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
__________________
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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