Soil phosphorus transformations following forest clearing for pasture in the Brazilian Amazon

Phosphorus limits grass production in pastures planted on most cleared mois t tropical forest, but little is known about soil P dynamics in these ecosy stems, We describe changes to total P and different soil P fractions that o ccurred after the conversion of forest to pasture in the Brazilian Amazon S tate of Rondonia. We used chronosequences of forest and pasture of differen t ages to document patterns of labile, occluded, and organic P pools using a sequential P fractionation technique, Phosphorus released from the aboveg round forest biomass substantially increased soil available P during the fi rst 3 to 5 yr after forest clearing and burning. During this period, nonocc luded forms of inorganic P increased by 2.0 to 2.7-fold in the resin-extrac table fraction and by 4 to 25-fold in the dilute HCl-extractable fraction. The introduction of grasses influenced the redistribution of soil P forms i n older pastures. Occluded P comprised a low er proportion of total P (40-5 5%) in 20-yr-old pastures compared with forests (63-65%), but the proportio n of organic P in these pastures increased (29-35%) compared with forests ( 20-21%). From the patterns in P transformations we developed a conceptual m odel in which we contrasted P transformations during slash and burn for pas ture with changes to soil P that occur during soil formation. On cleared la nds, the one-mag process by which P in primary minerals is converted to occ luded and organic forms is reset by the cutting and burning of plant biomas s, but instead of being released from primary minerals, P is released from the burned and decomposing biomass, Because this occurs in an already weath ered soil, P transformation from nonoccluded to occluded and organic forms occurs in <50 yr instead of the thousands of years required for these same transformations to occur during primary succession.