CHANGES IN PRODUCTIVITY AND NUTRIENT CYCLING FOLLOWING CONVERSION OF AMAZON RAINFOREST TO PASTURE (TROPICAL, SOIL FERTILITY, BRACHIARIA, GRAZING, LEACHING

The purpose of this study was to test 2 hypotheses: (1) Conversion of Amazon rainforest to pasture results in large losses of ecosystem nutrient stocks; and (2) Primary productivity of pasture is low, and declines further as ecosystem nutrient stocks are depleted.;Ecosystem nutrient stocks, primary and secondary productivity, nutrient retention and nutrient cycling were studied for 3.5 years after formation of pasture by cutting and burning mature rainforest and young secondary forest.;Conversion of rainforest to pasture resulted in a redistribution of nutrients from biomass to soil and litter. The initial pulse of mutrients to the soil following biomass combustion was as large as reported from other studies, but was followed by a more rapid soil nutrient decline than previously reported. Pasture formed from young secondary forest had a smaller input of nutrients to the soil following burning, and much less unburned woody residue.;Loss of nutrients from the ecosystem by leaching increased after pasture formation.;Total ecosystem nutrient stocks declined more rapidly than did storage in either soil or biomass. This is because decomposition of unburned woody residue continues to replace nutrients lost from other ecosystem components. Levels of nutrient stocks in the pasture formed from mature forest will decline to the levels of the pasture formed from young secondary forest after about 10 years.;Above-ground primary productivity in pasture formed from mature rainforest is similar to productivity of undisturbed forest. Most of this productivity is of the cultivated grass (Brachiaria decumbens), and there is no consistent change in the rate of productivity during the 3 years studied. However, productivity of pasture formed by cutting and burning young secondary forest was about 40% lower than pasture formed from mature forest. Weed productivity was similar on the 2 sites, but Brachiaria productivity was 80% lower on the pasture formed from secondary forest. I predict that continued nutrient depletion from pasture formed from mature rainforest will result in decreasing productivity, especially of the cultivated grass.;While conversion of rainforest to pasture on a moderate scale may not lead to rapid and complete ecosystem degradation, as had been predicted, the potential for long-term sustainable yield without fertilization seems small. Thus, pasture development is similar to traditional techniques of slash and burn agriculture.