5.1 Introduction

Nowadays, agriculture relies on chemical fertilizers in order to satisfy the demand of crops with a high yield potential and produce economically viable yields. The synthesis of these fertilizers requires high amounts of fossil fuels as an energy source. Fossil fuels are nonrenewable resources, and their oxidized products such as CO2 pose hazards to the environment and to human health. Moreover, fossil fuel reserves are finite and therefore unsustainable in long-term scale.

Phosphorus (P) is the second important element after nitrogen which is necessary to survival and growth of plants (Ogbo 2010). However, in the soil solution, it usually exists in very low quantities (a micromolar level) as compared with most of the other vital nutrient elements which are present in millimolar levels (Ozanne 1980). To ameliorate P deficiency, high amounts of chemical P fertilizers are used which can lead to the environmental degradation, pollution of natural resources, water eutrophication, and increased crop production cost. Moreover, a notable section of the P added into the soil as chemical fertilizers is rapidly converted to unavailable compounds such as calcium phosphate or other fixed forms. As reported by Gyaneshwar et al. (2002), about 75–90% of the chemical P fertilizers applied in agricultural soils become unavailable quickly due to P combination with other elements such as Fe, Al, Ca, and Mg depending on the soil pH level. Generally, in the alkaline soils, P is fixed by Ca or Mg, whereas in the acidic ones, it forms insoluble compounds via reaction with Fe or Al. Therefore, there are large reserves of P in most agricultural soils resulted from the massive use of the synthetic P fertilizers (Rodriguez and Fraga 1999); as in a global scale, these reserves can sustain crop yields in their maximum levels for about one century (Goldstein et al. 1993). On the other hand, major P chemical fertilizers are originated from rock phosphates as their mother materials which are known to be finite resources, and their reserves may be depleted during the next 100 years (Herring and Fantel 1993). Phosphate biofertilizers can play an important role in agroecosystems as renewable and ecofriendly nutrient suppliers for plants and are proposed as possible alternatives for conventional chemical P fertilizers. According to Raghuwanshi (2012), the use of these biofertilizers can be included as an efficient approach in Integrated Nutrient Management (INM) and Integrated Plant Nutrition System (IPNS). They can biologically transform soil P from unavailable to available forms.

These biofertilizers contain different types of microorganisms which increase the accessibility of plants to soil P reserves which are unavailable in normal conditions. This can be attributed to their ability to dissolve insoluble P compounds and extension of plant root system via establishment of a symbiotic relationship with the roots of different plant species. These microorganisms belong to different taxonomic groups especially fungi and bacteria.

5.2 Mycorrhiza

An important symbiotic relationship between soil fungi and vascular plant roots is called mycorrhizae which through it nutrients and energy are exchanged between two symbionts (Brundrett 2002). Roots of about 95% of plant species can be colonized by soil fungi and establish mutualistic relationships named arbuscular mycorrhizae (AM) (Smith and Read 2008). Terrestrial plants and AM fungi (AMF) have been evolved side by side during their evolutionary history. A symbiotic relationship between AM fungi and land plants has been distinguished in the fossils belonging to Ordovician era, approximately 460 million years before this (Redecker et al. 2000).

Plant roots are colonized by AM, and the fungi transmit nutrient elements such as P into the host plant in exchange for the photoassimilate produced by plant. Arbuscules are highly branched intracellular fungal structures which are formed in the cortex of host plant roots, and at the same time fungi constitute their mycelial network in the soil (Fig. 5.1). P uptake by plants can be enhanced due to symbiotic relationship with AM (Bolan 1991). Moreover, these beneficial microorganisms can increase nitrogen (Barea et al. 1991) and micronutrient (Burkert and Robson 1994) availability to host plants and aggregate soil particles leading to an improved soil structure (Tisdall 1994). However, supply host plant with P which is an extremely nonmobile macronutrient in most soils can be defined as the main benefit caused by AMF (Bucher 2007).

Fig. 5.1
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AM fungi form an extensive and dense mycelia network in soil by which cover the depletion zone around plant roots (Source: http://www.sarcozona.org/tag/mycorrhizae)

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5.2.1 Some Benefits of Mycorrhiza

As mentioned previously, increased phosphorus availability to plants is known as the main advantage resulted from the symbiosis with AMF. Because of low solubility and mobility, P is proposed as one of the most limiting essential soil elements needed for plant survival and growth. It is estimated that crop inoculation with AMF can reduce the use of P chemical fertilizers by 80% in field conditions (Jakobsen 1995). In a study, plants inoculated with AM showed a sixfold increase in Pi and fourfold increase in the other nutrients as compared with uninoculated fertilized plants. Other workers showed that inoculation with AM fungi increased plant ability to utilize soluble P from rock phosphate (Antunes and Cardoso 1991; Guissou et al. 2001).

