NITROGEN DYNAMICS IN SOILS OF FORESTS AND ACTIVE PASTURES IN THE WESTERN BRAZILIAN AMAZON BASIN

To investigate the influence of forest conversion to pasture on soil N transformations, we compared soil inorganic-N pools and net mineralization and nitrification rates along two chronosequences of upland (terra firme) forest and pastures ranging in age from 4 to 82 years in the state of Rondonia in the western Brazilian Amazon Basin. Forest and pasture soils had similar total extractable inorganic-N pools at 0-5 and 5-10 cm depths. Ammonium-N and NO3--N pools were of similar magnitude in forest soils (2-10 mu g N g(-1) dry soil), while NH4+-N dominated pasture soil inorganic-N pools. Annual average net N mineralization rates for the two chronosequences at 0-5 cm depth in the forests were 1.31-1.88 mu g N g(-1) d.s. d(-1) and exceeded the annual average net N mineralization rates measured in pastures of -0.11-0.02 mu g N g(-1) d.s. d(-1). Annual average net nitrification rates at 0-5 cm depth in forest(1.09-1.46 mu g N g(-1) d.s. d(-1)) were also higher than in pastures (0.24-0.25 mu g N g(-1) d.s. d(-1)). Pasture soils had lower net N mineralization and net nitrification rates than forest soils even though they had approximately equal or higher total C and total N content. Pasture age did not affect NH4+-N pools or net nitrification rates, but decreased NO3--N pools and net N mineralization rates. Net N mineralization rate was unaffected by soil moisture, but net nitrification rate decreased at higher soil moisture. Higher net mineralization and nitrification rates in forest soils suggest a higher potential for NO3--N losses either through leaching or gaseous emissions from intact forests compared with established pastures.