Interactive Effects of
Phosphate-Solubilizing Bacteria and Mycorrhizal Fungi at Increasing Plant P
Availability and Their Evaluation by Using Isotopic Techniques.
Departamento de
Microbiología y Sistemas Simbióticos. Estación
Experimental del Zaidín. CSIC. Profesor Albareda 1, 18008 Granada. SPAIN
E-mail:
josemiguel.barea@eez.csic.es
__________________
Microbial
populations are key components of the soil-plant systems where they are
immersed in a framework of interactions affecting plant developments [1],
therefore, rational exploitation of soil microbial activities is accepted as a
fundamental topic for sustainability issues [2]. Both saprophytes and
mutualistic symbionts are involved in microbial management approaches [3]. The
saprophytes include the so-called plant growth-promoting rhizobacteria (PGPR)
which participate in many key ecosystem processes such as those involved in the
biological control of plant pathogens, nutrient cycling and seedling
establishment [4, 5]. One group of PGPR, the phosphate-solubilizing bacteria
(PSB), are particularly important for the present study.
With regard to mutualistic symbionts, mycorrhizal
fungi must be considered [3]. These fungi, upon root colonization, develop an
external mycelium which is a bridge connecting the root with the surrounding
soil microhabitats. Therefore, the mycorrhizal symbiosis, by linking the biotic
and geochemical portions of the ecosystem, can contribute to nutrient capture
and supply, particularly, the arbuscular mycorrhizal (AM) symbiosis plays a
direct role in nutrient cycling rates and patterns in both agroecosystem and
natural environments [6].
It has been shown
that many soil microorganisms are able to solubilize phosphate ions from
sparingly soluble inorganic or organic P compounds in vitro [7]. Nevertheless,
the effectiveness of this process in soil is unclear because of the transient nature
of the compounds released by these microorganisms, responsible for phosphate
solubilization, and the possible re-fixation of phosphate ions on their way to
the root surface, if any solubilization does take place. The microbiologically
solubilized phosphate, however, would be taken up by a mycorrhizal mycelium,
thereby developing a synergistic microbial (mycorrhizosphere) interaction [3].
Because assimilable
P is scarce in soil, the phosphate stock must be restored in any agricultural
system. Therefore, current developments in sustainability involve the use of
less expensive sources of plant nutrients like rock phosphate (RP), however,
this sparingly soluble form of P, usually has a low effectiveness in many cases
[8]. Integrated approaches involving AM fungi and PSB interactions, have been
proposed to improve P bioavailability from RP sources, therefore, its agronomic
performance [9]. Although RP solubilization is difficult to occur in non-acidic
soils, it may take place when these soils are deficient in exchangeable Ca,
because this characteristic facilitates P solubilisation [10].
Radioactive P (32P)
has been applied as a means of evaluating the exchange rates governing
phosphate equilibrium between the soil solution and the solid phases of the
soil [11]. It can also be used to measure P availability in RP materials [8]
and to identify P sources for AM and nonmycorrhizal plants [12]. The isotopic
composition, or specific activity (SA), in plant growing in 32P-labelled
soil can be affected by treatments such as AM inoculation so that a lowering in
the SA compared to that in control plants would indicate that the plant is
using extra 31P solubilized from microbial activity, from otherwise
unavailable P sources [9]. It is assumed that all 'labile' P attain isotopic
exchange within the experimental period.
We have carried out
a number of micro/mesocosms experiments aimed at assessing the impact of a
biotechnological practice (PSB and AM fungal inoculation), in interaction with
a low-input technology (RP application), in improving sustainable nutrient
supply to plants. A triple interaction involving also Rhizobium spp. and legume
plants was also investigated. These experiments integrate 32P
isotopic dilution techniques and used several agricultural soils, all of them
with neutral pH and low Ca. The results from these assays allowed us to reach
some general conclusions. These are as follows:
Dual inoculation
with selected AM fungi and rhizobacteria improved N or P accumulation in both
the RP-added soil and in the non RP-amended controls. Whether or not RP was
added, AM-inoculated plants showed a lower specific activity (32P/31P)
than did their comparable non-mycorrhizal controls, suggesting that the
AM-plant was using otherwise unavailable P sources. The inoculated or naturally
existing phosphate-solubilizing, AM-associated, microbiota could in fact
release phosphate ions (31P), either from the added RP or from the
indigenous "less-available" soil phosphate. A low Ca concentration in
the test soil may have benefited P solubilization. In one particular experiment
[9] it was found that, at least 75% of the P in dually inoculated plants
derived from the added RP.
Further
developments [13] corroborated the interactive effects of PSB and AM fungi on P
acquisition by legume plants, where measurements of the 15N/14N
ratio in plant shoots also indicate an enhancement of the N2
fixation rates in Rhizobium-inoculated AM-plants, over that achieved by Rhizobium in non-mycorrhizal
controls.
Current experiments
[14] further corroborated that mycorrhizosphere interactions between bacterial
and fungal associated contributed to the biogeochemical P cycling, thus
promoting a sustainable nutrient supply to plants.
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