The Open Plant Science Journal




ISSN: 1874-2947 ― Volume 11, 2018
RESEARCH ARTICLE

The Presence of Beneficial Organisms Associated to N and P Economy in the Rhizosphere of Native Vegetation in an Oligotrophic Savanna of Guárico State, Venezuela



Edith Mora, Marcia Toro*, Danilo López-Hernández
Universidad Central de Venezuela. Instituto de Zoología y Ecología Tropical. Laboratorio de Estudios Ambientales, Apdo. 47058, Caracas 1041A, Venezuela

Abstract

Background:

In natural conditions, tropical plants are adapted to different ecological niches that can be associated to soil microorganisms which play a key role in nutrient cycling like Arbuscular mycorrhiza (AM), phosphate solubilizing bacteria (PSB) and/or nitrogen fixing rhizobia.

Methods:

We report a survey of the presence in a Trachypogon savanna located at Estación Experimental La Iguana (EELI) in Central Venezuela, of some beneficial plant-microorganism associations. In this savanna, plants present a high AM symbiosis affinity. The high mycorrhization and the presence of potential PSB suggest a synergic effect in plant P-uptake.

Results:

After screening the rhizospheres of 25 plant species from the zone, we could isolate a high proportion of potential PSB in relation to the total bacteria number from the rhizospheres of Centrosema venosum and Galactia jussiaeana.

Conclusion:

Therefore, the presence of potential PSB in the rhizosphere of those species constitutes an important finding to discover novel biofertilizers for crop plants.

Keywords: Arbuscular mycorrhizae, PGPR, N-fixation, Phosphorus, Nitrogen, Sustainability.


Article Information


Identifiers and Pagination:

Year: 2017
Volume: 10
First Page: 123
Last Page: 133
Publisher Id: TOPSJ-10-123
DOI: 10.2174/1874294701710010123

Article History:

Received Date: 14/07/2017
Revision Received Date: 15/09/2017
Acceptance Date: 30/10/2017
Electronic publication date: 17/11/2017
Collection year: 2017

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© 2017 Mora et al.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: https://creativecommons.org/licenses/by/4.0/legalcode. This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


* Address correspondence to this author at the Universidad Central de Venezuela. Instituto de Zoología y Ecología Tropical. Laboratorio de Estudios Ambientales, Apdo. 47058, Caracas 1041A, Venezuela, Tel: 50-212-6051394; E-mail: marcia.toro@ciens.ucv.ve




1. INTRODUCTION

In Venezuela, savanna ecosystems occupy 260.000 km2 (about 29% of the territory), located on dystrophic and well drained soils dominated by grasses such as Trachypogon plumosus Ness, locally known as Trachypogon savannas [1Ramia M. Tipos de sabanas en los llanos de Venezuela. Boletín de la Sociedad Venezolana de Ciencias Naturales 1967; 27: 264-88.]. The dominant grass, Trachypogon, is characterized by its low productivity, digestibility and palatability values, so the genus seems to be well adapted to acid and nutrient-depleted soils, particularly in nitrogen and phosphorus [2López-Gutiérrez JC, Toro M, López-Hernández D. Seasonality of organic phosphorus mineralisation in the rhizosphere of the native savanna grass, Trachypogon plumosus. Soil Biol Biochem 2004; 36: 1675-84.-5López-Hernández D, Hernández-Hernández RM, Hernández-Valencia I, Toro M. Nutritional stress in dystrophic savanna soils of the orinoco basin: Biological responses to low N and P availabilities. In: Parvaiz A Rasool S, Ed. Emerging technologies and management of crop stress tolerance Biological Techniques 2014; 343-68.].

Savanna soils are frequently burnt as a common agriculture practice, and this kind of management have contributed to accelerate carbon and nitrogen losses in the soil [6Abbadie L. Nitrogen inputs to and outputs from the soil-plant system. Lamto structure, functioning, and dynamics of a savanna ecosystem. In: Abbadie L, Gignoux J, Le Roux J, Lepage M, Eds. Ecological Studies 179 2006; 255-75., 7López-Hernández D. N biogeochemistry and cycling in two well-drained savannas: A comparison between the Orinoco Basin (Llanos-Venezuela) and Ivory Coast (Western-Africa). Chem Ecol 2013; 29: 280-95.
[http://dx.doi.org/10.1080/02757540.2012.744830]
]. Moreover, under increasing acidity, that characterizes the well-developed savanna soils, soil exchangeable aluminum (Al+3) tends to increase to toxic levels with a concomitant deficiency in available forms of phosphorus [8López-Hernández D. La Química del Fósforo en Suelos Ácidos Casa Editora: Ediciones de la Biblioteca 1977., 9Pinto FA, De Souza ED, Paulino HB, Curi N, Carbone CM. P-sorption and desorption in savanna Brazilian soil as a support for phosphorus fertilizer management. Cienc Agrotec 2013; 37: 521-30.]. Therefore, when savanna ecosystems are transformed in intensive agricultural lands, large amount of soluble and expensive P-fertilizers must be quenched by the high P-sorption capacities of savanna soils.

In natural conditions, most of the tropical plants are adapted to different ecological niches associated to soil microorganisms such as Arbuscular mycorrhiza-AM [10Janos DV. Mycorrhizas in humid tropical ecosystems. In: Safir G, Ed. Ecophysiology of VA mycorrhizal plants 1985; 107-33.,11Gemma JN, Koske RE, Habte M. Mycorrhizal dependency of some endemic and endangered Hawaiian plant species. Am J Bot 2002; 89: 337-45.], phosphate solubilizing bacteria (PSB) [12Toro M, Bazó I, López M. Micorrizas Arbusculares y bacterias promotoras de crecimiento vegetal, biofertilizantes nativos de sistemas agrícolas bajo manejo conservacionista. Agronomía Tropical 2008; 58: 78-83.] and symbiotic nitrogen fixing bacteria, as rhizobia [13Bala A, Giller KE. Relationships between rhizobial diversity and host legume nodulation and nitrogen fixation in tropical ecosystems. In: Bationo A, Ed. Advances in Integrated Soil Fertility Management in Sub-Saharan Africa: Challenges and Opportunities 2007; 691-702.], which can play a key role in nutrient cycling and in the protection of the plant to environmental stress.

