Making Microorganisms Mobilize Soil Phosphorus
CSIRO Plant Industry, PO Box 1600, Canberra ACT 2601, AUSTRALIA
Microorganisms are involved in a range of processes that affect the transformation of soil phosphorus (P) and are thus an integral component of the soil P cycle. In particular, soil microorganisms are effective in releasing P from inorganic and organic pools of total soil P through solubilization and mineralization. The microbial biomass in soil also contains a significant quantity of immobilized P that is potentially available to plants. Microorganisms therefore are critical for the transfer of P from poorly available soil pools to plant available forms and are important for maintaining P in readily available pools. These processes are likely to be most significant in the rhizosphere of plants.
Consequently, there has been longstanding interest in the manipulation of soil microorganisms to improve the P nutrition of plants, with the objective of increasing the overall efficiency of P-use in agricultural systems. This interest stems from the fact that P deficiency is widespread on soils throughout the world, that P fertilizer represents a major cost for agricultural production and that the efficiency of P-use by plants from soil and fertilizer sources is poor. Furthermore, P is a finite resource. Based on current rate of use, it is expected that the worlds’ known reserves of high quality rock phosphate will be depleted within the current century (Isherwood, 2000). Beyond this time the production of phosphate-based fertilizers will require the processing of lower-grade rock phosphates at significantly higher cost. Alternatively, the direct use of rock phosphates as fertilizers will require an effective means for solubilization. These issues are particularly relevant to soils throughout developing countries and on acidic soils in tropical and subtropical regions (Hedley et al., 1995). It is also imperative that management of P fertilizers in agricultural environments is improved (particularly in more highly P fertilized environments) so that any adverse environmental effects due to P losses are minimized (Tunney et al., 1997).
The concept of using soil microorganisms to improve mobilization of poorly available forms of soil P is not new. It is now some 50 years since Gerretsen (1948) first showed that pure cultures of soil bacteria could increase the P nutrition of plants through increased solubility of Ca-phosphates. Volumes of literature have since been published, a great deal has been promised, but it is fair to say that not much has been delivered. Clearly, microbial-plant interactions in soil environments are complex and, with few exceptions, have proven difficult to manipulate (reviewed by Richardson, 2001). Therefore, the challenge remains. Indeed, opportunities for exploiting soil microorganisms for P-mobilisation are improved as knowledge of the processes and understanding of the ecology of microorganisms in soil environments is gained. Such opportunities are further enhanced with the advent of new techniques. These include the possibility for direct manipulation of organisms through gene technology.
In this paper, recent issues concerning the mobilization of soil P by microorganisms are summarized and some opportunities for the future are discussed.
Phosphorus mobilization by soil microorganisms
Microorganisms directly affect the ability of plants to acquire P from soil through a number of structural or process-mediated mechanisms. These include (i) an increase in the surface area of roots by either an extension of existing root systems (eg, mycorrhizal associations) or by enhancement of root branching and root hair development (ie, growth stimulation through phytohormones), (ii) by displacement of sorption equilibria that results in increased net transfer of phosphate ions into soil solution or an increase in the mobility of organic forms of P and (iii) through stimulation of metabolic processes that are effective in directly solubilizing and mineralizing P from poorly available forms of inorganic and organic P. These processes include the excretion of hydrogen ions, the release of organic acids, the production of siderophores and the production of phosphatase enzymes that are able to hydrolyse soil organic P (Figure 1). In particular, organic acids and associated protons are effective in solubilizing precipitated forms of soil P (eg, Fe- and Al-P in acid soils, Ca-P in alkaline soils), chelating metal ions that may be associated with complexed forms of P or may facilitate the release of adsorbed P through ligand exchange reactions (Jones, 1998).
Figure 1. Schematic representation of major physiological factors associated with plant roots and soil microorganisms that influence the availability of soil P in the rhizosphere (from Richardson, 2001).
However, distinction between the roles of microbial processes, as distinct from direct effects of plant mechanisms, on P mobilization in soil is poorly understood. It is well established that plant roots effectively increase P acquisition through modified root growth and architecture and similarly produce metabolites that directly influence P availability (Raghothama, 1999). Processes such as rhizosphere acidification, exudation of organic acids and secretion of phosphatases from plant roots occur in response to P deficiency, and are established mechanisms by which plants acquire P (Randall et al., 2001). Furthermore, it has been suggested that microbial-mediated processes on their own may be insignificant in soil environments, and are unlikely to mobilize sufficient P for plant requirements (Tinker 1980). This argument remains to be resolved. On the other hand, the importance of the microbial biomass for P cycling in soil and the potential of this P to contribute to plant P nutrition is more difficult to deny.
