Nitrogen dynamics in Amazon forest and pasture soils measured by 15N 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 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 and an experimental slash-and-burn plot in Rondônia. 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 were similar to pools in the forest after 8.5 months. Rates of net N mineralization and net nitrification decreased from forest to 21-yr-old pasture. Rates of gross N mineralization were similar in forest, 4- and 10-yr-old pasture then declined in 21-yr-old pasture. These findings 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 nitrification alone would indicate.