The diversity and abundance of microorganisms, plant and pedofauna influence the diverse functions of the ecosystems such as soil nutrient cycling (nitrogen, phosphorus and carbon). The interaction between the specific host and the composition of the microbial communities might be affected by the root exudates [14López-Hernández D, Flores D. La desorción de fosfatos en suelos. Implicaciones fisioecológicas en el proceso. Acta Cient Venez 1979; 30: 23-35.], so the rhizosphere can be considered as an important domain to attract beneficial microorganisms nevertheless the exact role of the organisms are not yet fully understood [15Mora E, Toro M, López-Hernández D. A survey of arbuscular mycorrhizae, Rhizobium and phosphate solubilizing bacteria in low fertility savanna soils in Central Venezuela (Estación Experimental La Iguana). In: Miransari M, Ed. Soil Microbiology 2013; 97-114.].

Some soil microorganisms have a great potential to contribute to amend soil fertility problems and consequently they might be considered as promising biofertilizers [16Saharan BS, Nehra V. Plant growth promoting rhizobacteria: A critical review. Life Sciences and Medicine Research 2011; 1-30. 2011: LSMR-21]. Thus, the potential use of biofertilizers in tropical savannas with low nutritional levels are starting to be currently assayed in Venezuelan savannas as a convenient technique to improve plant nutrition and reduce the application of high commercial fertilizer doses [17Toro M, López-Hernández D. Potencialidades del manejo de las Micorrizas Arbusculares para el desarrollo sostenido de los sistemas agrícolas de bajos insumos del ecotono sabana-bosque amazónico. Memorias del IV Congreso Interamericano sobre el medio ambiente 1998; In: Colección Simposia.; Caracas, Venezuela. 1998; pp. 222-7.-20Laditi MA, Nwoke OC, Jemo M, Abaidoo RC, Ogunjobi AA. Evaluation of microbial inoculants as biofertilizers for the improvement of growth and yield of soybean and maize crops in savanna soils. AJAR 2012; 7: 405-13.].

In this contribution, we report a survey of the presence of some beneficial plant-microorganism associations in a typical Trachypogon savanna located in Central Venezuela, with particular emphasis on the populations of Arbuscular mycorrhiza, symbiotic N-fixing bacteria and free-living microorganisms that stimulate plant growth through phosphate solubilization. This preliminary report on beneficial microorganisms will be the basis to follow up more detailed research on specific treatments of savanna´s soils with potential native biofertilizers.

2. MATERIALS AND METHODS

2.1. Study Site

The study was carried out at Estación Experimental La Iguana (EELI), located in Guárico State in Northeastern Venezuela (8°25’N and 65°24’W). EELI is under the influence area of the Orinoco River watershed and corresponds to representative savannas of the Venezuelan Central Plains. These savannas are dominated by T. plumosus (Poaceae) with the presence of isolated trees and shrubs such as Curatella americana (Dellineaceae), Copernicia tectorum (Palmae), Byrsonima crassifolia (Malpigheaceae) and Bowdichia virgilioides (Papiloneaceae). The climate is markedly tropical isothermic, with a mean annual precipitation of 1342 mm, most of which falls during the rainy season (May to August), and a mean annual temperature of 27.9°C. EELI has different edaphic substrates in age and genesis [21Ponce M, González V, Brandín J, Ponce ME. Análisis de la vegetación asociada a una toposecuencia en los Llanos Centro-Orientales de Venezuela. Ecotrópicos 1994; 7: 11-22.].

2.2. Soil Characterization

In order to characterize the soil of the experimental site, in an area of 3 ha, samples from the surface (0-14 cm) and subsoil (14-28 cm) were collected in the month of July (middle of the rainy season). Within each depth, one composite mixed sample was taken from at least 6 cores (15 cm depth of sampling, and 9.5 of internal diameter) collected at random and then sieved through 2-mm mesh. After drying, duplicate samples were analyzed for soil pH (measured in a ratio 1:1 soil: water), organic carbon content [22Heanes DL. Determination of total organic-C in soils by an improved chromic acid digestion and spectrophotometric procedure. Commun Soil Sci Plant Anal 1984; 15: 1191-213.], total nitrogen (micro Kjeldahl method) according to Anderson and Ingram [23Anderson JM, Ingram I. Tropical soil biology and fertility: A handbook of methods 1992.]. Available phosphorus was extracted according to Olsen [24Olsen SR, Cole CV, Watanabe FS, Dean LA. A. Estimation of Available Phosphorus in Soils by Extraction with Sodium Bicarbonate U S Department of Agriculture Circular No. 939. 1954.] and phosphorus in the extracts determined using the Murphy and Riley method [25Murphy J, Riley JP. A modified single solution method for the determination of phosphate in natural waters. Anal Chim Acta 1962; 26: 31-6.]; exchangeable bases were determined by atomic absorption.

2.3. Experimental Design

At the experimental site at EELI, an area of 260 m x77 m was located; within it three 65 m equidistant transects were delimitated (Fig. 1). In the transects, at both sides, seven quadrats of 1 m2 were set up, 10 m apart from each other. At the middle of the rainy season (July), sampling was undertaken in the quadrats by collecting all the different plant species and the accompanying soil around the root (rhizospheric soil) inside the 21 chosen plots.

2.4. Sampling of Rhizospheric Soils and Root Systems

Rhizospheric soils and roots from each plant species located in the 21 plots were collected from the root zone (0-20 cm) by previously removing plant debris from the surface of the soil. Triplicate samples (1 g) of this rhizospheric soil located at 1 cm from the roots were used to isolate phosphate-solubilizing bacteria according to Varma [26Varma A. Mycorrhiza Manual 1998; 542.]. Therefore, microorganisms were characterized by sampling the rhizospheric soil of each one of the plant species collected following Barillot et al. [27Barillot C, Sarde C, Bert V, Tarnaud E, Cochet N. A standardized method for the sampling of rhizosphere and rhizoplan soil bacteria associated to a herbaceous root system. Ann Microbiol 2012.
[http://dx.doi.org/10.1007/s13213-012-0491-y]
]. Plant species were collected and preserved for later identification following Blackwelder [28Blackwelder RE. Taxonomy, A Text and Reference Book 1967.].

Fig. (1)
Scheme of the experimental plot with transects and quadrats sampled.


