Nitrogen fixation and denitrification in a floodplain forest near Manaus, Brazil

The Amazon floodplain (varzea) is seasonally affected by water level fluctuations of the Solimoes/Amazon River. The drastic environmental changes that occur also include microbiological processes, such as nitrogen (N-2) fixation and denitrification. Both processes were measured in the soil by the acetylene reduction assay and the acetylene block method in a varzea forest on Ilha de Marchantaria, Central Amazonia, Brazil. In the surface soil horizon (0-5 cm), N-2 fixation was highest during the exposed period (0.04-0.26 nmolN h(-1) g(-1) dry weight (dw)). In contrast, denitrification varied from 0 to 1.40 nmolN h(-1) g(-1) dw, with high rates during the submerged and the transition periods. No significant difference between locations with legume trees, with non-legume trees and without trees could be observed. N-2 fixation rates of incubations (litter down to 450 cm depth) for samples collected during the exposed period ranged from 0 to 0.11 nmolN h(-1) g(-1) dw, with highest rates in the surface soil horizon (0-5 cm). Denitrification ranged from 0 to 0.05 nmolN h(-1) g(-1) dw, with the highest rate at 250-300 cm depth, which was just below the water table. The maximum N-2 fixation rate (0.89 nmolN h(-1) g(-1) dw) and denitrification rate (0.09 nmolN h(-1) g(-1) dw) occurred in the litter layer. On average, at least three times as much N is lost from the surface soil horizon via denitrification than is gained by N-2 fixation annually, but the rates are strongly influenced by the flood pulse. Copyright (C) 2003 John Wiley Sons, Ltd.