Effect of grazer regulation and algal inoculation on photodependent nitrogen fixation in a wetland rice field

Summary A dry season field experiment conducted for two consecutive years highlighted problems of achieving increased populations of N₂-fixing blue-green algae (BGA) in wetland rice fields. Inoculation of non-indigenous BGA strains, either dried or as fresh viable inocula even at high levels of application, was unsuccessful. A limiting effect of grazing invertebrate populations on BGA establishment was evident, but other factors were involved. Reducing grazer pressure did not permit establishment of inoculated BGA; interspecific competition and environmental factors may explain the inoculation failure. Grazer regulation permitted the establishment of a fast-growing indigenous N₂-fixing Anabaena and the doubling of N₂-fixing activity over a control. Neither inoculation nor grazer control affected grain yields significantly.     

Inoculation of rice with nitrogen-fixing bacteria —problems and perspectives

Summary The present status and merits of inoculating rice with N₂-fixing bacteria are discussed in the light of recent advances. Bacterial inoculation improves plant growth and rice yield but not uniformly. The yield response to inoculation is more pronounced in the presence of moderate levels of fertilizer N. Evidence for the establishment and activity of the inoculated bacteria is limited, and the poor survival of the inoculum under field conditions further complicates the effects of inoculation. There is no clear evidence that improved growth and mineral content following inoculation are due to increased N₂ fixation. Beneficial effects of the inoculum on rice, such as plant growth promotion, N₂ fixation and antagonism effects against pathogens need to be further investigated under laboratory and field conditions. Improved management practices, such as organic amendments, suitable water and soil management, selection of efficient microbial strains, selection of effective breeding lines with high associative nitrogen fixation, and better management of agrochemicals are some of the measures suggested for deriving benefits from bacterial associations with rice.     

Estimating N2 fixation by Sesbania rostrata and S. cannabina (syn. S. aculeata) in lowland rice soil by the 15N dilution method

Summary A field experiment in concrete-based plots was conducted to estimate the contribution of N derived from air (Ndfa) or biological N₂ fixation in Sesbania rostrata and S. cannabina (syn. S. aculeata), using various references, by the ¹⁵N dilution method. The two Sesbania species as N₂-fixing reference plants and four aquatic weed species as non-N₂-fixing references were grown for 65 days after sowing in two consecutive crops, in the dry and the wet seasons, under flooded conditions. Soil previously labeled with ¹⁵N at 0.26 atom % ¹⁵N excess in mineralizable N was further labeled by ammonium sulfate with 3 and 6 atom % ¹⁵N excess. The results showed that ¹⁵N enrichment of soil NH ₄ ⁺ -N dropped exponentially in the first crop to half the original level in 50 days while in the second crop, it declined gradually to half the level in 130 days. The decline in ¹⁵N enrichment, in both N₂-fixing and non-fixing species, was also steeper in the first crop than in the second crop. Variations in ¹⁵N enrichment among non-fixing species were smaller in the second crop. The ratio of the uptake of soil N to that of fertilizer N in N₂-fixing and non-fixing species was estimated by the technique of varying the ¹⁵N level. In the second crop, this ratio in non-fixing species was higher than that in N₂-fixing species. Comparable estimates of % Ndfa were obtained by using ¹⁵N enrichment of various non-fixing species. There was also good agreement between the estimates obtained by using ¹⁵N enrichment of non-fixing species and those by using soil NH ₄ ⁺ -N, particularly in the second crop. By 25 days after sowing, the first crop of both Sesbania spp. had obtained 50% of total N from the atmosphere and the second crop had obtained 75%. The contribution from air increased with the age of the plant and ranged from 70% to 95% in 45–55 days. S. rostrata fixed substantially higher amounts of N₂ due to its higher biomass production compared with S. cannabina. Mathematical considerations in applying the ¹⁵N dilution method are discussed with reference to these results.     

Inoculation of rice plants with the endophytic diazotrophs Herbaspirillum seropedicae and Burkholderia spp.