2.5. AM Staining and Quantification

The whole radical system was extracted to avoid mechanical damage or losses of fine roots. Roots were preserved in a mixture containing 10 mL of formaldehyde, 5 mL of acetic acid, 50 mL of alcohol (95-96%) and 35 mL of distilled water. In order to be stained, roots were firstly, clarified in 10% KOH, washed with tap water, submerged in HCl 1N for 15-20 min, and then finally heated to dye with trypan blue 0.05% for 20 min [29Phillips JM, Hayman DS. Improved procedures for clearing roots and staining parasitic and vesicular-arbuscular mycorrhizal fungi for rapid assessment of infection. Trans Br Mycol Soc 1970; 55: 158-61.]. AM colonization was quantified using the grid intersect method and expressed as percentage of colonized root length [30Giovanetti M, Mosse B. An evaluation of techniques for measuring vesicular-arbuscular infection in roots. New Phytol 1980; 84: 489-500.]. In the case of the roots of legumes, rhizobial nodules were carefully removed, counted and kept in silicagel vials [31Herrmann L, Lesueur D, Giampieri F. Collecting symbiotic bacteria and fungi. In: Guarino L, Ramanatha Rao V, Goldberg E, Eds. Collecting plant genetic diversity: Technical guidelines-2011 Update 2011; 1-17.].

2.6. Identification and Frequency of Plant Species

All the plant samples collected in the experimental area were identified by using the corresponding taxonomy keys and compared with the collections already deposited at the Botanical Garden Herbarium, Caracas, Venezuela. The frequency of the different species and their families were established by the following formula:

Frequency (f):

ji the number of quadrats where the species appear

k total number of quadrats

2.7. Determinations of AM Infective Potential

AM propagules in the native soil were quantified using the most probable number (MPN) method [32Porter WM. The most probable number method for enumerating infective propagules of vesicular Arbuscular mycorrhizal fungi in soil. Aust J Soil Res 1979; 17: 515-9., 33Sieverding E. Vesicular-Arbuscular mycorrhiza management in tropical agrosystems 1991.]. A composite mixed sample of, at least, 5 subsamples were collected at random from the experimental area to obtain five kg of EELL soil (0-20 cm); it was sieved (< 1 cm) and steam sterilized for 1 h on 3 consecutive days. Ten-fold serial dilutions (1x up to a 10-9 dilution) of sterile/non-sterile soil were placed, by quintuplicates, in 250 g pots. A surface sterilized seed of Sorghum vulgare (Poaceae) was planted at a 2 cm depth in each pot and allowed to grow for 40 days. Sorghum roots were separated, washed and preserved as previously described for rhizospheric sampling to apply the trypan blue staining method [29Phillips JM, Hayman DS. Improved procedures for clearing roots and staining parasitic and vesicular-arbuscular mycorrhizal fungi for rapid assessment of infection. Trans Br Mycol Soc 1970; 55: 158-61.] to observe and register the presence or absence of AM structures [32Porter WM. The most probable number method for enumerating infective propagules of vesicular Arbuscular mycorrhizal fungi in soil. Aust J Soil Res 1979; 17: 515-9., 33Sieverding E. Vesicular-Arbuscular mycorrhiza management in tropical agrosystems 1991.]. Data are expressed as the number of infective AM propagules in 100 g of dry soil; confidence limits were assigned according to Fisher and Yates [34Fisher RA, Yates F. Statistical tables for biological, agricultural and medical research 1970.].

2.8. AM Root Colonization and Glomeromycota Spores Number

The roots of each one of the plant species collected at the experimental site were dyed [29Phillips JM, Hayman DS. Improved procedures for clearing roots and staining parasitic and vesicular-arbuscular mycorrhizal fungi for rapid assessment of infection. Trans Br Mycol Soc 1970; 55: 158-61.] and the root colonization was quantified using the grid intersect method [30Giovanetti M, Mosse B. An evaluation of techniques for measuring vesicular-arbuscular infection in roots. New Phytol 1980; 84: 489-500.]. In the case of the plants detected with a higher frequency (>0.50) and with a higher percent of infected root length (>50%) Glomeromycota fungal spores were counted. For technical reasons the spore number associated with each plant species was assessed by wet sieving and decanting [35Schenck NC. Methods and principles of mycorrhizal research 1982.] in only one rhizospheric soil sample and expressed as the number of viable AM spores per g of dry soil.

2.9. Isolation of Phosphate Solubilizing Bacteria (PSB)

The presence of calcium phosphate (CaHPO4) solubilizing bacteria in the rhizospheric soils was determined in Petri dishes by using the method of serial dilutions from 1 g of rhizospheric soil on two selective media; YED (0.5% yeast extract, 1% glucose, 0.2% calcium phosphate and 2% agar), according to Thomas et al. [36Thomas GV, Shantaram MV. Solubilization of inorganic phosphates for bacteria from coconut plantation soils. J Plant Crops 1986; 14: 42-8.], and PS (sucrose 0.5%, 0.05% magnesium sulphate, 0.05% potassium chloride, 0.1% potassium nitrate, calcium phosphate 0.3% and 1.5% agar), according to Wenzel et al. [37Wenzel CL, Ashford AE, Summerell BA. Phosphate solubilizing bacteria associated with proteoid roots of seedlings of waratah [Telopea speciosissima (Sm.) R.Br.]. New Phytologist 1994; 128: 487-496.], in both media 30 mg/L of cycloheximide was added in order to minimize the growth of other contaminant microbial groups, such as yeasts and fungi.

The plates were incubated between 4 and 15 days up to the emergence of a clear halo around the colony in the case of YED, whereas in the PS medium, a change of color from blue to yellow indicates the acidification and solubilization of phosphates. The total number of colony forming units was quantified per gram of rhizospheric soil (cfu/g rhizospheric soil), and the proportion of phosphate solubilizers, with respect to the total number of bacterial colonies present was calculated.

3. RESULTS

3.1. General Soil Characteristics and Plant Community of the Experimental Area

3.1.1. Soil Characteristics

The main physical and chemical characteristics of the soil in the experimental area are presented in Table 1. The soil is a Typic Plinthustuls, sand loamy, kaolinitic, isohyperthemic, with low natural fertility and organic matter, and acidic pH (4.15-4.60).

Table 1
Main physical and chemical characteristics of the soil in the experimental area.


3.1.2. Identification and Frequency of Plant Species

Dominant species in the experimental area (e.g. with a presence above 75%) are: Trachypogon sp., Fimbristylis sp., Hyptis sp., Rynchospora barbata, Mimosa pudica, Cassia cultrifolia. In the sampling, 25 species distributed in 10 families were found (Table 2) where the Poaceae, Ciperaceae and Leguminosae are the most abundant families.