Moreover, mycorrhizal roots can acquire nitrogen organic compounds, such as amino acids and small peptides, and transport them to host plants (Bajwa and Read 1985; Bajwa et al. 1985). Ericoid, a group of mycorrhizal fungi, can degrade organic nitrogen and transmit it to mycorrhizal plants in the experiments conducted in controlled environments (Abuzinadah and Read 1986; Read 1991; Read et al. 1989). Michelsen et al. (1996) also suggested that ericoid mycorrhizae enabled the host plants to access soil organic N sources under natural conditions.

AM fungi can also effectively protect soil against erosion. This can be achieved by their extraradical hyphae which are able to connect soil particles (Miller and Jastrow 1992) leading to an improved soil aggregate stability and consequently a lower soil erodibility. AMF can also produce a sticky glycoprotein named glomalin which cements soil particles (Wright and Upadhyaya 1998; Wright et al. 1999; Rillig et al. 2002) and improves the stability of soil aggregates via binding soil particles (Peters 2002).

5.2.2 Mechanisms by Which Mycorrhiza Interacts with Plants and Improves P and N Availability for Them

Before the physical contact between plants and AMF (i.e., at the pre-symbiotic stage), it is known that some molecular signals are exchanged between them. Some studies have been shown that on the one hand AMF modulate root gene expression (Kosuta et al. 2003; Weidmann et al. 2004), intracellular signaling (Navazio et al. 2007; Kosuta et al. 2008), development (Oláh et al. 2005), and metabolism (Gutjahr et al. 2009) via diffusion of some produced compounds. On the other hand, plants release some special biochemicals via their roots which stimulate fungi to establish a symbiotic relationship (Gianinazzi-Pearson et al. 1989; Siqueira et al. 1991; Tsai and Phillips 1991; Giovannetti et al. 1996; Buee et al. 2000). Strigolactones (SLs) have been distinguished as the main secondary metabolites which are produced by host plants and are able to stimulate the symbiont fungi (Akiyama et al. 2005; Besserer et al. 2006). Some important morphological and developmental events in AM fungi including spore germination, hyphal branching, and increasing fungal respiration and mitochondrial activity are usually induced by SLs (Besserer et al. 2006, 2008).

The soil volume exploited by plants can be extended by several times when plant roots are in association with AMF mycelial network (Fig. 5.2). Therefore, P uptake can be achieved more efficiently by a mycorrhizal than a non-mycorrhizal plant root system (Smith and Read 2008). In other words, mycorrhizal plants can access nutrients such as phosphorus which exist outside the rhizosphere zone where they are not accessible for non-mycorrhizal plants. This is achieved through the fungal mycelial network connected to plant root system (Friese and Allen 1991). For example, one centimeter of colonized roots might produce 50–150 cm of extraradical hyphae (Harley 1989). Moreover, in comparison with plant roots, fungal hyphae are much thinner (Bago et al. 1998), which can enable them to penetrate in the soil microscopic pores which are unavailable to plant roots.

Fig. 5.2
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The soil volume which can be explored by a plant can be increased by several times of magnitude via the network of fungal mycelium connected to AM roots (Source: http://www.dirtgoddessseeds.com/category_s/1901.htm)

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Another mechanism by which AMF increase P availability to plants is related to their ability to produce different organic acids (Lapeyrie 1988) which can transform soil mineral phosphates from insoluble to soluble forms. This inevitably leads to the higher plant access to acid-labile insoluble P compounds such as calcium phosphate. In addition, phosphatase produced by AMF can enable them to release P from organic phosphate forms (Koide and Shreinner 1992).