 Four experiments were performed under gnotobiotic conditions to select strains of the endophytic diazotrophs Herbaspirillum seropedicae and Burkholderia spp. as inocula of rice plants. Eighty strains of H. seropedicae originally isolated from rice, sorghum and maize plants, were tested in test tube cultures with N-free agar as the substrate. Rice plants showed medium and high increases in their fresh weight in response to inoculation with nineteen strains. These strains were tested again, and six strains were then selected to evaluate their contribution to the N of the plant via biological N₂ fixation (BNF) using an agar growth medium containing 5 mg N l^–1of ¹⁵N-labelled (NH₄)₂SO₄. The contribution of the strains to plant N via BNF varied from 54% when rice plants were inoculated with strain ZAE94, to 31% when strain ZAE67 was used. These results were confirmed in the fourth gnotobiotic experiment, which also included strains of the new N-fixing bacteria belonging to the genus Burkholderia, isolated from rice, as well as a strain of Burkholderia vietnamiensis, isolated from rice rhizosphere. Burkholderia spp. strains showed similar effects to those observed for H. seropedicae strains, while B. vietnamiensis fixed only 19% of plant total N. The best four strains were tested in a pot experiment where pre-germinated, inoculated rice seedlings were grown in soil labelled with ¹⁵N. The results confirmed the gnotobiotic experiments, although the levels of N in the rice plants derived from BNF of the selected H. seropedicae and Burkholderia spp. strains were lower. Nevertheless, there was an increase in N content in grains of inoculated plants, and the results showed that the method used for strain selection is very useful and can be applied to other strains of N₂-fixing bacteria and plants. Received: 4 May 1999     

Dual inoculation of Pisum sativum with Rhizobium leguminosarum and Penicillium bilaji

Summary To investigate the effect of single versus dual inoculation of peas (Pisum sativum L.) with Rhizobium leguminosarum biovar viceae and Penicillium bilaji (a soil fungi capable of solubilizing soil P) on N₂ fixation an experiment was carried out under controlled conditions. A sandy loam soil was selected which contained low levels of available N and P. P fertilizer [Ca(H₂PO₄)₂] and P. bilaji significantly increased dry matter production. Peas inoculated with R. leguminosarum showed only a small increase in dry matter, but the additional application of P significantly increased the yield. The total N accumulation was highly dependent on the presence of R. leguminosarum. Using the ¹⁵N method for estimating N₂ fixation, the highest level of N₂-fixing activity was observed in peas inoculated with R. leguminosarum and fertilized with inorganic P. Dual inoculation of peas with P. bilaji and R. leguminosarum significantly decreased the amount of N₂ fixed. Total P uptake was solely dependent on the P fertilizer.     

Isotope 15N enrichment of soil-ammonium N as a reference to estimate N2 fixation by stem and root-inoculated S. rostrata

Summary A pot experiment in the greenhouse was conducted to compare the contribution of N derived from the atmosphere or from biological N₂ fixation by Sesbania rostrata inoculated with Azorhizobium caulinodans, applied either to roots or to roots and stems (single or multiple stem inoculation). Two subsequent crops were grown for 50 days under flooded conditions. N derived from air was estimated by ¹⁵N dilution using ¹⁵N enrichment of soil NH _inf4 ^sup+ -N and of Echinochloa crusgalli as the non-N₂-fixing reference datum and compared with estimates obtained by the N-difference method. The first crop was grown to stabilize the ¹⁵N into the soil organic N fraction. The ¹⁵N enrichment of soil NH _inf4 ^sup+ -N in the second crop declined slowly. The extractability ratio (¹⁵N enrichment of extractable soil N to ¹⁵N enrichment of total soil N) decreased from 4.8 to 4.1 50 days after planting. The enrichment of soil NH _inf4 ^sup+ -N was comparable to that of E. crus-galli, resulting in similar estimates of N derived from air when either soil NH _inf4 ^sup+ -N or enrichment of E. crus-galli was used as a non-fixing reference. The N-difference method did not always provide reliable estimates of N derived from air; percentages ranged from 75 to more than 80 by 50 days after planting in both crops and did not differ among treatments. The study demonstrates the potential of using ¹⁵N enrichment of soil NH _inf4 ^sup+ -N as a non-N₂-fixing reference for reliable BNF estimates of crops in lowland puddled soil.     

