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DIVISION S-7-FOREST & RANGE SOILS

Soil Phosphorus Transformations Following Forest Clearing for Pasture in the Brazilian Amazon

^a The Ecosystems Center, Marine Biological Lab., Woods Hole, MA 02543 USA
^b Centro de Energia nuclear na Agricultura, Avenida Centenário 303, CEP 13416000, Piracicaba, Sao Paolo, Brazil

dgarcia{at}mbl.edu

Phosphorus limits grass production in pastures planted on most cleared moist tropical forest, but little is known about soil P dynamics in these ecosystems. We describe changes to total P and different soil P fractions that occurred after the conversion of forest to pasture in the Brazilian Amazon State of Rondônia. We used chronosequences of forest and pasture of different ages to document patterns of labile, occluded, and organic P pools using a sequential P fractionation technique. Phosphorus released from the aboveground forest biomass substantially increased soil available P during the first 3 to 5 yr after forest clearing and burning. During this period, nonoccluded forms of inorganic P increased by 2.0- to 2.7-fold in the resin-extractable fraction and by 4- to 25-fold in the dilute HCl-extractable fraction. The introduction of grasses influenced the redistribution of soil P forms in older pastures. Occluded P comprised a lower proportion of total P (40–55%) in 20-yr-old pastures compared with forests (63–65%), but the proportion of organic P in these pastures increased (29–35%) compared with forests (20–21%). From the patterns in P transformations we developed a conceptual model in which we contrasted P transformations during slash and burn for pasture with changes to soil P that occur during soil formation. On cleared lands, the one-way process by which P in primary minerals is converted to occluded and organic forms is reset by the cutting and burning of plant biomass, but instead of being released from primary minerals, P is released from the burned and decomposing biomass. Because this occurs in an already weathered 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.

Abbreviations: ANOVA, analysis of variance • i, inorganic • o, organic