Influence of arbuscular mycorrhizae and phosphorus fertilization on growth, nodulation and N2 fixation (15N) inMedicago sativa at four salinity levels

The rose of an isolate of the arbuscular mycorrhizal (AM) fungusGlomus mosseae in the protection ofMedicago sativa (+Rhizobium meliloti) against salt stress induced by the addition of increasing levels of soluble salts was studied. The interactions between soluble P in soil (four levels), mycorrhizal inoculum and degree of salinity in relation to plant growth, nutrition and infective parameters were evaluated. Salt stress was induced by sequential irrigation with saline water having four concentrations of three salts (NaCl, CaCl₂, and MgCl₂).¹⁵N-labelled ammonium sulphate was added to provide a quantitative estimate of N₂ fixation under moderate to high salinity levels. N and P concentration and nodule formation increased with the amount of plant-available P or mycorrhizal inoculum in the soil and generally declined as the salinity in the solution culture increased from a moderate to a high level. The mycorrhizal inoculation protected the plants from salt stress more efficiently than any amount of plant-available P in soil, particularly at the highest salinity level applied (43.5 dS m^–1). Mycorrhizal inoculation matched the effect on dry matter and nutrition of the addition in the soil of 150 mg P kg^–1. Nevertheless the highest saline solution assayed (43.5 dS m^–1) affected more severely plants supplemented with phosphorus than those with the addition of mycorrhizal inoculum. Such a saline-depressing effect was 1.5 (biomass), 1.4 (N) and 1.5 (P) times higher in plants supplied with soluble phosphate than with AM inoculum. Mechanisms beyond those mediated by P must be involved in the AM-protectioe effect against salinity. The¹⁵N methodology used allowed the determination of N₂ fixation as influenced by different P applications compared to mycorrhizal inoculation. A lack of correlation between nodule formation and function (N₂ fixation) was evidenced in mycorrhizal-inoculated plants. In spite of the reduced activity per nodule in mycorrhizal-inoculated In spite of the reduced activity per nodule in mycorrhizal-inoculated plants, the N contents determined indicated the highest acquisition of N occurred in plants with the symbiotic status. Moreover, N and P uptake increased while Ca and Mg decreased in AM-inoculated plants. Thus P/Ca ratios and cation/anion balance in general were altered in mycorrhizal treatments. This study therefore confirms previous findings that AM-colonized plants have optional and alternative mechanisms available to satisfy their nutritive requirements and to maintain their physiological status in stress situations and in disturbed ecosystems.     

Influence of mycorrhiza vs. soluble phosphate on growth,nodulation, and N2 fixation (15N) in alfalfa under different levels of water potential

Summary The legume Medicago sativa (+Rhizobium melilott) was grown under controlled conditions to study the interactions between soluble P in soil (four levels), or a mycorrhizal inoculum, and the degree of water potential (four levels) in relation to plant development and N₂ fixation. ¹⁵N-labelled ammonium sulphate was added to each pot for a qualitative estimate of N₂ fixation, in order to rank the effects of the different treatments.Dry-matter yield, nutrient content and nodulation increased with the amount of plant-available P in the soil, and decreased as the water stress increased, for each P-level. The mycorrhizal effect on dry matter, N yield, and on nodulation was little affected by the water potential. Since P uptake was affected by the water content in mycorrhizal plants, additional mechanisms, other than those mediated by P, must be involved in the mycorrhizal activity.There was a positive correlation between N yield and nodulation for the different P levels and the mycorrhizal treatment at all water levels. A high correlation between plant unlabelled N content and atom% ¹⁵N excess was also found for all levels of P. In mycorrhizal plants, however, the correlation between unlabelled N yield and ¹⁵N was lower. This suggests that mycorrhiza supply plants with other N sources in addition to those derived from the improvement on N₂ fixation.     

