Departamento
de Producción Vegetal. Instituto de Recursos Naturales y
Agrobiología CSIC. Cordel de Merinas 40-52, 37008 Salamanca. SPAIN
E-mail: igual@usal.es
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Phosphorus is
second only to N as the most limiting element for plant growth. Although bound
P is quite abundant in many soils, it is largely unavailable for plant uptake.
Crop yield on 40% of the world’s arable land is limited by P
availability. As currently practiced, agriculture will require 55-60 x 106
Mt of P fertilizer applied to agricultural soils to meet food production needs
in 2040. However, P is not a renewable resource and its future use in
agriculture will be impacted by declining availability and increased cost.
Moreover, the striking increase in the use of fertilizers by intensive
agriculture practices has led to degradation of air and water quality.
Therefore, sustainable management of P in agriculture requires strategies to
enhance its acquisition or uptake by plants. In this context, room is available
for research progress on the role of microorganisms in plant P acquisition.
Bacterial
involvement in the solubilization of inorganic phosphate is known since the
first decade of the past century. Most of the studies on phosphate
solubilization were done first by isolating the microorganisms from the soil
and then studying the solubilization in vitro. The investigations on
solubilization of phosphates under field conditions and on the uptake by plants
were however started later. Ectorhizospheric strains from pseudomonads and
bacilli, and endosymbiotic bacteria from rhizobia have been described as effective
phosphate-solubilizing bacteria (PSB). Beneficial effects of the inoculation
with PSB to many crop plants have been described by numerous authors. Rhizobia
are, perhaps, the most promising group of PSB on account of their ability to
fix nitrogen symbiotically with legumes and the capacity of some strains for
solubilizing insoluble inorganic phosphate compounds. Several publications have
demonstrated that phosphate-solubilizing strains of Rhizobium and Bradyrhizobium increase growth
and P content in both nonleguminous and leguminous plants. An alternative
approach for the use of PSB as microbial inoculants is either the use of mixed
cultures or the co-inoculation with other microorganisms. In this regard, some
results suggest a synergistic interaction between arbuscular mycorrhizae fungi
(AMF) and PSB, which allows for better utilization of poorly soluble P sources.
Similarly, plant growth can be increased by dual inoculation with PSB and Azospirillum or Azotobacter.
Phosphate-solubilizing
bacteria have already been used as biofertilizer for agriculture. For example,
in the former Soviet Union a commercial biofertilizer under the name
“phosphobacterin” was first prepared by incorporating Bacillus
megaterium var. phosphaticum and also widely used in East European countries and
India. In this last country, a carrier based preparation under the name
“Microphos” was developed by the Indian Agricultural Research
Institute, using efficient phosphate dissolving strains of Pseudomonas
striata, Bacillus polymyxa and Aspergillus awamori packed in a wood
charcoal and soil mixture. These cultures were tested in multilocal field
trials and were found to be, in general, effective.
Despite of these promising results, PSB-based biofertilizers has not
got widespread application in agriculture mainly because of the variable
response of plant species or genotypes to inoculation depending on the
bacterial strain used. Differential rhizosphere effect of crops in harbouring a
target PSB strain or even the modulation of the bacterial phosphate
solubilizing capacity by specific root exudates may account for the observed
differences. On the other hand, good competitive ability and high saprophytic
competence are the major factors determining the success of a bacterial strain
as an inoculant. Therefore, studies to know the competitiveness and persistence
of specific microbial populations in complex environments, such as the
rhizosphere, should be addressed in order to obtain efficient inoculants. In
this regards, research efforts in order to obtain appropriate formulations of
microbial inoculants, which protect the inoculant organism against
environmental stresses and at the same time enhance and prolong its activity,
may help in promoting the use of such beneficial bacteria in sustainable
agriculture.