基于风蚀模型的河北省土壤风蚀风险评价

[目的]对河北省空间范围开展土壤风蚀风险评价研究,以期为研究区基于土壤风蚀的土地退化和大气污染防治提供科学依据。[方法]在遥感和地理信息系统技术的支持下,采用第一次全国水力普查中推荐的风蚀模型,对研究区2009年的土壤风蚀风险进行评价。[结果]河北省土壤微度侵蚀面积所占比例最大,约占河北省总面积的65.36%,主要分布在河北平原、太行山地和冀北山地;其次为轻度侵蚀,约占河北省总面积的12.46%,主要分布在坝上高原和冀西北间山盆地;中强度侵蚀的风蚀面积最小,合计不足河北省总面积的0.1%,主要为分布在研究区北部的沙地类型;极强侵蚀和剧烈侵蚀没有分布。[结论]河北省土壤风蚀强度呈现出明显的空间差异性,干燥,风速大,植被覆盖度低的冀西北地区风蚀强度最大,湿润、风速小、植被覆盖度高的冀东北地区风蚀强度最小,南部平原和太行山区风蚀强度中等。     

山西省不同生态区NDVI时空变化及其影响因素

利用MODIS-NDVI数据研究了2000—2012年山西省生长季平均NDVI的时空变化特征,并分析了气候变化和人类活动对生长季平均NDVI变化的影响。结果表明:（1）13 a来山西省山地落叶阔叶林生态区、农业与草原生态区、汾河谷地农业生态区生长季平均NDVI均呈显著增加趋势,增长率分别为0.052/10 a,0.079/10 a,0.049/10 a。（2）山西省植被变化存在明显的空间差异,改善区占全省面积的73.76%,主要分布在太行山、太岳山、吕梁山、中条山等地区;而退化区占全省面积的3.29%,主要分布在太原盆地及临汾盆地的边缘地区。（3）NDVI变化是气候变化和人类活动共同影响的结果。不同生态区NDVI对气候变化的响应不同。其中,前一年11月至5月降水增多可能是山地落叶阔叶林生态区NDVI增加的主要原因,前一年12月至1月气温降低和2—5月降水增多可能是农业与草原生态区NDVI增加的主要原因,5—9月气温降低和前一年11月至8月降水增多可能是汾河谷地农业生态区NDVI增加的主要原因。而人类活动对NDVI变化的影响主要表现在退耕还林、防护林建设、采矿、城市扩张等方面。     

黑河上游植被覆盖时空变化特征及其未来趋势

[目的]分析黑河上游近14a来植被覆盖时空变化特征及其未来趋势,为区域生态保护提供决策参考。[方法]基于2001—2014年MODIS-NDVI数据,采用线性趋势和Hurst指数等数理方法进行定量分析。[结果]研究区植被覆盖整体处于缓慢增加趋势,年际变化呈现"波动—改善"趋势,空间分布呈现自东南向西北逐渐降低的分布特征,且具有明显经向地带性;植被覆盖改善区和退化区的面积分别占47.74%和21.81%,其中改善区主要分布在研究区中部和西北部,以高寒草原改善为主,退化区主要位于研究区东南部,以高寒草甸退化为主;未来的NDVI将向着改善的方向发展。[结论]近14a来黑河上游植被覆盖良好,且未来向着改善趋势发展,但对祁连、民乐和山丹县境内的持续退化区需要采取保护措施。     

藏西南高原植被NDVI时空变化及其与海拔梯度的关系

青藏高原植被分布不仅与区域水热条件密切相关,而且受海拔和地形的共同影响,认知植被与海拔梯度的关系对青藏高原生态保护具有重要科学和现实意义。基于MODIS NDVI数据和植被类型数据,分析了藏西南高原近21年来不同植被类型生长季NDVI时空变化特征,探讨了植被覆盖与海拔梯度的关系。结果表明:藏西南高原植被类型有森林、荒漠、草原、草甸、高山植被、栽培植被、灌丛和其他植被8种; 随时间推移,各植被类型NDVI均显著增加且在2017年达到最大值。研究区草原、草甸、灌丛和高山植被的增加速率依次为0.006/10 a,0.004/10 a,0.01/10 a和0.006/10 a。除了局地植被呈退化趋势外,绝大部分植被覆盖不断改善。草原、草甸、灌丛和高山植被主要集中分布在海拔4 000 m以上的地区,NDVI在各海拔梯度上均存在较大差异。不同植被类型NDVI随海拔升高均呈现不同的减小趋势,不同年份间同种植被NDVI随海拔梯度变化具有相似的变化趋势。研究结果可为藏西南高原生态建设、植被恢复和畜牧业发展提供一定的科学依据和决策支持。     

1999-2019年西南喀斯特地区NDVI时空变化及其气候驱动

[目的]西南喀斯特地区生态环境脆弱,对其植被覆盖变化及气候驱动机制进行研究具有重要意义。[方法]基于1999—2019年SPOT NDVI数据和同期209个气象站点的气温和降水数据,采用Theil-Sen+Mann-Kendall趋势分析法、偏相关分析和复相关分析法,探讨西南喀斯特地区NDVI时空变化及其气候驱动。[结果]1999—2019年西南喀斯特地区NDVI呈显著上升趋势,整体植被覆盖较好;NDVI变化主要以极显著上升趋势为主,仅5.73%的地区呈退化趋势。NDVI与气温和降水整体上均呈正偏相关关系,气温对NDVI的影响强于降水,且存在空间差异性。NDVI与气温和降水的复相关显著性通过0.05,0.01水平的面积分别占15.12%,5.68%;NDVI主要受气温驱动,占研究区面积的13.90%,其他气候因子驱动类型占比均未超过3%。[结论]揭示了西南喀斯特地区植被覆盖的时空变化特征,明确了气候因子对植被覆盖变化的驱动机制。     