Prospects and potentials for systems of biological nitrogen fixation in sustainable rice production

The N requirement of rice crops is well known. To overcome acute N deficiency in rice soils, this element is usually supplied to the rice crop as the commercially available fertilizer urea. But unfortunately a substantial amount of the urea-N is lost through different mechanisms causing environmental pollution problems. Utilization of biological N fixation (BNF) technology can decrease the use of urea-N, reducing the environmental problems to a considerable extent. Different BNF systems have different potentials to provide a N supplement, and it is necessary to design appropriate strategies in order to use BNF systems for efficient N supply to a rice crop. Research has been conducted around the world to evaluate the potential of different BNF systems to supply N to rice crops. This paper reviews salient findings of these works to assess all the current information available. This review indicates that the aquatic biota Cyanobacteria and Azolla can supplement the N requirements of plants, replacing 30–50% of the required urea-N. BNF by some diazotrophic bacteria like Azotobacter, Clostridium, Azospirillum, Herbaspirillum and Burkholderia can substitute for urea-N, while Rhizobium can promote the growth physiology or improve the root morphology of the rice plant. Green manure crops can also fix substantial amounts of atmospheric N. Among the green manure crops, Sesbania rostrata has the highest atmospheric N₂-fixing potential, and it has the potential to completely substitute for urea-N in rice cultivation.     

Dynamics of algal populations and acetylene-reducing activity in five rice soils inoculated with blue-green algae

Summary The dynamics of five inoculated strains of heterocystous blue-green algae (BGA) and indigenous algae were studied for 1 month in 1-m² microplots of five soils previously air-dried or oven-dried. The same soils were then dried and resubmerged for another 2 months to study the effect of controlling algal grazers with neem (Azardirachta indica) seeds on the revival and dynamics of indigenous and inoculated algae. During the month following inoculation, inoculated BGA multiplied to some extend in all soils but never dominated the total algal flora. They rarely dominated the indigeneous heterocystous BGA, and did so only when the growth of N₂-fixing BGA was poor or after the decline of blooms of indigenous strains. Once the soils were dried, two of the five inoculated strains did not reappear. During the 1st month following rewetting, the remaining inoculated strains again exhibited poor growth; however, after 2 months of submergence, inoculated Aulosira fertilissima developed an agronomically significant bloom in neem-treated plots of two soils. Correlations between acetylene-reducing activity and heterocystous BGA populations indicated a major contribution by indigenous BGA and a minor contribution by inoculated BGA to the N₂-fixing activity of the soils during the first experiment and the 1st month of the second experiment. The establishment of inoculated BGA exhibited clear differences among strains but was less affected by the nature of the soil and heat treatment. Neem application might have had a delayed positive effect on the late establishment of inoculated A. fertilissima and favored BGA growth and N₂ fixation by the total algal population.Visiting Scientist at IRRI     

Enhancing micronutrient uptake and yield of wheat through bacterial PGPR consortia

A pot experiment was undertaken under net house conditions, with three rhizobacterial strains AW1 (Bacillus sp.), AW5 (Providencia sp.) and AW7 (Brevundimonas sp.), applied along with 2/3 recommended dose of nitrogen (N) and full dose of phosphorus (P) and potassium (K) fertilizers (N₉₀P₆₀K₆₀). An enhancement of 14–34% in plant biometric parameters and 28–60% in micronutrient content was recorded in treatments receiving the combination of AW1?+?AW5 strains, as compared to full dose of fertilizer application. The treatment involving inoculation with AW5?+?AW7 recorded highest values of % P and N, with a two-fold enhancement in phosphorus and 66.7% increase in N content, over full dose application of P and K fertilizers. A significant correlation was recorded between plant biomass, panicle weight, grain weight, N, P and iron (Fe) with acetylene reduction activity, indicating the significance of N fixation in overall crop productivity. Our study illustrates the multiple benefits of plant growth promoting rhizobacteria (PGPR) inoculation in integrated nutrient management and biofortification strategies for wheat crop.     

