Factors Affecting the Solubilization of Insoluble Phosphates

Ely Nahas1

Departamento de Produção Vegetal. FCAV. Universidade Estadual Paulista-UNESP, 14870-000 Jaboticabal,SP, BRAZIL.

1Bolsista do CNPq.  E-mail: e-nahas@fcav.unesp.br

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South American soils are generally low in both total and available phosphorus and some of them sorb large amounts of applied phosphate fertilizer (6). Phosphorus deficiency is a consequence of the high weathering rate, causing a high degree of mineralogical decomposition, leaching of Ca, Mg, K, Na and P,  greater acidity, high solubilization of Al and Fe, deep profiles of soils, decreased contents of macro- and micronutrients, absence of minerals of open structure, and, therefore, low cation exchange capacity. Besides, rock phosphates are of igneous origin and present low agronomic potential (5).

The contents of total phosphorus in some Brazilian soils range  from 115.2 to 966.8 μg g-1 soil (average 560.6), organic P content ranges from 26.7 to 161.6 μg g-1 soil (average 55.4) and available P from 3.0 a 54.0 μg g-1 soil (average 12.5) (14). Most of the phosphorus was found as aluminum phosphate (22.7 to 265.7 ppm) and iron phosphate (74.2 to 144.9 ppm) rather than calcium phosphate (45.3 a 162.5 ppm) (2). Available phosphorus was only 2.2% of total P. In view of the phosphorus deficiency existing in Brazilian soils, the study of the factors that influence the microbial solubilization of mineral phosphates in the soil is important for agriculture.

The populations of phosphate-solubilizing bacteria and fungi from thirteen different soil types and vegetation ranged from 3.7 to 142.6 x 105 (16 % of total bacteria) and 2.3 to 57.7 x 103 g-1 soil (30 % of total fungi), respectively (13). These results indicated that the population of P-solubilizers is relatively high in these soils. Phosphate-solubilizing microorganisms dissolve insoluble phosphates by the production of inorganic or organic acids and/or by the decrease of the pH, producing available phosphate that can be uptake by the plants (3, 16). The solubilizing ability of a microorganism is related to its organic acid production, however the nature of the acid produced is more important than the quantity of the acid (1). Based on the effect of final pH of the culture medium or acid production on phosphate solubilization, controversial results have been obtained by the authors (10).

Production of acids was affected by the carbon sources. Among the carbohydrates tested, fructose, glucose, xylose, sucrose, and starch enhanced fluorapatite solubilization more than galactose and maltose, although total phosphorus accumulation was 78% with fructose as opposed to 59%-69% with the other carbon sources (3). A significant relationship was detected between solubilization measured as soluble phosphate and mycelium dry weight, final pH and titratable acidity in the culture medium. The amount of soluble phosphate was negatively correlated with pH and titratable acidity and positively correlated with mycelium dry weight. However, a significant correlation (P<0.05) was obtained between titratable acidity and soluble phosphate up to the third day of fungal growth (correlation coefficient, r=0.58) then point determinations may not reflect a behavior as a whole in the environment. Thus, other factors should also be considered, i.e., the time and conditions of growth of the microorganism. Fluorapatite solubilization was not enhanced when increasing amounts of fructose (0.25% - 4.00%, w/v) or vinasse were added to the culture medium as carbon sources. The maximum content of soluble phosphate produced corresponded to 1.2 % fructose and this was probably due to soluble phosphate uptake by the Aspergillus niger fungus (3, 12).

Phosphate solubilization was also related to the proton (H+) excretion  accompanying NH4+ assimilation (15). When ammonium nitrogen salts were added to culture medium inoculated with Aspergillus niger, fluorapatite solubilization was enhanced more than that of organic or nitrate sources (3). However, the addition of ammonium nitrate to vinasse used as culture medium enhanced A. niger growth but rock phosphate solubilization was severely reduced. This effect was due greater fungal growth, as well as to lower acid production and lower pH values compared to control (12).

Besides the effects of carbon and nitrogen sources, the solubilization of insoluble phosphates is affected by soluble phosphate levels in two ways. First, by the genetic mechanism of repression-derepression. The process of solubilization by the bacteria Escherichia coli and Erwinia herbicola was found to be regulated by the external phosphate levels, as also observed for phosphatases (4). A similar mechanism was found in A. niger (11). The production of acid phosphatase by A. niger was stimulated in low phosphate medium in the absence of fluorapatite. When fluorapatite was added to the medium, the enzyme was severely repressed, probably due to the enhanced soluble phosphate concentration. Acid phosphatase production and fluorapatite solubilization were decreased when the concentration of soluble phosphate was enhanced at 2 mM Pi. Second, soil fertilization with phosphorus is important to both plant growth increasing active exudation from roots, and microorganism growth (7), which provides a substantial amount of organic acids that enhance solubilization. Then, when soluble phosphate was exhausted, the solubilization process  is triggered, with a consequent  increase  in soluble phosphate in the soil.

Different factors related to the microbial isolate and to the type of insoluble phosphate (Gafsa, Araxá and Patos rock phosphates and calcium phosphate) affected the final pH values, the levels of titratable acidity and, consequently the amounts of solubilized phosphate (9). The lowest pH values were observed with the phosphates from Araxá and Patos. The levels of acids produced and the solubilization levels were in the following sequence: calcium phosphate>Gafsa>Araxá>Patos, i.e., they were in the same order of agronomic efficiency as these phosphates, indicating that the agronomic potential of rock phosphate may be related to its solubilizing ability or vice‑versa. Forty two soil isolates (31 bacteria and 11 fungi) were studied for their ability to solubilize rock phosphate and calcium phosphate in culture medium. Eight bacteria and 8 fungi presented high solubilizing ability (9). There was a correlation between final pH value and titratable acidity (r = ‑0.29 to -0.87) and between titratable acidity and soluble phosphate (r = 0.22 to 0.99). The correlation values ranged as a function of insoluble phosphate and of the group of microorganisms considered. A high correlation was observed between final pH and soluble phosphate only for the rock phosphates inoculated with the more solubilizing bacteria (r = -0.73 to ‑0.98). When each isolate was analyzed separately a correlation between the variables studied was observed in only some isolates. However, significant responses were obtained between titratable acidity and soluble phosphate for all phosphates used and showing that, in soil, the microbial interaction is more effective in the solubilization process. If there is a dependence of solubilization on the type of acid secreted (8), thus, the microorganisms as a whole would permit a better response because of a more diversity among the acids secreted.

In conclusion, it seems clear that some factors related to the nature or the contents of C, N and P sources affected the solubilization of insoluble phosphates by the P-solubilizing microbial populations. Some carbon sources found in soil enhanced P dissolution. Ammonium salts were more effective than nitrate or organic N sources. Phosphorus sources influenced both the growth of P-solubilizer microorganisms and  the repression of the genetic system related to the solubilization mechanism. Available phosphate was only 2.2% of total P, and the supply of phosphate to plants depends on microbial solubilization of insoluble phosphates.

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