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Abstract

In the várzea of Amazon River on Marchantaria Island. N₂ fixation was measured in the root-periphyton complex of three abundant macrophytes, i.e. Paspalum repens, Echinochloa polystachya and Eichhornia crassipes. The short-term study was set at a period of rising water when the biomass production of macrophytes is high. N₂ fixation was low in a main channel that links the river to the lakes, and it was high in Lakes Camaleão and Novo, the latter being a newly formed lake in the young sedimentation area of Marchantaria Island. The main factor controlling N₂ fixation on Marchantaria Island seemed to be O₂. The highest nitrogen input by N₂ fixation was observed in stands of Paspalum repens followed by Echinochloa polystachya and Eichhornia crassipes. Maximum rate of N₂ fixation reached 821.3 nmol N g DW⁻¹ h⁻¹ in the root-periphyton complex of Paspalum repens at Lake Camaleão. Under dark conditions there was always a decline in the rate of N₂ fixation pointing to the predominance of photoautotrophic cyanobacteria. Considering both N₂ fixation in the light and in the dark, related to the biomass of each macrophyte stand, N input in all three macrophyte stands was highest at Lake Novo. Upscaling the rates of N₂ fixation, the input at Lake Novo was 235 kg N ha⁻¹ yr⁻¹, 37 kg N ha⁻¹ yr⁻¹ and 1 kg N ha⁻¹ yr⁻¹ in the stands of Paspalum, Echinochloa and Eichhornia, respectively. It is concluded that Paspalum and its N₂ fixing periphyton has a fertilizing potential that plays an important role for autogenic succession in the várzea