3.1.3. AM Infective Potential in Savanna Soils

The number of AM infective propagules in the serial dilutions assay was of 4571 in 100 g of soil (intervals 2141-9765 at 95% confidence). This represents the presence of AM propagules like spores, Glomeromycota fungi mycelium and AM colonized rootlets that potentially can colonize new roots in soil.

Table 2
Frequency of native plant species in the experimental area.


3.1.4. Arbuscular Mycorrhizae Colonization (AM)

The levels of arbuscular mycorrhizae root colonization found allowed us to establish two categories among the species: a group with a high percentage of AM colonization (≥ 50%) (Table 3) and another with an intermediate percentage (30-50%) (Table 4). The Fabaceous-leguminous Desmodium sp. shows the highest colonization percentage followed by Hyptis suaveolens of the Lamiaceae family, whereas the leguminous Phaseolus vulgaris also shows a high mycorrhizal colonization percentage (Table 3).

Table 3
Plant species of the experimental area with a % AM root colonization of ≥ 50%.


Within the Poaceae, Trachypogon sp. presents the higher degree of colonization (68%). In the second category (30-50% root colonization), dominate the legumes Cassia cultrifolia and Mimosa pudica (Table 4).

Table 4
Plant species of the experimental area with a % AM root colonization of 30-50%.


3.1.5. Rhizobial Symbiosis

Legumes represented a high proportion of the plant species collected in the experimental area (43.5%); most of them were individuals of Cassia cultrifolia and Mimosa pudica. In total 10 leguminous species were collected, from which, 50% showed the presence of nodules located in the lateral roots. Nodulated species were: Indigosfera pascuorum and Stylosanthes sp. with the highest number of nodules followed by Cassia cultrifolia and Desmodium intortuo and Mimosa pudica. 50% of the legumes collected presented double symbiosis, Arbuscular mycorrhiza and rhizobia (Fig. 2).

3.1.6. Evaluation of Glomeromycota Fungi Spores Number Present in the Rhizosphere of Native Plants

Quantification of Glomeromycota spore number in the rhizospheric soil (soil around the root) was performed only in the plant species, which were detected in a higher frequency (> 0.50) and with % AM root colonization higher than 50%. The number of spores ranged from 100 to 1700 per 100 g soil (Fig. 3).

Fig. (2)
Percentage of AM root colonization and nodulation within leguminous.


3.1.7. Phosphate Solubilizing Bacteria (PSB)

The isolation of PSB was done in a total of 25 rhizospheres corresponding to the most important plant species present in the experimental area. From those rhizospheres, 8 were positive with the presence of potential PSB and correspond to the following species: Centrosema venosum, Galactia jussiaeana, Fimbristylis sp., Mimosa pudica, Ruellia geminiflora, Aeschynomene sp., Trachypogon sp. and Indigosphera pascuorum. When analysing the total amount of colony forming units per Petri dish with respect to the PSB we found that Centrosema venosum and Galactia jussiaeana present a high proportion of PSB (75% and 43%, respectively).

4. DISCUSSION

4.1. N-Fixation and Rhizobium-Legume Symbiosis

African and South American savannas are characterized by their great diversity of herbaceous and woody leguminous species and the proportion of leguminous tends to increase under moderate and over grazing [6Abbadie L. Nitrogen inputs to and outputs from the soil-plant system. Lamto structure, functioning, and dynamics of a savanna ecosystem. In: Abbadie L, Gignoux J, Le Roux J, Lepage M, Eds. Ecological Studies 179 2006; 255-75.]. In Orinoco´s savannas, few studies have done to document nitrogen fixation by native legumes under natural conditions, although evidence through natural abundance of 15N and relative abundance of ureids suggest N-fixation for a few species [38Izaguirre-Mayoral ML, Carballo O, Flores S, Sicardi de Mallorca M, Oropeza T. Quantitative analysis of symbiotic N2-fixation, non-structural carbohydrates and chlorophyll content in sixteen native legume species collected in different savanna sites. Symbiosis 1992; 12: 293-312., 39Medina E, Izaguirre ML. N2-fixation in tropical American savannas evaluated by the natural abundance of 15N in plant tissues and soil organic matter. Trop Ecol 2004; 45: 87-95.].

In the experimental area at ELLI the legumes represented almost half of the plant species collected that account for a good N-fixing plant presence; most of them were individuals of Cassia cultrifolia and Mimosa pudica. In total, ten leguminous were collected, from which, 50% showed the presence of nodules; Aristeguieta [40Aristeguieta L. Flórula de la Estación Biológica de los Llanos. Bol Soc Ven Ciênc Nat 1966; 26(110): 228-307.] reported also that Poaceae and Leguminosae were the more abundant families in a savanna located in Central, Venezuela with a proportion of 27 and 26%, respectively. Thus, nitrogen fixation by different mechanisms existing in savannas appears as an option to supply N to this ecosystem [6Abbadie L. Nitrogen inputs to and outputs from the soil-plant system. Lamto structure, functioning, and dynamics of a savanna ecosystem. In: Abbadie L, Gignoux J, Le Roux J, Lepage M, Eds. Ecological Studies 179 2006; 255-75.,7López-Hernández D. N biogeochemistry and cycling in two well-drained savannas: A comparison between the Orinoco Basin (Llanos-Venezuela) and Ivory Coast (Western-Africa). Chem Ecol 2013; 29: 280-95.
[http://dx.doi.org/10.1080/02757540.2012.744830]
,41Barrios S, González V. Rhizobial symbiosis on venezuelan savannas. Plant Soil 1971; 34: 707-19.,42López-Hernández D, Santaella S, Chacón P. Contribution of free-living organisms to N-budget in Trachypogon savannas. Eur J Soil Biol 2006; 42: 43-50.]. Consequently, under natural conditions the studied savanna presents a microbial community which might be metabolically adapted to different mechanisms able to profit from the scarce sources of N and P [2López-Gutiérrez JC, Toro M, López-Hernández D. Seasonality of organic phosphorus mineralisation in the rhizosphere of the native savanna grass, Trachypogon plumosus. Soil Biol Biochem 2004; 36: 1675-84.,3López-Gutiérrez JC, Toro M, López-Hernández D. Arbuscular mycorrhyza and enzymatic activities in the rhizosphere of Trachypogon plumosus in three acid savanna soils. Soil Agriculture and Environment 2004; 103: 405-11.,15Mora E, Toro M, López-Hernández D. A survey of arbuscular mycorrhizae, Rhizobium and phosphate solubilizing bacteria in low fertility savanna soils in Central Venezuela (Estación Experimental La Iguana). In: Miransari M, Ed. Soil Microbiology 2013; 97-114.,43López M. Eficiencia de absorción de fósforo por tres cultivares de sorgo de diferente toxicidad al aluminio Tesis de Maestría en el postgrado de Ciencias del Suelo 1997.].

