土地利用变化对乌江源区生态系统服务价值的影响

基于2000年和2015年乌江源区土地利用数据,利用CLUE-S模型,预测了2030年乌江源区土地利用数据。并利用生态系统服务价值系数法,分析了2000—2030年乌江源区土地利用变化对生态系统服务价值的影响。结果表明:2000—2015年和2015—2030年,乌江源区生态系统服务总价值均呈增加趋势。耕地面积减少导致耕地生态系统服务价值和食物生产服务价值下降,林地面积大幅增加使得大部分生态系统服务功能价值增加。生态系统服务价值变化空间破碎化严重与土地利用变化空间破碎有关。生态修复工程驱动下大量耕地转为林地是乌江源区生态系统服务价值增加的主要因素。     

基于RS与GIS的沙坡头区生态系统服务价值研究

以宁夏自治区沙坡头区为例,以2000年、2007年、2013年3期TM影像为数据源,综合运用土地利用动态度、GIS空间分析法以及中国陆地生态系统服务价值估算法对沙坡头区土地利用变化状况及其生态系统服务价值变化进行了分析研究。结果表明:2000—2013年沙坡头区土地利用变化显著,耕地和未利用地面积分别减少了39 951.90,241 289.24hm~2,而其他土地利用类型面积逐年增加,其中草地面积增幅最大,年变化率高达10.58%;研究期间沙坡头区生态系统服务价值呈增加趋势,草地和林地面积的增加起到了主导作用;从沙坡头区生态系统服务价值构成来看,草地和林地两者的生态系统服务价值占比在67%~87%,是生态系统服务价值的主要构成部分;各单项生态功能服务价值中土壤形成与保护、废物处理和生物多样性保护占研究区生态系统服务价值的比重最大,三者之和占生态系统服务总价值的55%以上;各土地利用类型生态系统服务价值敏感性指数均小于1,说明沙坡头区生态系统服务价值对生态服务功能价值系数缺乏弹性,研究结果可信。     

滇中城市群土地利用转型及生态系统服务价值交叉敏感性分析

为探清滇中城市群土地利用转型对生态系统服务功能的影响,基于2000年、2010年、2020年3期Landsat TM/OLI遥感土地利用解译数据,运用土地利用动态变化模型、生态系统服务价值计算模型和土地利用转型生态贡献度研究了不同土地利用转型过程和生态系统服务价值的时空演变特征,并结合交叉敏感性系数模型定量评估了土地利用转型对生态系统服务价值的响应程度并划定敏感性分区。结果表明:(1)滇中城市群土地利用类型以林地、草地、耕地为主,建设用地持续扩张及耕地、水域、未利用地面积不断减少,草地和林地经历了“先减少后增加”的变化过程;(2)研究期间生态系统服务价值呈“先减少后增加”的趋势,地均生态系统服务价值高值区主要分布在滇池、抚仙湖和星云湖三大高原湖泊流域;(3)近20 a来,退耕还林、还草、还湖对生态系统服务功能改善的贡献度达49.10%,毁林开荒、围湖造田对林地、草地和水域的占用导致生态系统服务价值减少约2.65亿元;(4)耕地、林地、草地和水域向其他地类转型对生态服务价值的响应较为敏感,耕地、林地和草地与建设用地间转型的高生态敏感区集中在曲靖和楚雄,耕地与林地间转型的高生态敏感区分布较小,林地向草地和水域转型的低生态敏感区空间分布范围最广。综上,该区不同土地利用转型对生态系统服务功能影响显著。     

黄土丘陵区生态退耕中的景观格局演变及其地形驱动

黄土丘陵区大规模生态退耕对该区域土地利用、景观格局及其生态系统服务产生了重要影响。基于野外调查和GIS分析,以典型黄土丘陵小流域大南沟为例,分析了1975—2007年生态退耕前后土地利用结构变化、景观格局演变及其地形驱动。结果表明流域大规模退耕前有小面积弃耕现象,耕地和荒草地稍有减少,林地增加,斑块数量增加,景观格局趋于复杂,破碎化严重。2000年大规模退耕以后,耕地大量减少,退耕地和荒草地约占流域面积75%,林地增加,各景观类型形状趋于简单、分布趋于均匀和集中。随退耕年限增加,林地持续增加,存在小面积的复垦现象,流域景观结构趋于复杂,但整体格局趋于合理。相对海拔和坡度是生态退耕最重要的两个地形驱动因子,大规模退耕后,耕地和果园趋向于中低海拔和坡度较缓的区域分布,荒草地基本不变,退耕地和林地趋向于高海拔和较陡的区域分布。     

