Soil, plant and cattle nutrient dynamics on pastures of the western Amazon of Brazil

Introduction Cattle production on Brachiaria pastures is a primary use of cleared forestland in the western Brazilian Amazon. About 6.8 million hectares in the States of Acre and Rondônia have been deforested, where 75% of land is now grazed (IBGE, 1998). The principal pasture species are B. brizantha and B. decumbens with the latter in decline from spittlebug susceptibility (Deois incompleta; Gonçalves et al., 1996). The general pattern of pasture establishment in the Amazon basin includes felling and burning forest biomass, planting annual crops for one to three years (especially on small farms), and then seeding to grasses. The conversion of tropical forest to pasture, the effects of that process on soil properties, and pasture degradation outcomes have been studied in the eastern Amazon region ( Serrao et al., 1978; Reiners et al., 1994; Sanchez and Salinas, 1981; Buschbacher et al., 1987; Moraes et al., 1996) but not in the remote western region. Most findings showed an initial increase in soil cation concentrations with pasture establishment, except P, which declined to nearly undetectable amounts; and losses in pasture productivity after about five years with little management and poorly matched grasses (Serrao et al., 1978). The extent to which this outcome is due to poor management practices (e.g., inappropriate grass species) is unknown, and few studies in the region have examined the impacts of stocking rates, soilplant nutrient relationships and burning frequencies on pasture degradation. Strategies that account for the cycling of major nutrients are needed to improve management of grasses and grass-legume associations. The objective of this study was to evaluate the hypothesis that low-input pasture use of land with Brachiaria spp. effectively sustains cattle production in the western Brazilian Amazon without deteriorating soil nutrient stocks . Three pasture land cover types (B. decumbens, B. brizantha and a grass association with Pueraria phaseoloides) were compared to primary forest and crops (maize and rice). Temporal (i.e., time post-deforestation) effects on the physical and chemical properties of soils and forages were evaluated. The nutrient pools in soil, plants and cattle herds and nutrients extracted in animal products were approximated to help understand the essential management to enhance cattle productivity and to avoid land degradation.