Nitrogen dynamics in Amazon forest and pasture soils measured by N pool dilution

Clearing tropical forests of the Amazon Basin for pasture alters rates of soil nitrogen cycling. Previous studies have shown that rates of soil net N mineralization and net nitri®cation are lower in established pastures than in forests. We compared soil inorganic N concentrations, rates of net and gross mineralization and net and gross nitri®cation in a chronosequence and an experimental slash-and-burn plot in Rondoà nia. Soils of pastures 4, 10 and 21-yr-old contained more NH4 + and less NO3 ÿ than soils of forest. Soil NH4 + and NO3 ÿ concentrations were elevated for 2 months after burning but were similar to pools in the forest after 8.5 months. Rates of net N mineralization and net nitri®cation decreased from forest to 21-yr-old pasture. Rates of gross N mineralization were similar in forest, 4and 10-yr-old pasture then declined in 21-yr-old pasture. These ®ndings indicate that when forests are converted to pasture, soil N turnover is maintained for a period of a decade or longer, but N turnover eventually slows in old pastures. As older pastures come to dominate deforested regions of the Amazon, the total N cycled in soils of the region is likely to decrease, but not as quickly as studies based on net mineralization and net nitri®cation alone would indicate. # 1999 Elsevier Science Ltd. All rights reserved.