Although, 50% of the legumes presented nodules and AM (Fig. 2), the lower nodulation reported, may be a consequence of the acidity and low fertility of the ultisols since rhizobia do not growth efficiently in acid soils [6Abbadie L. Nitrogen inputs to and outputs from the soil-plant system. Lamto structure, functioning, and dynamics of a savanna ecosystem. In: Abbadie L, Gignoux J, Le Roux J, Lepage M, Eds. Ecological Studies 179 2006; 255-75.,7López-Hernández D. N biogeochemistry and cycling in two well-drained savannas: A comparison between the Orinoco Basin (Llanos-Venezuela) and Ivory Coast (Western-Africa). Chem Ecol 2013; 29: 280-95.
[http://dx.doi.org/10.1080/02757540.2012.744830]
], whereas, on the contrary, mycorrhiza are more adapted to those environments [11Gemma JN, Koske RE, Habte M. Mycorrhizal dependency of some endemic and endangered Hawaiian plant species. Am J Bot 2002; 89: 337-45., 43López M. Eficiencia de absorción de fósforo por tres cultivares de sorgo de diferente toxicidad al aluminio Tesis de Maestría en el postgrado de Ciencias del Suelo 1997.-45Entry JA, Rygiewicz PT, Watrud LS, Donnelly PK. Arbuscular mycorrhizal response to adverse soil conditions. In: Johri BN, Ed. Arbuscular Mycorrhizae Interactions in Plants, Rhizosphere and Soils 2002; 135-58.]. Moreover, nodulation was lower, even though sampling was performed at the peak of the rainy season (July), it is well established that nodulation and nodule numbers in savannas are favoured during the wet season [13Bala A, Giller KE. Relationships between rhizobial diversity and host legume nodulation and nitrogen fixation in tropical ecosystems. In: Bationo A, Ed. Advances in Integrated Soil Fertility Management in Sub-Saharan Africa: Challenges and Opportunities 2007; 691-702.,46Sicardi M, Izaguirre-Mayoral ML. A comparative evaluation of the symbiotic N2-fixation and physiological performance of thirty-six native legume species collected. Symbiosis 1994; 16: 225-47.].

Fig. (3)
AM root colonization and glomeromycota spore number in rhizosperic soil's of the native species.


4.2. P-Uptake and Mycorrhizal Associations

Concerning the parameters related to mycorrhizal association, AM associations are relevant in this savanna soil, since the native plants present a high symbiosis affinity. Moreover, as expected, a good AM colonization was found in an important number of collected plants. As low fertility is reported for this soil by classical standard chemical methods [47Altieri M, Nicholls C. Agroecología Teoría y práctica para una agricultura sustentable 2000., 48Barea JM. Las micorrizas arbusculares componente clave en la productividad y estabilidad de agroecosistemas 2003.], the presence of phosphate solubilizer organisms and rhizobia are considered as good fertility indicators, which, in turn, can be considered an indication of “good soil quality”. In addition, the number of Glomeromycota fungi spores (100-1700 in 100 g soil) reported for the experimental area can be considered high for soils under natural conditions. In fact, López-Gutiérrez et al. [3López-Gutiérrez JC, Toro M, López-Hernández D. Arbuscular mycorrhyza and enzymatic activities in the rhizosphere of Trachypogon plumosus in three acid savanna soils. Soil Agriculture and Environment 2004; 103: 405-11.] reported values between 80 and 290 spores in 100 g soil in a nearby area, whereas Lovera and Cuenca [49Lovera M, Cuenca G. Diversidad de hongos micorrízicos arbusculares (HMA) y potencial micorrízico del suelo de una sabana natural y una sabana perturbada de la Gran Sabana, Venezuela. Interciencia 2007; 32: 108-14.] presented 120 spores in 100 g soil during the dry season in a natural savanna located in Gran Sabana, Venezuela. Similar low results (0-196 in 100 g soil) have been presented by Collins et al. [50Collins N, Pfleger F, Crookston R, Simmons S, Coipeland P. Vesicular-Arbuscular mycorrhizas respond to corn and soybean cropping history. New Phytol 1991; 117: 657-63.] in an evaluation of AM in soybean and maize, and by Douds et al. [51Galvez L, Janke RR, Wagoner P. Effect of tillage and farming system upon populations and distribution of vesicular-arbuscular mycorrhizal fungi. Agric Ecosyst Environ 1995; 52: 111-8.] for cropping systems (wheat, soybean and maize) under different tillage managements. However, a higher value was also reported (1000 spores in 100 g dry soil) by Howeler et al. [52Howeler RH, Sieverding E, Saif S. Practical aspects of mycorrhizal technology in some crops and pastures. Plant Soil 1987; 100: 249-83.] for introduced Brachiaria decumbens. In conclusion, although under controlled conditions spore populations are high, the results presented in this survey indicate also a good establishment of Glomeromycota fungal populations in savanna under acid conditions.

