Soil texture can
modify the content and dynamic of soil phosphorus (P). The objective of this
work was to study the P form contents associated to physical fractionation of
SOM in soils with different texture. Soil samples (0-0.15 m) were obtained from
27 sites with different texture in a twenty five years old pasture located in
the Experimental Station of Bordenave, Argentina (63°01’20”W; 37°51’55”S). Soil
particle size fractions were obtained by wet sieving separating the fine (0-100
µm) and coarse (100-2000 µm) fractions. Soil organic matter was determined in
each fraction, mineral associated (MOM) and particulate organic matter (POM),
respectively. Extractable (Pe), organic (Po), inorganic (Pi) and total
extractable (Pt) phosphorus was determined and occluded P (Pocl) was calculated
as the difference [Pt - (Po+Pi)] in the whole soil and the particles fractions.
In these soils, texture determines P reserves and the equilibrium of its
different forms. Available P forms (estimated by Pe) was related to the
inorganic form present the fine fraction of the soil. Phosphorus content in its
different forms is closely associated with soil fractions. Soils with higher
content of fine fraction present the coarse fraction enriched in Po. All the
studied P forms were higher in fine textured soils than in coarse ones,
however, P forms in particle size fractions showed different tendencies. In coarse
textured soils, Po in MOM was lower than fine textured ones. However, P content
in MOM was richer and in POM was poorer than in fine textured soils. The
available P, estimated with Pe content, comes predominantly from the inorganic
pool and it is associated to the fine fraction of the soil. Phosphorus content
in its different forms is closely associated with soil size fractions. It was
proposed a conceptual model in which size fractionation of SOM and P forms
determination can be represented to a better understanding of soil P
equilibriums.
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