Nitrogen dynamics in Amazon forest and pasture soils measured by N-15 pooldilution

Clearing tropical forests of the Amazon Basin for pasture alters rates of s oil nitrogen cycling. Previous studies have shown that rates of soil net N mineralization and net nitrification are lower in established pastures than in forests. We compared soil inorganic N concentrations, rates of net and gross mineralization and net and gross nitrification in a chronosequence an d an experimental slash-and-burn plot in Rondonia. Soils of pastures 4, 10 and 21-yr-old contained more NH4+ and less NO3- than soils of forest. Soil NH4+ and NO3- concentrations were elevated for 2 months after burning but w ere similar to pools in the forest after 8.5 months. Rates of net N mineral ization and net nitrification decreased from forest to 21-yr-old pasture. R ates of gross N mineralization were similar in forest, 4- and 10-yr-old pas ture then declined in 21-yr-old pasture. These findings indicate that when forests are converted to pasture, soil N turnover is maintained for a perio d of a decade or longer, but N turnover eventually slows in old pastures. A s older pastures come to dominate deforested regions of the Amazon, the tot al N cycled in soils of the region is likely to decrease, but not as quickl y as studies based on net mineralization and net nitrification alone would indicate. (C) 1999 Elsevier Science Ltd. All rights reserved.