Change of phosphorus forms in soils of the semiarid pampean region cultvated with wheat (Triticum aestivum).

The effect of two soil managements with wheat on the distribution of P in granulometric fractions of an Entic Haplustoll of the semiarid region was evaluated. The soil (0-0.15 m depth) under continuous wheat (TT) and under wheat-grazing (TP, wheat every two years) sequences during 9 years were studied. A reference, non cultivated plot close to each sequence was use as a check. Organic (Po), inorganic (Pi), total acid extractable (Pte), and extractable (Pe) in whole soil, in the fine (FF,< 100 µm) and in the coarse (FG,> 100µm) soil fractions were determined. Continuous wheat decreased the contents of all P forms during the studied period. Fertilization decreased the P losses compared with the reference plots and increased Pe from 11,8 to 27, 4 µg P g soil-1 . The wheat-grazing sequence had higher concentration of P forms than continuous wheat. Phosphorus application did not change the contents of Po, Pte and Pe, but decreased Pi. The most important changes of P forms were detected in the soil coarse fractions, where the Po associated to the organic matter (ca. 95% of total Po) and the Pi (25% in TP and 44 % in TT) decreased substantially.

Key words: phosphorus forms, granulometric fractions, wheat sequences rotations, fertilization, semiarid region