和生态系统服务价值变化。结果表明,河北省耕地、水域、湿地和未利用土地面积在减少,而园地、林地、牧草地和建设用地面积在增加,因水域和湿地单位面积生态系统服务价值较高,7年间河北省区域生态系统服务价值显著减少,河北省生态系统服务价值从1993年的157.3116亿美元减少到2000年的152.3503亿美元,减少率为3.15%,净减少值4.9613亿美元。     

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