Inoculation withGlomus mosseae improves N2 fixation by field-grown soybeans

Summary A field study carried out in a sandy, relatively acid Senegalese soil with a low soluble P content (7 ppm) and low vesicular-arbuscular mycorrhizal (VAM) populations showed that soybean responded toGlomus mosseae inoculation when the soluble P level in the soil had been raised by the addition of 22 kg P ha^–1. In P-fertilized plots, N₂ fixation of soybean, assessed by the A value method, was 109 kg N₂ fixed hat when plants were inoculated withRhizobium alone and it reached 139 kg N₂ fixed ha^–1 when plants were dually inoculated withRhizobium andGlomus mosseae using an alginate bead inoculum. In addition to this N₂ fixation increase (+28%),Glomus mosseae inoculation significantly improved grain yield (+13%) and total N content of grains (+16%). This success was attributed mainly to the low infection potential of the native VAM populations in the experimental site. In treatments without solubleP or with rock phosphate, no effect of VAM inoculation was observed.     

Influence of inoculation with arbuscular mycorrhizal fungi on stable isotopes of nitrogen in Phaseolus vulgaris

The interactions between Phaseolus vulgaris, Rhizobium spp. strains nodulating P. vulgaris, and arbuscular mycorrhizal (AM) fungi were assessed under greenhouse conditions in a nonsterilized Typic Haplustalf soil from Cauca, Colombia. Our results indicate a specific involvement of AM fungal species in nitrogen acquisition by the legume plants from symbiotic nitrogen fixation and from soil. A significant specific influence of inoculation with Glomus spp. on the ¹⁵N/¹⁴N ratio in plant shoots was dependent on the inoculated rhizobial strain, but AM fungal inoculation had no significant effect on shoot dry weight or nodule occupancy in the two different rhizobial strain treatments. The results imply that in low P soils the effects of an improved mycorrhizal symbiosis may include improved symbiotic N₂ fixation efficiency and/or improved soil N uptake. Received: 11 May 1996     

Uptake and metabolism of nitrate in mycorrhizal plants as affected by water availability and N concentration in soil

We compare the effect of arbuscular mycorrhizal (AM) colonization and PO₄^?3 fertilization on nitrate assimilation, plant growth and proline content in lettuce plants growing under well‐watered (?0.04 MPa) or drought (?0.17 MPa) conditions. We also tested how AM‐colonization and PO₄^?3 fertilization influenced N uptake (¹⁵N) and the percentage of N derived from the fertilizer (% NdfF) by plants under a concentration gradient of N in soil. Growth of mycorrhizal plants was comparable with that of P‐fertilized plants only under well‐watered conditions. Shoot nitrogen content, proline and nitrate reductase activity were greater in AM than in P‐fertilized plants under drought. The addition of 100 μg g^?1 P to the soil did not replace the AM effect under drought. Under well‐watered conditions, AM plants showed similar (at 3 mmol N), greater (at 6 mmol N) or lesser (at 9 mmol N) %NdfF than P‐fertilized plants. Comparing a control (without AM inoculation) to AM plants, differences in % NdfF ranged from 138% (3 mmol N) to 22.6% (6 mmol N) whereas no differences were found at 9 mmol N. In comparison with P fertilization, mycorrhizal effects on %NdfF were only evident at the lowest N levels, which indicated a regulatory mechanism for N uptake in AM plants affected by N availability in the soil. At the highest N level, P‐fertilized plants showed the greatest %NdfF. In conclusion, AM symbiosis is important for N acquisition and N fertilizer utilization but this beneficial mycorrhizal effect on N nutrition is reduced under large quantities of N fertilizer.     

Variable inhibition of iron uptake by oat phytosiderophore in five soybean cultivars

