The dynamics of soil P forms and particle size fractions was studied under three wheat-based cropping sequences in production systems of Argentina. The whole soil and its coarse (100–2000？μm) and fine (0–100？μm) fractions were analyzed to determine Bray-Kurtz extractable (Pe), organic (Po), inorganic (Pi), and total (Pte) phosphorus. The reference soil was determined at time 0 and compared to a four-year period (time 9 to 12) in three crop sequences: wheat (Triticum aestivum L.)-cattle grazing on natural grasses (WG), continuous wheat (WW), and wheat-legume (WL). Levels of Pe showed differences over time, from 10 to 16？μg？g？1 in WG, in line with agriculture and cattle grazing alternate sequences. In WW, P level increased with time, while in WL systems a significant decrease in P from 33.7 to 10.4？μg P？g？1 was found during the legume period. Soil P values varied between reference soil and soil samples in year nine and between treatments. Pi was significantly lower in WW, and its concentration increased with time. The coarse fraction of the reference plots had significantly higher levels of Po and Pi than the cultivated treatments, probably a consequence of the particulate organic matter decomposition and coarse mineral particle weathering. The observed changes in Pi content could be attributed to differences in occluded P equilibrium under different soil environments (mainly pH) and crop-tillage-climatic interaction. 1. Introduction Plants absorb mainly inorganic phosphorus (Pi) but the organic phosphorus (Po) is also an important reservoir for plant nutrition [1, 2]. Several studies have reported reduction in soil organic P fraction during crop growth [3, 4]. The Po can be found in chemically or physically protected forms, which can be slowly mineralized into available forms for plant uptake, mainly as a product of soil organic matter (SOM) decomposition or by the action of specific enzymes [5, 6]. A continuous loss of the soil P reservoir due to crop harvesting can rapidly consume Po and Pi forms [5, 7, 8], which may eventually lead to plant P deficiencies. Several soil studies have shown P deficiencies in the semiarid and semihumid regions of Argentina, and that extractable P content is lower due to soil pedogenetic characteristics and the agricultural history of the region [9–11]. Agricultural and cattle breeding systems of the semiarid Pampas region revealed negative P balances [12]. While P in the soil has been widely studied, little information is available on its organic fractions [13]. In the western area of the Province of Buenos Aires, Po
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