Nitrous oxide emissions from grain legumes as affected by wetting/drying cycles and crop residues

Grain legume production with rhizobial inoculation has drawn attention not only because of the economic value of nitrogen (N) fixation by grain legumes, but also because of the concern that N₂ fixation by grain legumes may enhance emissions of nitrous oxide (N₂O), a powerful greenhouse gas. However, the relationship between N₂O emissions and biological N₂ fixation by grain legumes is not well understood. The objective of this study was to quantify N₂O emissions associated with N₂ fixation by grain legumes as affected by wetting/drying cycles and crop residues. Two grain legumes, lentil (Lens esculenta Moench) and pea (Pisum sativum L.), either inoculated with two Rhizobium leguminosarum biovar viciae strains, 99A1 and RGP2, respectively, or fertilized with ¹⁵N-labeled fertilizer were grown in a controlled environment under three wetting/drying cycles. Profile N₂O concentrations and surface N₂O emissions were measured from the soil–plant systems, which were compared with those from a cereal, spring wheat (Triticum aestivum L. ac. Barrie). After harvest, crop residues were incorporated into soils that were seeded to spring wheat to evaluate the effect of crop residues on N₂O emissions. Results indicated that: (1) inoculating grain legumes with non-denitrifying rhizobia did not enhance N₂O emissions and the presence of grain legumes did not increase N₂O emissions compared with the cereal crop, and (2) profile N₂O accumulation and surface emissions were not related to the type of crop residues added to the soil, but related to the residual N applied previously as N fertilizer. This suggests that N₂O emissions are not directly related to biological N₂ fixation by grain legumes, and on a short time scale, N rich residues of N₂-fixing crops have a limited impact on N₂O emissions compared with N fertilization.     

Nutrient accumulation in aboveground biomass of planted tropical trees: a meta-analysis

Efficient nutrient use is essential for biomass production by tropical trees growing in infertile soils. Accumulation of nitrogen (N) and phosphorus (P) in the aboveground biomass of four groups of tree stands [Acacia, Eucalyptus, N₂-fixing trees excluding Acacia, and other non-N₂-fixing broadleaved (ONNFB) trees] were investigated using meta-analyses of a range of biomass data to test the hypothesis that fast-growing Acacia and Eucalyptus trees accumulate fewer nutrients. Data for 83 tropical tree stands were selected from the literature. Standardized major axis regressions were applied between the log₁₀-transformed biomass and N or P accumulation. Nutrient use efficiency was compared with aboveground biomass and topsoil conditions. The slope of the regression between aboveground biomass and N accumulation for Eucalyptus was significantly smaller than the slopes for the N₂-fixing trees (excluding Acacia) and the ONNFB trees. N use efficiency of Eucalyptus increased with biomass more than that of N₂-fixing trees (excluding Acacia) and the ONNFB trees, because their stems and twigs tended to accumulate less N than in the other groups as biomass increased. The regressions between aboveground biomass and P accumulation had a common slope, and the intercepts of Acacia and Eucalyptus were significantly lower than that of ONNFB trees. P use efficiency of Acacia was consistently higher than that of the ONNFB trees. P use efficiency is more affected by other factors like soil conditions than is N use efficiency, and the differences in the tree groups significantly affect the use efficiency of both nutrients. These results explained some aspects of the general suitability of Acacia and Eucalyptus species for tropical plantations on infertile soils.     

Biological nitrogen fixation in Azospirillum strain-maize genotype associations as evaluated by the 15N isotope dilution technique

