1. Echinochloa polystachya forms extensive monotypic stands on the lower levels of the Amazon floodplains. During its annual growth cycle c. 100t (dry mass) ha-1 of biomass is formed as the floodplain is being submerged (December-September) and a phase of death and decomposition occurs when the water has retreated (October-November). This study examines the mineral nutrient dynamics of this plant and its potential significance to the nutrient status of the floodplain. 2. Echinochloa polystachya was sampled monthly from a study site in the central Amazon. N, P and K contents for different plant organs were determined and net uptake calculated from concurrent measurements of dry matter production and turnover. 3. Leaf N, P and K contents were c. 20, 1.7 and 19 g kg-1, values typical of nutrient-replete stands of C4 plants. Stem concentrations were c. 12% of those of the leaves. Net N and P uptake followed the rise in the river level, whilst K appeared independent of water level. 4. The vegetation accumulated 377, 51 and 1136 kg ha-2 of N, P and K, respectively, during the growth phase. Over a possible 5000 km2 of these stands in the Varzea, this represents a massive sequestration of nutrients in the flood phase and a high release during the following low-water period. It is suggested that the E. polystachya stands could have a role in maintaining the nutrient status of the Amazon floodplain.
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