Abstract

Greenhouse experiment was conducted to evaluate the effect of arbuscular mycorrhizal fungi (AMF) on plant growth, and nutrient uptake in saline soils with different salt and phosphorus (P) levels. The following treatments were included in this experiment: (i) Soil A, with salt level of 16.6 dS m^?1 and P level of 8.4 mg kg^?1; (ii) Soil B, with salt level of 6.2 dS m^?1 and P level of 17.5 mg kg^?1; and (iii) Soil C, with salt level of 2.4 dS m^?1 and P level of 6.5 mg kg^?1. Soils received no (control) or 25 mg P kg^?1 soil as triple super phosphate and were either not inoculated (control) or inoculated with a mixture of AM (AM1) and/or with Glomus intraradices (AM2). All pots were amended with 125 mg N kg^?1 soil as ammonium sulfate. Barley (Hordeum vulgar L., cv. “ACSAD 6”) was grown for five weeks. Plants grown on highly saline soils were severely affected where the dry weight was significantly lower than plants growing on moderately and low saline soils. The tiller number and the plant height were also lower under highly saline condition. The reduced plant growth under highly saline soils is mainly attributed to the negative effect of the high osmotic potential of the soil solution of the highly saline soils which tend to reduce the nutrient and water uptake as well as reduce the plant root growth. Both the application of P fertilizers and the soil inoculation with either inoculum mixture or G. intraradices increased the dry weight and the height of the plants but not the tiller number. The positive effect of P application on plant growth was similar to the effect of AM inoculation. Phosphorus concentration in the plants was higher in the mycorrhizal plant compared to the non mycorrhizal ones when P was not added. On the other hand, the addition of P increased the P concentration in the plants of the non mycorrhizal plants to as high as that of the mycorrhizal plants. Iron (Fe) and zinc (Zn) uptake increased with AM inoculation. The addition of P had a positive effect on micronutrient uptake in soil with low level of soil P, but had a negative effect in soil with high level of soil P. Micronutrient uptake decreases with increasing soil salinity level. Inoculation with AMF decreases sodium (Na) concentration in plants grown in soil of the highest salinity level but had no effect when plants were grown in soil with moderate or low salinity level. The potassium (K) concentration was not affected by any treatment while the K/Na ratio was increased by AM inoculation only when plant were grown in soil of the highest salinity level.     

Influence of soil inoculation with vesicular-arbuscular mycorrhiza and a phosphate-dissolving bacterium on plant growth and 32P-uptake

The influence of inoculation of soil with a vesicular-arbuscular mycorrhizal fungus (Glomus fasciculatus) and a phosphate-dissolving bacterium (Bacillus circulans) on phosphate solubilization, growth of finger millet (Eleusine coracana) and phosphorus uptake from ³²P-labelled tricalcium phosphate and superphosphate were studied. The mycorrhizal plants produced more dry matter and removed more ³²P from the soil than non-mycorrhizal plants, but did not show increased ³²P activity per unit plant mass. The 30 mm NH₄F-HCl extractable ³²P (available ³²P) in soil, plant ³²P activity and total P uptake were enhanced by soil inoculation with the bacterium. In the treatment receiving both inocula a synergistic effect was recorded with increased P uptake and dry matter production.     

Response of a wild-type and modern cowpea cultivars to arbuscular mycorrhizal inoculation in sterilized and non-sterilized soil

Cowpea is an important crop that serves as a legume and vegetable source to many smallholder farmers in sub-Saharan Africa. Soil fertility is a significant limitation to its production thus; inoculation with beneficial soil biota such as arbuscular mycorrhizal fungi (AMF) could improve its performance. However, plant–AMF interaction could vary based on crop cultivar hence affecting overall crop production. The present study aimed at determining the effect of AMF inoculation and soil sterilization on root colonization and growth of a wild-type and three modern cowpea cultivars grown by smallholder farmers in Kenya. Potted cowpea plants were inoculated with a commercial AMF inoculum comprising of Rhizophagus irregularis, Funneliformis mosseae, Glomus aggregatum and Glomus etunicatum and maintained in a greenhouse for 40 days. After harvesting, mycorrhizal colonization, nodule number and dry weight, root and shoot dry weights, nitrogen (N,) phosphorus (P) and potassium (K) content were determined. Interestingly, the modern cultivars showed significantly (p < 0.001) higher root colonization, nodulation, shoot P and N compared to the wild-type cultivar. Moreover, a strong positive correlation between AMF root colonization and shoot P (r² = 0.73, 0.90, p < 0.001), AMF root colonization and shoot N (r² = 0.78; 0.89, p < 0.001) was observed in both sterilized and non-sterilized soil, respectively. Soil sterilization affected root colonization and growth parameters with plants grown in non-sterilized soil performing better than those grown in sterilized soil. This study provides major evidence that modern cowpea cultivars are still responsive to mycorrhizal inoculation suggesting that modern breeding programs are not deleterious AMF symbiosis.     