Although the plant growth-promoting effect of AMF is mainly attributed to their ability to dissolve insoluble P compounds and increase phosphate uptake by plants, there are some evidences on the effectiveness of these fungi to increase nitrogen accessibility to plants (Ames et al. 1984; Azcón-Aquilar et al. 1993). Matsumura et al. (2013) reported that under different amino acid treatments, the nitrogen content of mycorrhizal plants was notably higher than that for non-mycorrhizal plants. In another study, Hobbie and Hobbie (2006) observed that in arctic tundra, 61–86% of the nitrogen acquired by plants was resulted from an ectomycorrhizal symbiotic relationship. Govindarajulu et al. (2005) also found that AM fungi are able to obtain soil inorganic nitrogen by their extraradical mycelium which then is converted to arginine and translocated to the intra-radical fungal mycelium located in the roots of host plant.

Some studies have demonstrated that the nitrogen present in the soil organic compounds can be accessible to AMF (Hodge et al. 2001; Whiteside et al. 2009; Hodge and Fitter 2010). Hodge and Fitter (2010) showed that decomposing soil organic materials are responsible for 31% of the nitrogen acquired by AMF hyphae system. This can be explained by the AMF ability to produce a diverse range of hydrolytic enzymes including cellulase, pectinase, and xyloglucanase in their external mycelial network (Garcia-Romera et al. 1991; Garcia-Garrido et al. 1992). It clearly is known that these enzymes are responsible to decompose the soil organic matters.

Chitinases are another group of metabolites produced by AMF species which are proposed as one of the factors involved in plant root protection against soil pathogens (Azcón-Aguilar and Barea 1997; Gianinazzi-Pearson 1996). Whiteside et al. (2012) showed that recalcitrant (i.e., a molecule with relatively large and complex structure) organic N compound such as chitosan can be absorbed by AMF in situ.

5.3 Phosphate Solubilizing Microorganisms (PSMs)

P is proposed as one of the most important elements participant in growth, development, and biological processes of different organisms. It is also known as an essential limiting factor for plants due to its insufficient solubility and mobility in soils (Vessey 2003) especially in extraordinary pH conditions. However, insoluble P compounds such as calcium phosphate and apatite can be solubilized by phosphate solubilizing microorganisms (PSMs) mainly bacteria and fungi which are in association with plant roots. Bacillus and Pseudomonas are known as the most important genera of mineral phosphate solubilizing bacteria (PSB) (Illmer and Schinner 1992), while main genera of fungi involved in P solubilization process are Aspergillus and Penicillium (Motsara et al. 1995). In soil, bacterial and fungal PSMs form 1–50 and 0.1–0.5% of the total soil phosphate solubilizing microorganisms, respectively. It means that the number of PSB is higher by 2–150 times than that for fungal solubilizing agents (Kucey 1983). Generally, production of organic acids and chelating factors by PSMs can explain their ability to solubilize insoluble phosphate compounds (Deinum et al. 1996; Dong and Pierdominici 1995).

However, there are some evidences which indicate inorganic acids can also be produced by PSMs. For example, the bacteria belonging to the genus Acidithiobacillus produce sulfuric acid via reaction with elemental sulfur (Garcia Junior 1992). This biologically produced acid plays an effective role in natural P solubilizing process via reducing soil pH which consequently leads to the improved plant growth (Stamford et al. 2002). However, it has been shown in both liquid and solid media that fungi have a higher ability to produce organic acids and therefore are more efficient to solubilize insoluble P compounds when compared with PSB (Venkateswarlu et al. 1984).

In a soil with P limited resources, PSMs can notably increase plant accessibility to this important element. According to Mohammadi et al. (2015), in a weedy condition along with a reduced sowing uniformity (i.e., when high intra- and interspecific competitions were intensified), phosphate biofertilizers containing fungi and bacteria could significantly improve soybean yield indicating the essential role of these microorganisms to support plants in a P limited condition.

5.4 Mechanisms by Which PSMs Improve P Availability for Plants

The improvement of P availability by PSMs can be achieved through different mechanisms (Fig. 5.3). However, it seems that the production of different organic acids by PSMs is the main reason explaining their solubilizing activity (Alam et al. 2002). Diverse organic acids such as gluconic, ketogluconic, oxalic, citric, succinic, fumaric, tartaric, α-ketobutyric, lactic, itaconic, isovaleric, isobutyric, acetic, malic, glyoxylic, and malonic can be produced by PSMs. The results of some studies show that the most efficient organic acid involved in P solubilization process is gluconic acid which is produced by Gram-negative bacteria (Goldstein et al. 1993; Kim et al. 1998). Khan et al. (2009) also suggested that gluconic and ketogluconic are the main low molecular weight organic acids produced by PSMs which are able to solubilize insoluble phosphate compounds in soil. The glucose oxidative metabolism by glucose dehydrogenase in the presence of a cofactor named pyrroloquinoline quinone (PQQ) is the mechanism by which gluconic acid is produced by PSMs (Fig. 5.4).

