Here you go Zarnon...just one of about 2 dozen nice articles I have on "bio-organics':
Making Microorganisms Mobilize Soil Phosphorus
Alan E. Richardson
CSIRO Plant Industry, PO Box 1600, Canberra ACT 2601, AUSTRALIA
[email protected]
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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.