Mineral Uptake of Mycorrhizal Wheat (Triticum aestivum L.) under Salinity Stress

The hypothesis was that arbuscular mycorrhizal (AM) fungi are able to alleviate salt stress on plant growth by enhancing and adjusting mineral uptake. The objectives were to determine (1) the effects of soil salinity on mineral uptake by different wheat genotypes and (2) the effectiveness of different mycorrhizal treatments on the mineral uptake of different wheat (Triticum aestivum L.) genotypes under salinity. Wheat seeds of Chamran and Line 9 genotypes were inoculated with different species of AM fungi including Glomus mosseae, G. intraradices, and G. etunicatum and their mixture at planting using 100 g inoculum. Pots were treated with the salinity levels of 4, 8, and 12 dS/m before stemming. Different arbuscular mycorrhizal treatments, especially the mixture treatment, increased wheat mineral uptake for both genotypes. Although Line 9 genotype resulted in greater nutrient uptake under salinity stress, Chamran was more effective on adjusting sodium (Na⁺) and chloride (Cl^?) uptake under salt stress.     

Lichen mycobionts of mycorrihizal symbiosis with nitrogen‐fixation of showy partridge pea

Effective mycorrhizal colonization is characteristic for nodulated Cassia genera that are adaptive to subhumid areas throughout the world. Growth, regeneration, and nitrogen (N) fixation occurs within regions of extreme soil and climatic environments that preclude persistent survival of other Leguminosae. Objectives of this study were to determine effective mycobiont components and adjunctive soil fertility factors governing growth, nodulation, and symbiotic N fixation of the important forage species, Showy Partridge Pea [Cassia Chamaecrista fasciculate (L.) Michx.] The perennial foliose lichen, Parmelia incurva, ubiquitous within extreme harsh drought and temperature regions, was utilized for mycorrizal mycobionts. Largest above ground plant growth, nodulation, and nitrogen fixation resulted with mycorrhizal colonization within lichen amended soil that received no other soil fertility treatments. Responses attained with phosphorus (P) and calcium (Ca) plant nutrient soil additions, without mycorrhizal mycobiont additions, were approximately half or less of effective mycorrhizal colonized plants. In general, yield response of mycorrhizal plants was reduced with plant nutrient additions throughout this study. Nitrate reductase (NR) and nitrate‐nitrogen (NO₃‐N) levels were significantly higher within nodule cytosol of nonmycorrhizal plants. Ureidoglycolate enzyme transformers and nodule cytosol ureide components were significantly greater for mycorrhizal colonized plants. These included urease (URC), allantoinase (ALTH), allantoicase (ALTC), and total ureides. However, differences were not significant for cytosol contents of pyruvate, amine‐amide N, aspartate transaminase (AST), glutamate dehydrogenase (GDH), glutamine synthetase (GS), and glutamate oxoglutarate trasaminase (GOGAT). Representative histological microscopy of mycorrhizal colonized Showy Partridge Pea are presented. Effective mycobiont propagules associative with lichen associations are apparently opportune commensal species and only functional as site specific sycophants governed by variable environmental conditions with lichen dissipation.     