延安市退耕还林前后土壤保持生态服务功能评价

为更好地探究延安市退耕还林前后土地利用变化对区域土壤保持生态服务功能的影响,采用了InVEST模型对延安市退耕还林前后2000年和2015年的土地利用变化及土壤侵蚀量、土壤保持量进行了评估,该模型在计算生态系统土壤保持服务功能时,考虑了地块泥沙输移、沉积这一重要水文过程,在计算土壤保持量时更为精确。结果表明:(1)15年间各地类面积均发生了不同程度的变化,其中林地面积的增加量(主要来源为耕地和草地)和耕地面积的减少量(主要流向为林地和草地)分别达到了3 142.25km2及3 011.27km2,退耕还林工程取得了巨大成效。(2)延安市2000年和2015年的土壤侵蚀强度均以轻度和中度侵蚀为主,侵蚀总量分别为2.20亿t和1.38亿t,15年间各侵蚀强度类型均有向低等级侵蚀强度类型转换的趋势。(3)延安市2000年与2015年土壤保持总量分别为7.72亿t和8.50亿t,且土壤保持量呈东北向西南增加的趋势,各地类中林地和草地的土壤保持效果最佳,说明退耕还林工程的推进是增加区域土壤保持功能的重要途径。     

陕西渭北旱塬区土地利用与生态系统服务价值变化——以陇县为例

以陕西省陇县为研究对象,基于该区域2000年、2005年、2010年、2014年四期遥感影像,探讨了陇县土地利用变化情况,并参照生态系统服务价值当量估算法定量分析了研究期内陇县的生态系统服务价值变化特征,以期为陇县生态环境保护和土地利用调控提供科学参考。结果表明:（1）近14年,陇县土地利用结构发生了明显变化,耕地、未利用地、水体的面积出现减少,林地、草地、建设用地的面积均有不同程度增加,草地面积增加最多,增长了15.18%,耕地面积减少最多,减少了2.68%。（2）研究期内,林地生态系统服务价值最高,依次为耕地、草地、水体和未利用地。陇县总生态系统服务价值增加了1.617×10⁷元,林地和草地的贡献度最大。耕地和水体面积减少引起食物生产、废物处理、水源涵养和娱乐文化的生态服务价值在不同时期出现下降。（3）不同时期,各地类的生态系统服务价值敏感系数均小于1,表明生态系统服务价值对价值系数变化缺乏弹性,研究结果具有可信度。     

陕西省神木县土地利用格局和生态服务价值变化

在对土地利用信息提取的基础上,运用土地利用变化强度、动态度、土地利用程度、生态系统服务价值对陕西省神木县土地利用时空演变特征及其生态响应进行了分析。结果表明,2000-2010年神木县土地利用类型转换较快,土地利用程度明显减弱,耕地、草地、林地、水域、工矿用地、建设用地和未利用土地的变化强度分别达到-1.687%,0.782%,0.469%,-0.033%,0.067%,0.086%和0.316%。2000-2010年神木县生态系统服务价值呈增长态势,10 a间增加了0.94亿元,同期耕地生态价值减少了3.95亿元,林地和草地分别增加了3.47和1.92亿元,而水域和未利用土地的生态价值变化不大。空间上,北部乡镇生态系统服务价值以下降为主,而南部乡镇以增长为主。     

陕北地区土地利用变化对生态服务功能价值的影响

以陕北地区榆林、延安2市作为研究区域,利用土地利用变化数据,对陕北地区用地类型的变化进行分析,并使用合适的生物量因子,修正生态服务功能价值当量表,使当量表更符合陕北地区的实际情况,结合已有研究得到的二级用地类型当量因子比例关系,细化生态系统服务功能当量因子表,以便更准确的估算陕北地区生态系统服务功能价值.结果表明:1985-2013年间,主要用地类型变化为耕地、林地和草地,耕地面积减少2 811 km2,减少率达到9.98％,林地和草地面积分别增加1 392和2417 km2,增幅分别达到12.85％和7.10％;其中,2000-2013年面积急剧变化,林地和草地面积的增加量分别是1985-2000年的7倍和2倍,退耕还林(草)效果明显;生态系统服务功能价值大量增加,林地和草地的生态系统服务功能价值增量分别为25.2亿和13.9亿元,2000-2013年的增量是1985-2000年增量的3倍.生态服务功能价值的变化与全面推行退耕还林(草)的时间相对应,表明退耕还林(草)政策,对陕北地区生态系统服务功能价值有积极的影响.     

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。     

退耕还林（草）对延河下游流域生态系统服务价值的影响

为定量评估延河下游流域退耕还林（草）对生态环境的影响,在分析处理退耕前（1997年）TM影像和退耕后（2005年）CBERS卫片的基础上,根据Costanza及我国学者相关研究成果,计算并比较了延河下游流域退耕前后的生态系统服务价值。研究结果表明,通过退耕还林（草）,延河下游流域的生态资产总值增加3．6％,说明该区域的生态环境状况有所改善;退耕前后不同土地利用类型生态系统服务价值大小排序基本不变,依然是草地〉耕地〉林地,草地的贡献率增加10％,耕地贡献率减少11％;退耕前后生态系统服务功能价值结构比例基本未变。     

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