湖北省植被覆盖时空分异季节变化研究

本文从年际变化、季节变化、主要生长季、空间分布特征研究了湖北省1998年4月至2014年3月近16年植被覆盖时空分异变化情况,近16年年均NDVI整体呈上升趋势,多年NDVI季节变化春季与秋季的变化曲线相似,主要生长季NDVI与多年年际总体变化趋势是一致的。湖北省植被覆盖变化具有明显的空间差异,大部分区域植被呈现转好趋势,但局部地区也有退化的现象,其中退化面积只占总面积的3.25%。     

基于CA-Markov的渭河流域NDVI时空变化模拟及预测

为综合分析流域植被覆盖的时空变化特征,进而为生态环境保护提供科学参考,以渭河流域作为研究区域,首先基于MODIS NDVI中国月合成数据计算研究区年际NDVI值,并进行等级划分;而后利用CA-Markov模型,以NDVI等级作为元胞类型,计算了不同时期各等级的转移矩阵,由此模拟了2015年NDVI的空间分布;对比模拟NDVI结果和原始影像数据,评价模拟精度,并预测了2020年和2025年NDVI的空间分布状况。结果表明:(1)利用CA-Markov模型模拟渭河流域NDVI的空间变化,得到2015年模拟结果的Kappa系数为0.785 0,叠置分析的准确度为73.4%,符合精度要求,可以用于NDVI空间分布的预测。(2)渭河流域植被覆盖存在明显的空间差异性,低度植被覆盖区、较低植被覆盖区主要分布于陇中和陕北黄土高原地区;中度植被覆盖区主要分布在泾河和北洛河河谷地区,较高植被覆盖区主要分布在关中平原、子午岭等地,高度植被覆盖区主要分布在秦岭、六盘山等山区。(3) 2015—2025年,预测流域植被覆盖状况将进一步改善。其中,低度植被覆盖区、较低植被覆盖区和中度植被覆盖区面积将有所减少,空间上整体向北迁移;较高植被覆盖区和高度植被覆盖区面积进一步增加,空间上向北扩张。     

青藏高原植被生长季NDVI时空变化及对气候因子的响应分析

为了探究青藏高原植被覆盖时空演变特征及其驱动因子,对青藏高原的生态环境保护提供科学依据,基于1982—2015年青藏高原内部及其周边139个气象站点的气象数据和同期的GIMMS NDVI数据,研究了青藏高原生长季植被NDVI的时空变化特征及其与气候因子的响应关系。结果表明：(1)在研究期内,青藏高原生长季NDVI总体呈上升趋势,不同干湿地区生长季NDVI变化趋势有所差异,湿润地区植被退化面积占比相对较大,干旱地区植被改善面积占比相对较大。(2)研究区植被未来总体向改善方向发展,植被未来趋向改善面积占62.25%,趋向退化面积占37.58%。(3)研究区植被对各气候因子的响应存在一定的滞后性,草原、草甸、高山植被和灌丛4种主要植被对气温和相对湿度主要当月响应,对降水主要当月或滞后1个月响应,对日照时数主要滞后3个月响应。(4)气温、降水、相对湿度及日照时数4个气候因子对青藏高原植被NDVI变化的相对贡献率分别为37.19%,27.53%,20.30%和14.97%,其中,气温和降水是湿润/半湿润地区、半湿润地区、大部分半干旱地区及干旱地区植被NDVI变化的主要气候驱动因子,日照时数和相对...     

河北省生态环境支撑区生态系统服务价值评估

[目的]研究和评估不同生态区主要生态系统服务功能和价值的变化,为区域的生态保护、自然资源核算和生态补偿等决策提供理论依据和参考。[方法]基于1980,2000和2015年的土地利用数据,采用单位面积价值当量因子法构建土地利用二级分类当量表,计算3个时段河北省各生态环境支撑区的生态系统服务价值。[结果]①1980—2015年河北省土地利用的变化主要表现为农田转为城乡建设用地,其他生态用地转为农田的趋势。②以2015年的价格计算,1980—2015年河北省总生态系统服务价值的年平均值约为3.56×10~(11)元。③1980—2015年河北省生态系统服务价值减少约7.00×10~9元。④单位面积生态系统服务价值最高的生态区为燕山—太行山,其次是坝上高原、海域海岸、京津保,最低的为低平原区。⑤1980—2015年京津保生态区生态系统服务价值下降幅度最大为28.1亿元,降低15.0%。[结论] 2000年后的生态环境建设对恢复和提升河北省生态系统服务功能发挥了重要作用。     

开都—孔雀河流域NDVI动态变化及其与气温和降水的联系

基于2000—2014年开都—孔雀河流域的MODIS NDVI数据,采用最大合成法、偏相关分析法,结合气象站同期降水与气温数据,研究流域近15年NDVI时空动态,并分析了其与气温和降水的关系。结果表明:(1)研究区NDVI多年平均呈北部和中部高,南部和东部低的空间分布态势;(2)近15年开都—孔雀河流域植被退化区域大于植被改善区域,退化区域面积占研究区总面积的9.1%,主要集中在和静县山区;改善区域面积占总面积的6.33%,主要集中于博湖绿洲和库尔勒市及周边;84.57%的区域NDVI基本保持不变,主要集中于尉犁县;(3)植被覆盖2002年与2010年有明显改善,2001年、2004年有明显退化,其他年份退化和改善程度不一;(4)研究区NDVI对降水和气温变化均具滞后性且滞后水平存在显著的空间差异;(5)气温对NDVI的影响明显高于降水对NDVI的影响。     

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