Elevated carbon dioxide level along with phosphorus application and cyanobacterial inoculation enhances nitrogen fixation and uptake in cowpea crop

Climate change, as a result of increase in the concentration of greenhouse gases, influences growth and productivity of leguminous crops. A study was carried out to analyse the impacts of elevated carbon dioxide (CO₂) and cyanobacterial inoculation on growth, N₂ fixation and N availability and uptake in cowpea crop, under different doses of phosphorus. Cowpea crop was grown under ambient (400 µmol mol^?1) and elevated (550 ± 20 µmol mol^?1) CO₂ levels using Free-Air Carbon dioxide Enrichment facility. Elevated CO₂ level increased chlorophyll content in leaves, improved nodulation and nitrogen fixation by the crop. Increase in P dose up to 16 mg kg^?1 soil enhanced nodule development and N₂ fixation under high CO₂ condition. Cyanobacterial inoculation increased nodule weight, leghaemoglobin content in nodules and total nitrogenase activity. Although nitrogen concentration in cowpea seeds decreased in high CO₂ treatment, higher N uptake was recorded. Under elevated CO₂ condition, cyanobacterial inoculation and higher P doses led to enhanced root growth and N₂ fixation and availability of soil nitrogen. The study illustrated the synergistic effect of high CO₂ and cyanobacterial inoculation in enhancing crop growth and availability of soil N, mediated by biological N₂ fixation in cowpea under different levels of P.     

Vesicular-arbuscular mycorrhiza and growth in barley: Effects of irradiation and heating of soil

The influence of soil irradiation (0.25–4.0 Mrad) and soil heating on mycorrhizal survival, establishment and development after reinoculation, and on plant growth, was investigated.The lowest radiation dose applied, completely eliminated the infectivity of a soil with a high number of mycorrhizal propagules.Mycorrhiza developed more slowly after inoculation in irradiated soils than in untreated soils. This could have been due to the small amounts of inoculum used, but the high concentrations of nutrients released by irradiation of the soil were probably of greater significance particularly the increased amounts of plant-available N as indicated by incubation experiments. Inorganic N was increased to similar levels by the various treatments. Available soil P increased with increasing irradiation dose.Incubation of inoculum in soil for 40 days before sowing increased mycorrhizal infection.     

Varied rates of mycorrhizal inoculum on growth and nutrient acquisition by barley grown with drought stress

Effectiveness of arbuscular mycorrhizal fungi (AMF) is crucial for maximum plant growth and acquisition of mineral nutrients under drought. The objective of this research was to determine effects of varied rates of AMF inoculum on plant growth and acquisition of phosphorus (P), zinc (Zn), copper (Cu), and manganese (Mn) by barley (Hordeum vulgare L. cv. SLB‐6) grown with and without drought stress (WS and nonWS). Plants inoculated with four inoculum rates [control (M₀), 120 (M₁), 240 (M₂), and360 (M₃) spores per 100 g dry soil] of Glomus mosseae were grown in a low P silty clay (Typic Xerochrept) soil (pH=8.0) mix in a greenhouse for 45 days. Root AMF colonization increased as inoculum rate increased in plants grown with WS and nonWS. Leaf area and shoot and root dry matter (DM) increased as inoculum rate increased up to M₂ regardless of soil moisture. Shoot concentrations of P, Cu, and Mn were generally higher for mycorrhizal (AMF) than for nonmycorrhizal (nonAMF) plants grown with both WS and nonWS. Shoot contents of P, Zn, Cu, and Mn were higher for AMF than for nonAMF plants grown with nonWS, and shoot contents of P were higher for AMF than for nonAMF plants with WS. For plants grown with WS and nonWS, contents of P, Zn, Cu, and Mn were generally higher for plants inoculated with M₂ compared to other rates of inoculum. The results of this study indicated that plant responses to root colonization with AMF were dependent on AMF rate and soil moisture. Based on enhancements in plant DM and mineral acquisition traits, M₂ inoculum was the most effective rate of inoculation for this AMF isolate.     

EFFECTS OF MYCORRHIZAL FUNGI ON TOLERANCE CAPABILITY OF CORN GROWN UNDER SALT STRESS CONDITION

A greenhouse experiment was conducted to determine the effect of salinity on the efficacy of two arbuscular mycorrhizal fungi (AMF), Glomus mossea and natural mycorrhiza, of Glomus species, was investigated in terms of growth and nutrition of corn plant (Zea mays L). Plants were grown under different salinity levels imposed by 2.0, 2.5, 3.5, 5.0, 8.0, 12.0 dS m^?1of Hoagland's Solution [sodium chloride (NaCl), sodium sulfate (Na₂SO₄)_, Calcium dichloride (CaCl₂), and magnesium sulfate (MgSO₄) 7:9:3:1 ratio, respectively]. Both types of mycorrhizal fungi did not display significant protection in the host plant against the detrimental effects of the soil salinity. The effect of inoculation on growth varied only with the level of salinity. Maximum root colonization and spore numbers were observed in plants cultivated with low salinity levels. It was found that significant interaction between AMF x Salinity level for calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn), and manganese (Mn) of shoot, and for Zn and Mn, of root.     

