SOIL CARBON AND NITROGEN STOCKS FOLLOWING FOREST CLEARING FOR PASTUREIN THE SOUTHWESTERN BRAZILIAN AMAZON

Tropical soils contain large stocks of carbon and nitrogen that can be altered by clearing for agriculture. In the Brazilian Amazon, cattle pasture is the predominant use for cleared forest lands. We examined c hanges to soil bulk density and C and N stocks in seven chronosequence s, each consisting of an intact forest and pastures of different ages created directly from cleared forest (7 forests, 18 pastures), along a 700-km transect in Rondonia in the southwestern Amazon Basin. The tra nsect included sites with a similar climate but a range of soil types. We used soil delta(13)C distributions to determine the origin of soil C and to infer changes to soil C cycling patterns after forest cleari ng. Soil bulk density increased under pasture; these increases were si gnificant in 6 of 18 pastures examined. Changes in C stocks to a depth of 30 cm under pasture ranged from a loss of 0.72 kg/m(2) to an incre ase of 1.77 kg/m(2). Soil C stocks increased in 14 of 18 pastures, but these increases were significant in only 4 pastures. Changes in soil N stocks to a depth of 30 cm ranged from a loss of 0.25 kg/m(2) to a g ain of 0.23 kg/m(2) and showed a similar pattern to C, except in one s ite where we measured significant N loss. Five of 18 pastures accumula ted significant amounts of N, and one pasture lost a significant amoun t of N. Soil delta(13)C values were greater in pastures than in the or iginal forests, and delta(13)C values increased with a longer time und er C, pasture vegetation. Bulk density increases were greater on soils with higher clay contents. Carbon accumulation increased with pasture age but was independent of soil texture. Soil C increases to a depth of 30 cm of up to 1.77 kg/m(2) amounted to an increase of >50% of the original soil C stock and represented up to 12% of the C in the biomas s of forest vegetation. In contrast, changes to soil N stocks in the r ange of 0.25 kg/m(2) approximately equaled the N stock in the original forest vegetation. Our results indicated that when site history was c ontrolled by considering only pastures formed directly from cleared fo rest, C and N accumulation was the dominant trend in pasture soils. Ab sence of a correlation between C and N accumulation and soil texture s uggested that site history and management may be more important than s oil type as determinants of the direction and magnitude of changes in soil C and N stocks.