Fig. 5.3
figure 3

Different mechanisms by which PSMs improve P availability for plants (Adapted from Zaidi et al. 2009)

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Fig. 5.4
figure 4

Molecular structure of pyrroloquinoline quinone (PQQ) which acts as a cofactor in gluconic acid production process (Adapted from Matsumura et al. 2014)

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In general, the reduced soil pH caused by organic acids produced by PSMs can explain their ability to dissolve insoluble P compounds (Nahas 1996). However, it appears that increasing P solubilization rate cannot be achieved by acidifying reaction alone (Subha Rao 1982). According to Kucey (1988), another major factor influencing solubilization process is the capacity of organic acids to chelate insoluble P compounds; as in a study when 0.05 M EDTA was added to the medium, solubilization rate was the same as inoculation with Penicillium bilaii.

PSMs can also produce inorganic acids, synthesize exopolysaccharides, and release H+ as other important mechanisms contributing to inorganic P solubilization process (Gamalero and Glick 2011). Moreover, phosphatase produced by PSMs can play a key role in solubilization of organic P compounds (Park et al. 2011).

5.5 Factors Influencing the Efficiency of P-Related Microorganisms

The effectiveness of PSMs as biofertilizers can be influenced by diverse factors. Ho and Ko (1985) showed that after artificial introduction of PSMs into the soil, the size or density of their populations was decreased quickly. The success level of PSMs after introducing them into the soils highly depends upon their ability to compete with other soil microorganisms and the presence of a notable PSM saprophytic capacity. According to Kucey et al. (1989), the effectiveness of the inoculated PSMs to improve plant growth and yield can be varied in relation to several factors including:

  1. 1.

    If inoculated PSM can survive and colonize in the plant rhizosphere.

  2. 2.

    Its competitive ability with native microorganisms.

  3. 3.

    Essence and characteristics of the inoculated soils and plant varieties.

  4. 4.

    Inadequate rhizospheric nutritional level which can lead to the sufficient organic acid production by PSMs to dissolve insoluble P compounds.

  5. 5.

    PSM infirmity to dissolve soil P.

It is concluded that extensive studies should be carried out to distinguish the PSM strains with high durability and competitive ability under the environments with high complexity such as a plant rhizosphere in order to access to highly efficient P biofertilizers.

5.5.1 Soil Factors

Edaphic factors including soil composition (Bashan et al. 1995), physiological condition, temperature, pH, water content (Van Elsas et al. 1991), and the existence of recombinant plasmids (Van Veen et al. 1997) can significantly affect the survival of the inoculated PSMs. While competition, predation, and the growth of plant roots which supplies the substrates needed to PSMs form the main biotic factors influencing PSM survival as inoculants. Since the survival of AMF as obligatory endosymbionts only depends on the carbohydrates produced by the root cells of host plants, all edaphic agents determining the metabolism and growth of host will certainly affect AMF efficiency.

The soils with high buffering capacity can notably reduce PSM efficiency to solubilize insoluble P compounds, especially when PSM strains are not able to release acceptable levels of organic acids. Khan et al. (2007) also found that the presence of diverse environmental conditions is an important reason which can explain the variation in PSM efficiency. The low effectiveness of PSMs can be related to an unsuitable soil environment as may be observed in high alkaline soils. As in the soils with high alkalinity level that are commonly found in arid and semiarid climatic conditions (e.g., many areas of Iran) and usually have high temperatures and salinity levels, PSMs may colonize plant roots poorly resulting in a low P solubilizing activity. Therefore, it seems that searching for PSM strains with high efficiency in unfavorable environmental conditions is necessary.

5.5.2 Agronomic Practices

Sole cropping, conventional tillage, and fertilizer application are some of the common techniques to produce yield in most modern agricultural systems which can negatively affect AMF abundant and diversity in soils (Helgason et al. 1998; Oehl et al. 2005).

5.5.2.1 Tillage Practices

Tillage operations have been shown to reduce the number of AMF spores present in the soil (Kabir et al. 1998) and AM fungi colonization in some agricultural crops (Jasper et al. 1989; Miller et al. 1995; McGonigle and Miller 1996). Annual soil disturbances produced by conventional tillage systems showed reducing effects on AMF colonization when compared with reduced tillage practices (Miller and Jastrow 1992; Miller et al. 1995; Al-Karaki 1998; Miller 2000).

