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 Rondônia 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 μg 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 μg N g−1 d.s. d−1 and exceeded the annual average net N mineralization rates measured in pastures of −0.11-0.02 μg N g−1 d.s. d−1. Annual average net nitrification rates at 0–5 cm depth in forest (1.09–1.46 μg N g−1 d.s. d−1) were also higher than in pastures (0.24–0.25 μg 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.