The role of periphytic N2 fixation for stands of macrophytes in the whitewater floodplain (várzea)

Abstract

In the varzea 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, Echinochlou 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.