Nitrogen fixation and CO2 exchange in soybeans (Glycine max L.) inoculated with mixed cultures of different microorganisms

N₂ fixation, photosynthesis of whole plants and yield increases in soybeans inoculated with mixed cultures of Bradyrhizobium japonicum 110 and Pseudomonas fluorescens 20 or P. fluorescens 21 as well as Glomus mosseae were found in pot experiments in gray forest soil carried out in a growth chamber. The effects of pseudomonads and vesicular-arbuscular (VA) mycorrhizal fungus on these parameters were found to be the same. Dual inoculation of soybeans with mixed cultures of microorganisms stimulated nodulation, nitrogenase activity of nodules and enhanced the amount of biological nitrogen in plants as determined by the ¹⁵N dilution method in comparison to soybeans inoculated with nodule bacteria alone. An increased leaf area in dually infected soybeans was estimated to be the major factor increasing photosynthesis. P. fluorescens and G. mosseae stimulated plant growth, photosynthesis and nodulation probably due to the production of plant growth-promoting substances. Increasing phosphorus fertilizer rates within the range of 5–40 mg P 100 g^-1 1:1 (v/v) soil: sand in a greenhouse experiment led to a subsequent improvement in nodulation, and an enhancement of N₂ fixation and yield in soybeans dually inoculated with B. japonicum 110 and P. fluorescens 21. These indexes were considerably higher in P-treated plants inoculated with mixed bacterial culture than in plants inoculated with nodule bacteria alone.     

Comparative salinity tolerance of symbiotically dependent and nitrogen-fed pigeonpea (Cajanus cajan) and its wild relative Atylosia platycarpa

Summary Once symbiosis between the pigeonpea cultivar ICPL 227 and the Rhizobium sp. strain IC 3024 is established, it is efficient in fixing N₂ under saline conditions and can support growth comparable to N-fed plants in growth media with up to 6 dS m^-1 salinity. However, the early stages of establishment of the pigeonpea-IC 3024 symbiotic system have proved sensitive to salinity. The present study showed that the number of nodules was markedly reduced at 8 dS m^-1 salinity; however, nodule development and functioning were not affected by salinity in the pigeonpea-IC 3024 symbiosis. The symbiotic system of Atylosia platycarpa and Rhizobium sp. strain IC 3087 was established successfully even at 12 dS m^-1 and supported growth comparable to that of N-fed plants. P levels in leaves were increased under saline conditions in N-fed and N₂-fixing pigeonpea and A. platycarpa. There were no consistent differences in the leaf Na and chloride levels between N-fed and N₂-fixing plants of pigeonpea and A. platycarpa. The present study suggests that the rhizobial symbiosis may not be a necessary factor for initial screening of pigeonpea and related wild species for salinity tolerance.Submitted as JA No. 964 by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT)     

Effect of Magnesium Chloride–Induced Salinity on Carbon Dioxide Evolution and Nitrogen Mineralization in a Silty Clay Loam Soil

A laboratory incubation experiment was conducted to evaluate the effect of magnesium chloride–induced salinity on carbon dioxide (CO₂) evolution and nitrogen (N) mineralization in a silty loam nonsaline alkaline soil. Magnesium chloride (MgCl₂) salinity was induced at 0, 4, 8, 12, 16, 20, 30, and 40.0 dS m^?1 and measured CO₂ evolution and N mineralization during 30 days of incubation. Both CO₂ evolution and N mineralization decreased significantly with increasing salinity. The cumulative CO₂ evolution decreased from 235 mg kg^?1 soil at electrical conductivity (EC) 0.65 dS m^?1 to 11.9 mg kg^?1 soil at 40 dS m^?1 during 30 days of incubation. Similarly, N mineralization decreased from 185.4 mg kg^?1 at EC 0.65 dS m^?1 to 34.45 mg kg^?1 at EC 40.0 dS m^?1 during the same period. These results suggested that increasing magnesium chloride salinity from 4 dS m^?1 adversely affect microbial activity in terms of carbon dioxide evolution and N mineralization.     

