N-15 NATURAL ABUNDANCE IN PLANTS OF THE AMAZON RIVER FLOODPLAIN AND POTENTIAL ATMOSPHERIC N2 FIXATION

The N-15 natural abundance values of various Amazon floodplain (varzea) plants was investigated. Samples of young leaf tissues were collected during three different periods of the river hydrography (low water, mid rising water and high water) and during one period in the Madeira River (high water). A large variation of N-15 abundance was observed, both among the different plant types and between the different flood stages. This variation probably, reflected, in part, the highly variable nature of the floodplain, sometimes dry and oxygenated and at other times inundated and anaerobic and, in part, changes in plant nitrogen metabolism. Comparison of the nitrogen isotopic composition of leguminous plants with that of non-leguminous plants showed that, on average, the N-15 abundance was lower in the legumes than non-legumes, suggesting active N-fixation. Also, the N-15 natural abundance in aquatic grasses of the genera Paspalum, was in general, lower than the N-15 abundance of aquatic grasses of the genera Echinochloa. As both of these grasses grow in the same general habitat, it appears that Paspalum grasses may also be nitrogen fixers.