In some cases, a good correlation has been reported between the number of spores of Glomeromycota fungi in the rhizosphere and the percentage of colonized root length [52Howeler RH, Sieverding E, Saif S. Practical aspects of mycorrhizal technology in some crops and pastures. Plant Soil 1987; 100: 249-83.-54Serralde A, Ramírez A. Análisis de poblaciones de micorrizas en maíz (Zea mays) cultivado en suelos ácidos bajo diferentes tratamientos agronómicos. Corpoica 2004; 5: 31-40.]. In the experiments here presented such association was found only in the case of the Trachypogon sp., which showed a high % CRL and also an important spore number (Fig. 3 and Table 3), no doubt those traits account for the remarkable adaptation of this species in the unfertile Orinoco’s savannas [2López-Gutiérrez JC, Toro M, López-Hernández D. Seasonality of organic phosphorus mineralisation in the rhizosphere of the native savanna grass, Trachypogon plumosus. Soil Biol Biochem 2004; 36: 1675-84.,3López-Gutiérrez JC, Toro M, López-Hernández D. Arbuscular mycorrhyza and enzymatic activities in the rhizosphere of Trachypogon plumosus in three acid savanna soils. Soil Agriculture and Environment 2004; 103: 405-11.,5López-Hernández D, Hernández-Hernández RM, Hernández-Valencia I, Toro M. Nutritional stress in dystrophic savanna soils of the orinoco basin: Biological responses to low N and P availabilities. In: Parvaiz A Rasool S, Ed. Emerging technologies and management of crop stress tolerance Biological Techniques 2014; 343-68.]. The density of spores depends on climatic conditions, on the physiology of the plant and the phosphorus availability at the moment of collection [55Carrillo L, Varela L, Orellana R. Variación estacional en la densidad de esporas de hongos micorrizógenos arbusculares y en el porcentaje de colonización micorrízica de tres palmeras Yucatanenses. In: Alarcón Ferrera-Cerrato R, Ed. Ecología, fisiología y biotecnología de la micorriza arbuscular 2000; 39-45., 56Giovanetti M. Seasonal variations of vesicular-arbuscular mycorrhizas and endogonaceous spores in maritime sand dune. Trans Br Mycol Soc 1985; 84: 679-84.].

The number of AM infective propagules measured was of 4571 in 100 g of dry soil (intervals 2141 – 9765 at 95% confidence). These values were six times higher than those reported for other natural savannas located near the experimental area [57López-Gutiérrez JC, Toro M, López-Hernández D. Micorrizas arbusculares y actividades enzimáticas en la rizósfera de Trachypogon plumosus Nees en tres sabanas de suelos ácidos. Acta Biol Venez 2001; 21: 49-97.]; however they are similar to the values presented by Toro and Sieverding [58Toro TS, Sieverding E. Evaluación cuantitativa y cualitativa de hongos formadores de Micorriza Vesiculo Arbuscular en la región de Mondomo, Colombia. Suelos Ecuatoriales 1986; 16: 122-9.] in Colombian savannas under management, which favor AM potentiality, those results suggest that the studied savanna has enough AM propagules to colonize native plant species. In addition, the presence of potential PSB and high root colonization by AM in this savanna soil suggest that a synergic effect might work in the plant uptake of phosphorus, as previously reported by Barea et al. [59Barea JM, Toro M, Orozco MO, Campos E, Azcón R. The application of isotopic (32P and 15N) dilution techniques to evaluate the interactive effect of phosphate-solubilizing rhizobacteria, mycorrhizal fungi and Rhizobium to improve the agronomic efficiency of rock phosphate for legume crops. Nutr Cycl Agroecosyst 2002; 63: 35-42.]. If that constitutes an important nutritional mechanism of biological origin in this dystrophic soil [5López-Hernández D, Hernández-Hernández RM, Hernández-Valencia I, Toro M. Nutritional stress in dystrophic savanna soils of the orinoco basin: Biological responses to low N and P availabilities. In: Parvaiz A Rasool S, Ed. Emerging technologies and management of crop stress tolerance Biological Techniques 2014; 343-68., 60Toro M, Azcón R, Barea JM. Improvement of arbuscular mycorrhiza development by inoculation of soil with phosphate-solubilizing rhizobacteria to improve rock phosphate bioavailability (32P) and nutrient cycling. Appl Environ Microbiol 1997; 63: 4408-12., 61Toro M, Azcón R, Barea JM. The use of isotopic dilution techniques to evaluate the interactive effects of Rhyzobium genotype, mycorrhizal fungi, phosphate solubilizing rhizobacteria and rock phosphate on nitrogen and phosphorus acquisition by Medicago sativa. New Phytol 1998; 138: 265-73.], it deserves further research. Moreover, we have found that the rhizosphere of Centrosema venosum and Galactia jussiaeana presents a high proportion of PSB, specifically for Burkholderia cepacia, which constitutes an important material to look for potential biofertilizers; an information that is presented in a forthcoming publication [62Mora E, Toro M, Flores E, López-Hernández D. Plant growth promoting abilities of phosphate solubilizing bacteria native from a high Psorbing ultisol. Annals Advanced Agricultural Science 2017; 1: 1-10.].

Fig. (4)
Percentage of colony forming unit (cfu) of solubilizing bacteria respect to total bacterial counts (cfu ToB) in rhizopheric soil of field plants.


CONCLUSION

In a descriptive study of the soil microbial community in a typical Trachypogon savanna located in Central Llanos, Venezuela, we have found that AM associations are relevant in this savanna soil, since plants present a high AM symbiosis affinity. The isolation of PSB was performed in a total of 25 rhizospheres of the plant species present in the experimental area. From those rhizospheres, 8 were positive to the presence of potential PSB. When analyzing the total amount of bacteria colony forming units respect the PSB we found that the rhizosphere of Centrosema venosum and Galactia jussiaeana present a high proportion of PSB, therefore the presence of PSB in the rhizosphere of those species constitutes an important material to look for potential biofertilizers.

ETHICS APPROVAL AND CONSENT TO PARTICIPATE

Not applicable.

HUMAN AND ANIMAL RIGHTS

No Animals/Humans were used for studies that are base of this research.

CONSENT FOR PUBLICATION

Not applicable.

CONFLICT OF INTEREST

The authors declare no conflict of interest, financial or otherwise.

ACKNOWLEDGEMENTS

Fondo Nacional de Ciencia y Tecnología (Proyecto S1-2000000649) and Consejo de Desarrollo Científico y Humanístico-UCV (Proyecto 03.31.4109.98) partially financed the study. We thank José Hernández, Mauricio Ramia and Shingo Nozawa for their assistance in the identification of plant samples. We would also like to acknowledge Dr. Rosvel Bracho of Biology Department University of Florida, Gainesville, USA for his comments and revision of an early version of the manuscript.