Few studies of the inoculation of cereal crops with N₂-fixing bacteria have included more than one or two plant genotypes. In a recent study performed in Argentina using 12 different maize genotypes, it was found in 2 consecutive field experiments that several of them responded consistently, either negatively or positively, to inoculation with a mixture of strains of Azospirillum spp. The present study in post was performed to investigate the effect of inoculation of individual strains (and a mixture) of Azospirillum spp., and their nitrate reductase negative (NR-) mutants, on the growth of four of these maize genotypes. Two of these genotypes were grown in ¹⁵N-labelled soil with the aim of quantifying any contributions of biological N₂ fixation. Two genotypes (Morgan 318 and Dekalb 4D-70) produced similar increases in grain yield when they were inoculated with a mixture of Azospirillum spp. strains or fertilized with the equivalent of 100 kg N ha^-1. The other genotypes (Dekalb 2F-11 and CMS 22) showed little response to inoculation or N fertilization. The Morgan 318 and Dekalb 4D-70 genotypes showed a large increase in total N accumulation, suggesting that the response was due to increased N acquisition, but not due to bacterial nitrate reductase as the NR- mutants generally caused plant responses similar to those of the parent strains. Despite problems with the stabilization of the ¹⁵N enrichment in the soil, the ¹⁵N isotope dilution results indicated that there were very significant biological nitrogen fixation (BNF) contributions to the Dekalb 4D-70 and CMS 22 maize genotypes.Dedicated to Professor J.C.G. Ottow on the occasion of his 60th birthday     

Performance of Pseudomonas spp. containing ACC-deaminase for improving growth and yield of maize (Zea mays L.) in the presence of nitrogenous fertilizer

Some plant-growth-promoting rhizobacteria (PGPR) promote plant growth by lowering the endogenous ethylene synthesis in the roots through their 1-aminocylopropane-1-carboxylate (ACC)-deaminase activity. However, in the vicinity of the roots may decrease the efficiency of these PGPR by stimulating ACC-oxidase activity resulting in greater ethylene production by the roots. This study was designed to assess the performance of PGPR containing ACC-deaminase for improving growth and yield of maize grown in N-amended soil. Several strains of rhizobacteria containing ACC-deaminase were screened for their growth-promoting activity in maize roots under gnotobiotic conditions. Six strains were selected and their effectiveness in soil amended with N at a concentration of 175 kg ha⁻¹ (1050 mg pot⁻¹) was investigated by conducting a pot trial on maize. Significant increases in plant height, root weight and total biomass were observed in response to inoculation. Based upon the results of pot trials, the three most efficient strains were selected and tested in the field for their effectiveness in the presence and absence of N fertilizer. Results of the field trial revealed that the inoculum performed relatively better in the absence of N-fertilizer application. Pseudomonas fluorescens biotype G (N₃) was the most effective strain both in the presence and absence of N fertilizer. Results may imply that even in the presence of optimum levels of nitrogenous fertilizers, inoculation with rhizobacteria containing ACC-deaminase activity could be effective to improve the growth and yield of inoculated plants.     

Effects of Plant Growth Promoting Rhizobacteria on Yield and Some Fruit Properties of Strawberry

Effects of plant growth promoting rhizobacteria (PGPR) [(Pseudomonas BA-8 (biological control agent), Bacillus OSU-142 (N₂-fixing), and Bacillus M-3 (N₂-fixing and phosphate solubilizing)] on yield and some fruit properties of strawberry cultivar ‘Selva’ in the province of Erzurum, Turkey in 2002–2003. Foliar + root application of PGPR strains significantly increased yield per plant as compared with the control. Root application of PGPR strains significantly increased total soluble solids, total sugar and reduced sugar, but decreased titratable acidity. It was also determined that bacteria applications have no important effect on the average fruit weight and pH. The results of this study suggested that Pseudomonas BA-8, Bacillus OSU-142 and Bacillus M-3 have potential for increasing yield in strawberry plant.     

Co-inoculation of selected nodule endophytic rhizobacterial strains with Rhizobium tropici promotes plant growth and controls damping off in common bean