Soil microbial biomass phosphorus and contribution to plant nutrition
The microbial biomass in soil contains a significant amount of P (typically 10 to 50 kg P/ha, but as high as 100 kg P/ha) and generally accounts for 2 to 5% of the total P and around 10 to 15% of the soil organic P. Importantly, microbial P is a dynamic component of the soil P cycle and is responsive to soil fertility, seasonal conditions and management practices. Whilst the P content of microbial biomass may vary considerably in relation to microbial C, it is evident that significant pools are maintained even in soils considered to be P deficient for plant growth (Oberson et al., 2001). This indicates that microorganisms in soil are highly efficient in acquiring P to meet their own requirements. In addition, it has been shown that soil microorganisms are capable of rapidly assimilating P supplied from fertilizer or as plant residues. For instance, McLaughlin et al (1988) showed that some 25% of P in labelled crop residues was incorporated into microbial biomass within 7 days.
A number of studies have highlighted the potential importance of microbial P in providing available P to plants. Seasonal dynamics indicate that significant amounts of P are released from the biomass in response to soil moisture deficiency and it is estimated that soil microbial P is completely turned over at least annually (He et al., 1997). More recent studies have found that the rate of P-flux through the microbial biomass is, in fact, considerably greater (Odel et al., 2000, Oberson et al., 2001). Incubation studies using labelled phosphate have shown both a rapid incorporation of P into biomass (within 2 to 3 days) and concomitant release of the P back to soil solution. Significantly, these transfers of P occurred in the absence of any significant changes in the size of the microbial P pool. Highest rates of P cycling through the biomass were evident in P-deficient soil and in soils that received organic inputs, as distinct from those that were P-fertilized. The capacity of the microbial biomass to immobilize P was also increased by the provision of soluble C, which resulted in an increase in both the size of the microbial P pool and its rate of turnover.
These observations have important implications concerning the contribution of microbial P to plant nutrition. First, the significance of P immobilization within the soil microflora and its effect on the ‘short-term’ availability of P to plants is not clear. Likewise, processes that affect the release of P from the microbial biomass and its subsequent availability to plants require further investigation. Although P in microorganisms occurs predominantly in organic forms (or as polyphosphates), the P appears to be rapidly mineralised and is readily available for uptake by plant roots (Macklon et al., 1997). However, in soil environments the availability of released P will be influenced by spatial and temporal factors and will also be subject to further immobilization (by both soil micro- and macro-flora and fauna) and other physico-chemicals reactions of P in soil. The actual contribution that P-turnover through microorganisms makes to the mobilization of soil P therefore remains to be fully determined. Nevertheless, it is well known that soil P is significantly depleted in close proximity to roots, that roots release significant quantities of C that is available to soil microorganisms and that microbial populations in the rhizosphere are enhanced by many orders of magnitude (Bowen and Rovira, 1999). Therefore, the potential for turnover of P by rhizosphere microorganisms is substantial, and further work needs to be undertaken to quantify it in terms of enhancing plant P nutrition.
Using microorganisms to improve soil phosphorus availability
Recognition that microorganisms are important for P mobilization in soil has led to research effort directed at improving plant P nutrition. Essentially, there are two major strategies for manipulating soil microorganisms:
(i) Management of existing microbial populations to optimize their capacity to mobilize P.
Success with this approach requires detailed knowledge of how soil management practices (eg, crop rotations, soil amendments, cultivation, etc.) impact on microbial abundance, diversity and presence of various functional groups and how these relate to the magnitude and availability of different soil P fractions. The manipulation of VA mycorrhizas in soil through crop rotation is one example of how populations might be managed to increase the availability of soil P to plants (Thompson 1994). Increased mineralisation of organic P generally occurs in response to soil cultivation and crop rotation has been shown to increase the rate of P cycling through the microbial biomass. For example, incorporation of organic residues through legume rotation resulted in higher biological activity and increased microbial P uptake and release (Oberson et al., 2001). Although the contribution of P released through these processes needs to be evaluated in relation to plant uptake, such observations indicate that management opportunities do exist for increasing the cycling of P and its maintenance in plant-available pools. Elucidation as to whether or not the availability of this P can be synchronized with plant requirements, or be targeted to the rhizosphere, remains a significant challenge.
(ii) The use of specific microbial inoculants to increase P mobilization.