REFERENCES

[1] Ramia M. Tipos de sabanas en los llanos de Venezuela. Boletín de la Sociedad Venezolana de Ciencias Naturales 1967; 27: 264-88.
[2] López-Gutiérrez JC, Toro M, López-Hernández D. Seasonality of organic phosphorus mineralisation in the rhizosphere of the native savanna grass, Trachypogon plumosus. Soil Biol Biochem 2004; 36: 1675-84.
[3] López-Gutiérrez JC, Toro M, López-Hernández D. Arbuscular mycorrhyza and enzymatic activities in the rhizosphere of Trachypogon plumosus in three acid savanna soils. Soil Agriculture and Environment 2004; 103: 405-11.
[4] López-Hernández D, Hernández-Hernández RM, Brossard M. Historia del uso reciente de tierras de las sabanas de América del Sur. Interciencia 2005; 30: 623-30.
[5] López-Hernández D, Hernández-Hernández RM, Hernández-Valencia I, Toro M. Nutritional stress in dystrophic savanna soils of the orinoco basin: Biological responses to low N and P availabilities. In: Parvaiz A Rasool S, Ed. Emerging technologies and management of crop stress tolerance Biological Techniques 2014; 343-68.
[6] Abbadie L. Nitrogen inputs to and outputs from the soil-plant system. Lamto structure, functioning, and dynamics of a savanna ecosystem. In: Abbadie L, Gignoux J, Le Roux J, Lepage M, Eds. Ecological Studies 179 2006; 255-75.
[7] López-Hernández D. N biogeochemistry and cycling in two well-drained savannas: A comparison between the Orinoco Basin (Llanos-Venezuela) and Ivory Coast (Western-Africa). Chem Ecol 2013; 29: 280-95.
[http://dx.doi.org/10.1080/02757540.2012.744830]
[8] López-Hernández D. La Química del Fósforo en Suelos Ácidos Casa Editora: Ediciones de la Biblioteca 1977.
[9] Pinto FA, De Souza ED, Paulino HB, Curi N, Carbone CM. P-sorption and desorption in savanna Brazilian soil as a support for phosphorus fertilizer management. Cienc Agrotec 2013; 37: 521-30.
[10] Janos DV. Mycorrhizas in humid tropical ecosystems. In: Safir G, Ed. Ecophysiology of VA mycorrhizal plants 1985; 107-33.
[11] Gemma JN, Koske RE, Habte M. Mycorrhizal dependency of some endemic and endangered Hawaiian plant species. Am J Bot 2002; 89: 337-45.
[12] Toro M, Bazó I, López M. Micorrizas Arbusculares y bacterias promotoras de crecimiento vegetal, biofertilizantes nativos de sistemas agrícolas bajo manejo conservacionista. Agronomía Tropical 2008; 58: 78-83.
[13] Bala A, Giller KE. Relationships between rhizobial diversity and host legume nodulation and nitrogen fixation in tropical ecosystems. In: Bationo A, Ed. Advances in Integrated Soil Fertility Management in Sub-Saharan Africa: Challenges and Opportunities 2007; 691-702.
[14] López-Hernández D, Flores D. La desorción de fosfatos en suelos. Implicaciones fisioecológicas en el proceso. Acta Cient Venez 1979; 30: 23-35.
[15] Mora E, Toro M, López-Hernández D. A survey of arbuscular mycorrhizae, Rhizobium and phosphate solubilizing bacteria in low fertility savanna soils in Central Venezuela (Estación Experimental La Iguana). In: Miransari M, Ed. Soil Microbiology 2013; 97-114.
[16] Saharan BS, Nehra V. Plant growth promoting rhizobacteria: A critical review. Life Sciences and Medicine Research 2011; 1-30. 2011: LSMR-21
[17] Toro M, López-Hernández D. Potencialidades del manejo de las Micorrizas Arbusculares para el desarrollo sostenido de los sistemas agrícolas de bajos insumos del ecotono sabana-bosque amazónico. Memorias del IV Congreso Interamericano sobre el medio ambiente 1998; In: Colección Simposia.; Caracas, Venezuela. 1998; pp. 222-7.
[18] Bhattacharyya PN, Jha DK. Plant growth-promoting rhizobacteria (PGPR): Emergence in agriculture. World J Microbiol Biotechnol 2012; 28: 1327-50.
[19] Babajide PA, Fagbola O, Alamu LO. Influence of biofertilizer-fortified organic and inorganic nitrogenous fertilizers on performance of sesame (Sesamum indicum Linn.) and soil properties under savanna ecoregion. IJAAR 2012; 8: 108-16.
[20] Laditi MA, Nwoke OC, Jemo M, Abaidoo RC, Ogunjobi AA. Evaluation of microbial inoculants as biofertilizers for the improvement of growth and yield of soybean and maize crops in savanna soils. AJAR 2012; 7: 405-13.
[21] Ponce M, González V, Brandín J, Ponce ME. Análisis de la vegetación asociada a una toposecuencia en los Llanos Centro-Orientales de Venezuela. Ecotrópicos 1994; 7: 11-22.
[22] Heanes DL. Determination of total organic-C in soils by an improved chromic acid digestion and spectrophotometric procedure. Commun Soil Sci Plant Anal 1984; 15: 1191-213.
[23] Anderson JM, Ingram I. Tropical soil biology and fertility: A handbook of methods 1992.
[24] Olsen SR, Cole CV, Watanabe FS, Dean LA. A. Estimation of Available Phosphorus in Soils by Extraction with Sodium Bicarbonate U S Department of Agriculture Circular No. 939. 1954.
[25] Murphy J, Riley JP. A modified single solution method for the determination of phosphate in natural waters. Anal Chim Acta 1962; 26: 31-6.
[26] Varma A. Mycorrhiza Manual 1998; 542.
[27] Barillot C, Sarde C, Bert V, Tarnaud E, Cochet N. A standardized method for the sampling of rhizosphere and rhizoplan soil bacteria associated to a herbaceous root system. Ann Microbiol 2012.
[http://dx.doi.org/10.1007/s13213-012-0491-y]
[28] Blackwelder RE. Taxonomy, A Text and Reference Book 1967.
[29] Phillips JM, Hayman DS. Improved procedures for clearing roots and staining parasitic and vesicular-arbuscular mycorrhizal fungi for rapid assessment of infection. Trans Br Mycol Soc 1970; 55: 158-61.
[30] Giovanetti M, Mosse B. An evaluation of techniques for measuring vesicular-arbuscular infection in roots. New Phytol 1980; 84: 489-500.
[31] Herrmann L, Lesueur D, Giampieri F. Collecting symbiotic bacteria and fungi. In: Guarino L, Ramanatha Rao V, Goldberg E, Eds. Collecting plant genetic diversity: Technical guidelines-2011 Update 2011; 1-17.