Impact of 12-year field treatments with organic and inorganic fertilizers on crop productivity and mycorrhizal community structure

The effect of manure and mineral fertilization on the arbuscular mycorrhizal (AM) fungal community structure of sunflower (Helianthus annuus L.) plants was studied. Soils were collected from a field experiment treated for 12 years with equivalent nitrogen (N) doses of inorganic N, dairy manure slurry, or without N fertilization. Fresh roots of tall fescue (Festuca arundinacea Schreb.) grass collected from the field plots without N fertilization and unfumigated field soils were used as native microbial inoculum sources. Sunflower plants were sown in pots containing these soils, and three different means of manipulating the microbial community were set: unfumigated soil with fresh grass roots, fumigated soil with fresh grass roots, or fumigated soil with sterilized grass roots. Assessing the implications with respect to plant productivity and mycorrhizal community structure was investigated. Twelve AM fungal OTUs were identified from root or soil samples as different taxa of Acaulospora, Claroideoglomus, Funneliformis, Rhizophagus, and uncultured Glomus, using PCR-DGGE and sequencing of an 18S rRNA gene fragment. Sunflower plants grown in manure-fertilized soils had a distinct AMF community structure from plants either fertilized with mineral N or unfertilized, with an abundance of Rhizophagus intraradices-like (B2). The results also showed that AM inoculation increased P and N contents in inorganic N-fertilized or unfertilized plants, but not in manure-fertilized plants.     

Lettuce response to organic and phosphate fertilizers and root mycorrhization

The response of lettuce to production system, organic and phosphate fertilizers and root mycorrhization, was evaluated in two pot trials with factorial treatment combination of: (i) soil type (from organic and from conventional production systems) and organic fertilizer (0, 2 and 4 t ha^?1) in the first trial; and (ii) mycorrhizal inoculation (mycorrhized and non-mycorrhized plants) and Gafsa phosphate (0, 100 and 200 kg P₂O₅ ha^?1) in the second. Lettuce growth decreased with increasing rates of the organic fertilizer because of its very high electrical conductivity (50.1 dS m^?1) and lack of maturation. However, the fertilizer harmful effects were minimized in the soil from organic production. The application of Gafsa phosphate significantly increased lettuce yield and nutrient uptake. However, for the highest rate of phosphate, mycorrhized lettuce yield decreased compared to non-mycorrhized lettuce, suggesting that high soil available P may have harmful effects on the activity of mycorrhizal fungi.     

Influence of soil on the interactions between endomycorrhizae and Azospirillum in sorghum

Sorghum (S. bicolor L. Moench cv. Bok 8) plants were grown in soil or sand-perlite low in plant-available N and P. Plants were inoculated with a vesicular-arbuscular mycorrhizal (VAM) fungus, or a strain of Azospirillum brasilense or both endophytes together. Plants received a nutrient solution which did not contain N or P. Increases in plant dry weight, shoot-to-root ratios, and the N content of dually-infected plants could be accounted for by summing the VAM and Azospirillum effects. For sorghum inoculated with both endophytes, the presence of A. brasilense in the rhizosphere increased VAM colonization and biomass, while the N input due to Azospirillum decreased, possibly due to competition for carbohydrates.Comparisons between sorghum grown with or without VAM-fungal infection in four growth media showed that edaphic factors other than P availability determined the host response to VAM infection. The P-fixing capacity of the soil, rather than the amount of available (NaHCO₃-extractable) P, influenced the balance between mutualistic and parasitic VAM-fungal growth.