Production of common bean(Phaseolus vulgaris)is limited by the occurrence of damping off(rhizoctoniosis),which is caused by the fungus Rhizoctonia solani.However,the co-inoculation of plant growth-promoting rhizobacteria(PGPR)involved in biological control along with diatomic nitrogen(N2)-fixing rhizobia can enhance N nutrition and increase production.In this context,finding microorganisms with synergistic effects that perform these two roles is of fundamental importance to ensure adequate yield levels.The aim of this study was to evaluate the effects of co-inoculation of nodule endophytic strains of the genera Bacillus,Paenibacillus,Burkholderia,and Pseudomonas with Rhizobium tropici CIAT 899,an N2-fixing rhizobial strain,on the biocontrol of damping off and growth promotion in common bean plants.Greenhouse experiments were conducted under axenic conditions using the common bean cultivar Pérola.The first experiment evaluated the potential of the 14 rhizobacterial strains,which were inoculated alone or in combination with CIAT 899,for the control of R.solani.The second experiment evaluated the ability of these 14 rhizobacterial strains to promote plant growth with three manners of N supply:co-inoculation with CIAT 899 at low mineral N supply(5.25 mg N mL^-1),low mineral N supply(5.25 mg N mL^-1),and high mineral N supply(52.5 mg N mL^-1).The use of rhizobacteria combined with rhizobia contributed in a synergistic manner to the promotion of growth and the control of damping off in the common bean.Co-inoculation of the strains UFLA 02-281/03-18(Pseudomonas sp.),UFLA 02-286(Bacillus sp.),and UFLA 04-227(Burkholderia fungorum)together with CIAT 899 effectively controlled damping off.For the common bean,mineral N supply can be replaced by the co-inoculation of CIAT 899 with plant growth-promoting strains UFLA 02-281/02-286/02-290/02-293.Nodule endophytes UFLA02-281/02-286 are promising for co-inoculation with CIAT 899 in the common bean,promoting synergy with rhizobial inoculation and protection against disease.     

Developed microbial biofilms can restore deteriorated conventional agricultural soils

Nitrogen fixing bacteria play a key role in the growth and persistence of effective microbial communities in the soil by supplying N through biological nitrogen fixation (BNF). In the long run, chemical inputs, particularly N fertilisers are known to adversely affect N₂ fixers and hence maintenance of soil fertility and crop productivity. This study examined the effect of developed microbial biofilms with N₂ fixers on restoration of soils deteriorated by conventional agricultural practices in tea cultivation. Just reducing recommended chemical fertiliser use by 50% significantly increased soil microbial biomass and BNF, and decreased soil NO₃⁻ and pest infestation. The lower chemical fertiliser addition coupled with the biofilm-based biofertilisers known as biofilmed biofertilisers (BFBFs) further increased BNF significantly. The combined application significantly increased soil organic C by ca. 20%, and reduced leaf transpiration by ca. 40%. It also supported plant growth, rhizoremediation and soil moisture conservation in comparison to the 100% chemical fertilisation. Those improved performances were observed to be proportional to the increased density of soil bacteria, and have several agronomic and environmental implications. It is apparent from this study that replenishing the depleted soil microbial communities by applying such biofertilisers is likely to be beneficial in agroecosystems with chemical N fertiliser use, if they are to be sustained for crop production.     

Biological nitrogen fixation by common beans (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.) increases with bio-char additions

This study examines the potential, magnitude, and causes of enhanced biological N₂ fixation (BNF) by common beans (Phaseolus vulgaris L.) through bio-char additions (charcoal, biomass-derived black carbon). Bio-char was added at 0, 30, 60, and 90 g kg⁻¹ soil, and BNF was determined using the isotope dilution method after adding ¹⁵N-enriched ammonium sulfate to a Typic Haplustox cropped to a potentially nodulating bean variety (CIAT BAT 477) in comparison to its non-nodulating isoline (BAT 477NN), both inoculated with effective Rhizobium strains. The proportion of fixed N increased from 50% without bio-char additions to 72% with 90 g kg⁻¹ bio-char added. While total N derived from the atmosphere (NdfA) significantly increased by 49 and 78% with 30 and 60 g kg⁻¹ bio-char added to soil, respectively, NdfA decreased to 30% above the control with 90 g kg⁻¹ due to low total biomass production and N uptake. The primary reason for the higher BNF with bio-char additions was the greater B and Mo availability, whereas greater K, Ca, and P availability, as well as higher pH and lower N availability and Al saturation, may have contributed to a lesser extent. Enhanced mycorrhizal infections of roots were not found to contribute to better nutrient uptake and BNF. Bean yield increased by 46% and biomass production by 39% over the control at 90 and 60 g kg⁻¹ bio-char, respectively. However, biomass production and total N uptake decreased when bio-char applications were increased to 90 g kg⁻¹. Soil N uptake by N-fixing beans decreased by 14, 17, and 50% when 30, 60, and 90 g kg⁻¹ bio-char were added to soil, whereas the C/N ratios increased from 16 to 23.7, 28, and 35, respectively. Results demonstrate the potential of bio-char applications to improve N input into agroecosystems while pointing out the needs for long-term field studies to better understand the effects of bio-char on BNF.     