In general, conservation tillage practices have positive effects on AM fungi parameters and other soil factors. Positive consequences caused by no tillage consisted of higher soil carbon, nitrogen, sulfur, and phosphorus quantities and a greater AM fungal propagules remaining in the soil as compared with conventional tillage, as well as a simultaneously increased phosphorus accessibility for subsequent crops.

In a study, the amount of total glomalin produced by AMF enhanced in the soils under reduced tillage and no tillage than conventional tillage and soil carbon content was known as an important factor determining this enhancement (Borie et al. 2006).

5.5.2.2 Fertilization

AMF diversity and abundance have increasingly been declined in response to mineral nutrient application in agroecosystems (Lin et al. 2012; Liu et al. 2012). Among the mineral nutrients, Pi and nitrate have solely shown adverse effects on AMF, while these beneficial fungi were not negatively affected even by high levels of other essential elements including potassium, calcium, magnesium, sulfate, and iron.

Although the adverse effect of Pi on AMF has been recognized for a long time (Abbott et al. 1984; Thomson et al. 1986; Amijee et al. 1989; Breuillin et al. 2010; Balzergue et al. 2011), the increased AMF-plant symbiotic relationship caused by N deficiency can significantly overcome the reducing influence resulted from high P levels on AMF. This indicates that symbiosis can be enhanced by plants as long as there are limiting levels of one of these two important elements in rhizosphere.

5.5.2.2.1 Phosphorus

Crop production through the extensive use of chemical P fertilizers can notably decline AMF existence and abundance in soils (Johnson 1993). In a P-enriched environment, plant roots are not usually colonized severely by AMF (Amijee et al. 1989) as it has been indicated that when adequate accessible P is present in the soil, the growth of certain plant species may be reduced due to AMF colonization (Son and Smith 1995).

Pi can systemically suppress AM development which is in relation to the nutritional condition of host plant shoot. Inasmuch as a notable section of the photosynthate produced by host plant is usually used by AMF (Smith et al. 2009; Douds et al. 2000), the inhibiting effect of the elevated Pi levels on AMF development may be attributed to an energy-saving negative feedback mechanism in the environments in which the P needed for plant can adequately be provided in the absence of a symbiotic relationship with fungi. In other words, at a high level of phosphorus, plant preferentially adopts a nonexpensive and direct approach to acquire P (Nagy et al. 2008), and therefore, the plant root colonization by AMF can significantly be declined.

Moreover, long-term previous P applications can also affect AM fungi colonization of subsequent crops (Kahiluoto et al. 2000; Dekkers and van der Werff 2001). Dekkers and van der Werff (2001) reported that after 10 years without P fertilization, AM fungi colonization of winter wheat (Triticum aestivum) and barley (Hordeum vulgare) was greater when previous long-term annual P fertilization ranged from 0 to 17.5 kg ha−1 compared to when the rate of P application was 52.5 kg ha−1.

The main metabolites including amino acids and carbohydrates which are secreted by host plant roots and are usable for AMF can be reduced in a P-enriched soil (Graham et al. 1981; Thomson et al. 1986). The genes involved in carotenoid biosynthesis and those responsible for symbiotic relationship, e.g., PT4, were suppressed in the presence of Pi (Breuillin et al. 2010). In contrast, the roots exposed to a P-deficient condition can exude some essential flavonoid signals which induce the growth and activity of AMF at the pre-symbiotic phase (Nair et al. 1991).

Based on some conducted studies, low quantities of strigolactones (key factor to trigger plant-AMF symbiosis) can usually be produced and exuded by different plant species when the soil phosphorus is high (Yoneyama et al. 2007a, b; López-Ráez et al. 2008), and strigolactones may not be present in the plant root exudates exposed to high P levels, and consequently these plants don’t show stimulating effects on AMF.

Therefore, it can be assumed that the suppressive effect of P-enriched soils on AMF symbiosis is related to the decreased plant ability to produce sufficient levels of strigolactones in these conditions (Bouwmeester et al. 2007; Yoneyama et al. 2007b). Balzergue et al. (2011) reported that the exudates extracted from the plant roots developed in a P-enriched soil were not able to induce branching of fungal hyphae.