[32] Porter WM. The most probable number method for enumerating infective propagules of vesicular Arbuscular mycorrhizal fungi in soil. Aust J Soil Res 1979; 17: 515-9.
[33] Sieverding E. Vesicular-Arbuscular mycorrhiza management in tropical agrosystems 1991.
[34] Fisher RA, Yates F. Statistical tables for biological, agricultural and medical research 1970.
[35] Schenck NC. Methods and principles of mycorrhizal research 1982.
[36] Thomas GV, Shantaram MV. Solubilization of inorganic phosphates for bacteria from coconut plantation soils. J Plant Crops 1986; 14: 42-8.
[37] Wenzel CL, Ashford AE, Summerell BA. Phosphate solubilizing bacteria associated with proteoid roots of seedlings of waratah [Telopea speciosissima (Sm.) R.Br.]. New Phytologist 1994; 128: 487-496.
[38] Izaguirre-Mayoral ML, Carballo O, Flores S, Sicardi de Mallorca M, Oropeza T. Quantitative analysis of symbiotic N2-fixation, non-structural carbohydrates and chlorophyll content in sixteen native legume species collected in different savanna sites. Symbiosis 1992; 12: 293-312.
[39] Medina E, Izaguirre ML. N2-fixation in tropical American savannas evaluated by the natural abundance of 15N in plant tissues and soil organic matter. Trop Ecol 2004; 45: 87-95.
[40] Aristeguieta L. Flórula de la Estación Biológica de los Llanos. Bol Soc Ven Ciênc Nat 1966; 26(110): 228-307.
[41] Barrios S, González V. Rhizobial symbiosis on venezuelan savannas. Plant Soil 1971; 34: 707-19.
[42] López-Hernández D, Santaella S, Chacón P. Contribution of free-living organisms to N-budget in Trachypogon savannas. Eur J Soil Biol 2006; 42: 43-50.
[43] López M. Eficiencia de absorción de fósforo por tres cultivares de sorgo de diferente toxicidad al aluminio Tesis de Maestría en el postgrado de Ciencias del Suelo 1997.
[44] Azcón R, Barea JM. Micorrizas. Investig Cienc 1980; 47: 8-16.
[45] Entry JA, Rygiewicz PT, Watrud LS, Donnelly PK. Arbuscular mycorrhizal response to adverse soil conditions. In: Johri BN, Ed. Arbuscular Mycorrhizae Interactions in Plants, Rhizosphere and Soils 2002; 135-58.
[46] Sicardi M, Izaguirre-Mayoral ML. A comparative evaluation of the symbiotic N2-fixation and physiological performance of thirty-six native legume species collected. Symbiosis 1994; 16: 225-47.
[47] Altieri M, Nicholls C. Agroecología Teoría y práctica para una agricultura sustentable 2000.
[48] Barea JM. Las micorrizas arbusculares componente clave en la productividad y estabilidad de agroecosistemas 2003.
[49] Lovera M, Cuenca G. Diversidad de hongos micorrízicos arbusculares (HMA) y potencial micorrízico del suelo de una sabana natural y una sabana perturbada de la Gran Sabana, Venezuela. Interciencia 2007; 32: 108-14.
[50] Collins N, Pfleger F, Crookston R, Simmons S, Coipeland P. Vesicular-Arbuscular mycorrhizas respond to corn and soybean cropping history. New Phytol 1991; 117: 657-63.
[51] Galvez L, Janke RR, Wagoner P. Effect of tillage and farming system upon populations and distribution of vesicular-arbuscular mycorrhizal fungi. Agric Ecosyst Environ 1995; 52: 111-8.
[52] Howeler RH, Sieverding E, Saif S. Practical aspects of mycorrhizal technology in some crops and pastures. Plant Soil 1987; 100: 249-83.
[53] Douds DDJr, Galvz L, Bécard G, Bécard G, Kapulnik Y. Regulation of arbuscular mycorrhizal development by plant host and fungus species in alfalfa. New Phytol 1990; 138: 27-35.
[54] Serralde A, Ramírez A. Análisis de poblaciones de micorrizas en maíz (Zea mays) cultivado en suelos ácidos bajo diferentes tratamientos agronómicos. Corpoica 2004; 5: 31-40.
[55] Carrillo L, Varela L, Orellana R. Variación estacional en la densidad de esporas de hongos micorrizógenos arbusculares y en el porcentaje de colonización micorrízica de tres palmeras Yucatanenses. In: Alarcón Ferrera-Cerrato R, Ed. Ecología, fisiología y biotecnología de la micorriza arbuscular 2000; 39-45.
[56] Giovanetti M. Seasonal variations of vesicular-arbuscular mycorrhizas and endogonaceous spores in maritime sand dune. Trans Br Mycol Soc 1985; 84: 679-84.
[57] López-Gutiérrez JC, Toro M, López-Hernández D. Micorrizas arbusculares y actividades enzimáticas en la rizósfera de Trachypogon plumosus Nees en tres sabanas de suelos ácidos. Acta Biol Venez 2001; 21: 49-97.
[58] Toro TS, Sieverding E. Evaluación cuantitativa y cualitativa de hongos formadores de Micorriza Vesiculo Arbuscular en la región de Mondomo, Colombia. Suelos Ecuatoriales 1986; 16: 122-9.
[59] Barea JM, Toro M, Orozco MO, Campos E, Azcón R. The application of isotopic (32P and 15N) dilution techniques to evaluate the interactive effect of phosphate-solubilizing rhizobacteria, mycorrhizal fungi and Rhizobium to improve the agronomic efficiency of rock phosphate for legume crops. Nutr Cycl Agroecosyst 2002; 63: 35-42.
[60] Toro M, Azcón R, Barea JM. Improvement of arbuscular mycorrhiza development by inoculation of soil with phosphate-solubilizing rhizobacteria to improve rock phosphate bioavailability (32P) and nutrient cycling. Appl Environ Microbiol 1997; 63: 4408-12.
[61] Toro M, Azcón R, Barea JM. The use of isotopic dilution techniques to evaluate the interactive effects of Rhyzobium genotype, mycorrhizal fungi, phosphate solubilizing rhizobacteria and rock phosphate on nitrogen and phosphorus acquisition by Medicago sativa. New Phytol 1998; 138: 265-73.
[62] Mora E, Toro M, Flores E, López-Hernández D. Plant growth promoting abilities of phosphate solubilizing bacteria native from a high Psorbing ultisol. Annals Advanced Agricultural Science 2017; 1: 1-10.

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