A range of soil microorganisms able to solubilize precipitated forms of P or mineralize organic P has been characterized. Typically, such organisms have been isolated using cultural procedures, with species of Pseudomonas and Bacillus bacteria and Aspergillus and Penicillium fungi being predominant. These organisms are commonly associated with the rhizosphere and, when inoculated onto plants, often result in improved growth and P nutrition with responses being observed under both glasshouse and field conditions (see reviews by Kucey et al., 1989; Rodríguez and Fraga, 1999; Whitelaw, 2000). Despite this, there are few examples of successful application of microbial inoculants. Essentially, a lack of consistent performance under different environmental conditions in the field has precluded their wider use. A number of factors can be identified to explain this variable performance (Richardson, 2001). They include (i) poor understanding of the actual mechanisms involved in plant growth promotion where, in fact, P mobilization may not necessarily be the primary mechanism involved, (ii) selection of microorganisms by laboratory screening may be insufficiently rigorous when organisms are required to mobilize P in soil environments, (iii) the apparent lack of any specific association between phosphate solubilizing microorganisms and host plants, (iv) poor understanding of interactions between physical and chemical characteristics of soil and how these interact with biological P availability, (v) poor knowledge of how to deliver microorganisms into soil environments and of how to establish them as dominant components of complex microbial communities and, in particular, of their capability of colonizing the rhizosphere, and (vi) in most instances the benefits of microbial mobilization of P may in fact be indirect. In short, whilst microorganisms may directly solubilize P to meet their own requirements, subsequent benefits to plants may only occur following turnover of the microbial biomass.
It is evident therefore that the proposition for developing inoculants for routine application remains problematic and a number of issues still need to be addressed. Nevertheless, some microorganisms show consistent plant growth promotion under glasshouse and field conditions and have been developed as commercial inoculants (eg., Penicillium spp., Leggett et al., 2001). Although growth promotion in such cases is generally associated with increased plant P nutrition, it is difficult in many cases to ascertain whether increased P-mobilization is either the cause or the consequence of the response. Similarly, many other organisms (including mixed populations of soil bacteria and fungi, that often are only poorly characterized) have been promoted as commercial inoculants with claims that they increase plant growth through P-mobilization. Unfortunately, in most cases detailed evidence to support these claims does not exist.
Prospects for enhancing phosphorus mobilization by soil microorganisms
There seems little doubt that soil microorganisms are essential for the cycling of P in terrestrial ecosystems and as such, play an important role either directly or indirectly in mediating phosphate availability to plants. However, attempts to capitalise on microbial processes to increase plant access to P from soil and/or fertilizer sources have generally met with limited success. It is reasonable to assume that future opportunities will increase as understanding of the processes of P mobilization and the ecology of microorganisms in soil environments improves. Development of novel techniques and access to new technologies will be important. Recent developments in microbial community analysis that do not rely on cultural procedures will provide better understanding of how microorganisms interact in complex environments. For example, elegant procedures are now available for detection and visualization of specific microorganisms in the rhizosphere (eg, Ranjard et al., 2000, Theron and Cloete, 2000).
Opportunity also exists for genetic manipulation of soil microorganisms. It is now a reality that gene technologies can be used to enhance specific traits that may increase an organism’s capacity to mobilize soil P directly, enhance its ability to colonize the rhizosphere (ie, rhizosphere competence, Lugtenberg et al., 2001), or even to form specific associations with plant roots (Bowen and Rovia, 1999). Alternatively, microorganisms may provide a novel source of genes for directly modifying plants. For example, it has been reported that, when expressed in roots, a bacterial citrate synthase gene increases the exudation of organic acids and significantly improves plant access to soil P (López-Bucio et al., 2000). The credibility of this report has however been questioned (Delhaize et al., 2001). The ability of plants to use P from phytate, which is a predominant form of organic P in most soils, has been shown to be dependent on the presence of soil microorganisms and utilisation of phytate-P was significantly improved when an Aspergillus phytase gene is expressed directly in plant roots (Richardson et al., 2001a, Richardson et al., 2001b). Although such approaches require further validation, particularly in soil environments, they highlight that new opportunities do exist. However, it is also important to recognize that any approach to increase the mobilization of soil P through genetic modification, whether it be of soil microorganisms or plants themselves, will also need to satisfy a range of community issues.
The promise of exploiting soil microorganisms to increase mobilization of soil P remains. Whether or not this will be achieved through better management of soil microbial communities, by development of more effective microbial inoculants, through the genetic manipulation of specific organisms or with a combination of these approaches is not known. Nevertheless, what is clear is that soil microorganisms play an important role in the mobilization of soil P and that detailed understanding of their contribution to the cycling of P in soil-plant systems is required for the development of sustainable agriculture.
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