Other researchers also demonstrated that there is a negative correlation between the levels of strigolactones produced by host plant and soil available phosphorus (Yoneyama et al. 2007a, b; López-Ráez et al. 2008) and these metabolites could not be detected in the root exudates obtained from the plants developed in P-enriched environments. However, the number of the roots colonized by AMF and plant ability to produce strigolactones is mainly determined by shoot Pi content compared to the externally soil available phosphorus or the Pi levels which locally exist in plant roots (Balzergue et al. 2011).

5.5.2.2.2 Nitrogen

Previous studies in controlled environments and the field have found that low N levels (20 mM N) increased mycorrhizal infection (Goulart et al. 1995, 1996; Stribley and Read 1976). Whiteside et al. (2012) suggested that increasing nitrogen accessibility can decrease plant tendency to establish a symbiotic relationship with AMF, because the cost-effectiveness of fungal association is significantly reduced under this condition. Consequently, in the soils with high N levels, a decreased AMF frequency can be expected as is usually happened in different ecosystems (Treseder 2004). Cappellazzo et al. (2008) also reported that the ability of AM fungus G. mosseae to transport amino acids was notably declined in the presence of high inorganic nitrogen levels. The suppressing effects of N-enriched environments on AM colonization and activity have been demonstrated in several works. For example, Whiteside et al. (2012) observed a lower AMF ability to organically derived nitrogen uptake when accessible nitrogen was increased. In their study, the use of nitrogen fertilizer notably reduced the rate of specific uptake (i.e., per unit biovolume) of labile organic N by AMF.

However, if soil available N is so low that it reduces plant growth, establishment of the mycorrhizal association may be affected. In a study, the limited N supply to the host plants could have resulted in a reduced C supply to support mycorrhizal association, thus leading to a reduced mycorrhizal infection level (Yang et al. 2002). Other studies showed that serious nitrogen deficiency in plants may contribute to low root carbohydrate content which lowers infection levels in vesicular-arbuscular mycorrhizal associations (Hepper 1983; Same et al. 1983). It can be concluded that the presence of a critical N level to achieve an efficient plant-AM association in soil is necessary.

5.5.2.3 Rotation

Since the development of AM fungi is biotrophic (Morton 1990), the absence of mycorrhizae hosts could cause a decrease in soil residual AM propagules and their vitality for crops seeded afterward in a rotation.

Including non-mycorrhizal crops in rotation might affect the concentration and vitality of indigenous AM species in soil, thereby affecting the growth of AM-dependent crops following in the rotation (Dalpè and Monreal 2004). Gavito and Miller (1998) reported that intra-radical AM colonization of corn (Zea mays L.) was delayed in field plots when canola rather than corn was the previous crop.

In general, the crops belonging to Chenopodiaceae, Brassicaceae, and Caryophyllaceae (Barker et al. 1998) families don’t form symbiotic associations with AM fungi, and thus including them in rotations can significantly reduce the absence and activity of AM fungi in agroecosystem soils. Moreover, since AM fungi are obligate symbionts and their survival is fully dependent to live hosts, including black fallow in a rotation has negative effects on these beneficial microorganisms.

Conclusion

In general, phosphate biofertilizers can be proposed as suitable alternatives to synthetic chemical fertilizers which are extensively applied in modern agricultural ecosystems. Maintaining and invigorating these beneficial microorganisms via adoption of appropriate agronomic practices and introducing them into the agricultural soils intentionally can notably reduce fossil fuel consumption and environmental hazards caused by chemical inputs used in cropping systems while reclaiming the soil ecosystem. These microorganisms which mainly belong to fungi and bacteria groups can increase crop accessibility to nutrient reserves in soil via different mechanisms such as formation of a dense and extensive mycelial network connected to crop roots and production of a wide range of organic acids and chelating metabolites. However, some conventional operations which are extensively used in crop production systems today have shown negative effects on these beneficial microorganisms which consequently have been led to the increased dependency of these systems to external inputs.

It is concluded that in order to attain the self-sufficient and sustainable agricultural systems, the essential role of phosphate-related microorganisms as efficient nutrient suppliers for crops should seriously be considered. Moreover, the reasonable crop production practices including the use of conservation tillage (no or reduced tillage), organic manures instead of synthetic chemical fertilizers, and suitable and black fallow-free crop rotations as well as the artificial introduction of these microorganisms as biofertilizers into the agricultural soils should be included in cropping system management programs.