AMF和PGPR修复甲胺磷污染土壤的效应

丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)和根围促生细菌(plant growthpromoting rhizobacteria,PGPR)能降解有毒有机物,但分解土壤中残留甲胺磷农药尚未见报道。本试验旨在测定AMF和PGPR矿化甲胺磷的效应。试验设甲胺磷0、50、100和150μg g-1下,对番茄(Lycospersicon esculentum,品种金冠)接种AMF Glomus mosseae(Gm)、Glomus etunicatum(Ge)、PGPR Bacillus subtilis(Bs)、Bacillus sp.B697(Bsp)、Pseudomonas fluorescens(Pf)、Gm+Bs、Gm+Bsp、Gm+Pf、Ge+Bs、Ge+Bsp、Ge+Pf和不接种对照,共48个处理。结果表明,接种Gm显著增加了根区土壤和根内PGPR定殖数量,而Pf处理显著提高了AMF侵染率,表明Gm与Pf能够相互促进。甲胺磷100μg g-1水平下,Gm+Pf处理的番茄株高显著高于其他处理,地上部干重显著高于其他处理(Ge+Pf除外),根系干重显著高于对照、PGPR各处理和Ge处理;而根内甲胺磷浓度则显著低于其他处理,茎叶中的则显著低于其他处理(Gm+Bs、Gm+Bsp和Ge+Pf除外)。AMF、PGPR或AMF+PGPR处理均显著降低番茄体内甲胺磷浓度。甲胺磷50~100μg g-1水平下,Gm+Pf显著降低根区土壤中甲胺磷残留量,矿化率达52%~60.6%。AMF和PGPR显著提高了根区土壤中甲胺脱氢酶活性,其中以Gm+Pf组合处理的酶活性最高。表明AMF和PGPR均能促进土壤中残留甲胺磷的降解,Gm+Pf是本试验条件下的最佳组合。     

Influence of pruning frequency of Albizia lebbeck, Gliricidia sepium and Leucaena leucocephala on nodulation and potential nitrogen fixation

The influence of four pruning frequencies on biomass, nodulation and N₂ fixation was investigated on Albizia lebbeck, Gliricidia sepium and Leucaena leucocephala grown in the screenhouse for 16 months, using acetylene reduction and ¹⁵N dilution methods. Frequent prunings at 4-month intervals had no deleterious effect on symbiotic N₂ fixation, which increased in Gliricidia and Leucaena in particular. Nodulation and nitrogenase activity varied inconsistently within species, and were not influenced by pruning frequency. Cumulative assessment of pruning effect showed higher biomass, N yield and N₂-fixing capacity of the woody species than at last harvest, and appeared to have more practical relevance. Across species, cumulative total dry matter, N yields, and both percentage and absolute amount of N₂ derived from atmosphere increased with pruning frequency, except when trees were pruned 3 times. Of the three species, G. sepium had the lowest biomass production, N₂ fixation and N accumulation. Received: 25 October 1995     

A bradyrhizobial-<Emphasis Type="Italic">Penicillium</Emphasis> spp. biofilm with nitrogenase activity improves N<Subscript>2</Subscript> fixing symbiosis of soybean

A bradyrhizobial-fungal biofilm (i.e. Bradyrhizobium elkanii SEMIA 5019-Penicillium spp.) developed in vitro was assayed for its nitrogenase activity and was evaluated for N₂-fixing symbiosis with soybean under greenhouse conditions. The biofilm showed nitrogenase activity, but the bradyrhizobial strain alone did not. Shoot and root growth, nodulation and N accumulation of soybean increased significantly with an inoculum developed from the biofilm. This study concludes that such biofilmed inoculants can improve N₂-fixing symbiosis in legumes, and can also directly contribute to soil N fertility in the long term. Further studies should be conducted to investigate the performance of these inoculants under field conditions.