Phosphorus Dynamic in the Soil-Plant System ... 27increased interest due to its contribution to the current environmental,
agronomic and economic issues (Sharpley & Tunney, 2000).
Soil P-Cycling
Phosphates are absorbed from the soil solution by plants and are then
quickly restored through labile P-forms. When they become depleted, the rate
of mobility from less soluble forms such as primary and secondary minerals
will determine P-concentration in the soil solution. Phosphorus availability
over time will depend on the capacity of these forms to keep P-concentration
in the soil solution by replacing the ions absorbed around the roots. Mobility
of less available P-forms towards the soil solution is very slow (years) and it
depends on the physical and chemical characteristics of soil, on the content of
the various P-forms and on biological activity (Tiessen et al., 1994). Labile
forms are regarded as those which are available to plants and soil
microorganisms in the short term. Weathering of primary minerals produces
simple P-forms which are incorporated into the soil solution. Phosphorus can
thus be absorbed by plants and enter the organic cycle. The unabsorbed P
remaing in equilibrium with labile forms or it may precipitate into secondary
minerals and, eventually, convert into occluded forms. The P absorbed by
plants is incorporated into the soil in the form of organic compounds with
different characteristics, and they develop together with soil organic matter
(SOM); so much so that lability of organic forms will vary according to the
characteristics of organic matter (form crop residues to humic substances), the
soil environment and microbial activity. Forms of this Po existing in
chemically or physically protected forms may be slowly mineralized as a
result of mineralization of SOM or by the action of a specific enzyme in
response to the need for P. Thus, organic matter turnover as well as Pi
concentration and the demand for P by microbial and plant components will be
factors controlling Po lability (McGill & Cole, 1981). Organic P-forms are an
important contribution to P in the soil solution. In this turnover process, soil
microorganisms are continually regulating P-input, by mineralization of soil
organic matter (SOM) or production of enzymes, and –output, by assimilation
during its growing, in the soil solution depending on the organic materials
available as an energy source.
Reduction in the P concentration of the soil solution can be restored from
labile and moderately labile soil P forms. The evidence shows that
understanding P dynamics as a whole and improving the interpretation of the