Abstract The N and P budgets in littoral floating soils – embalsados – of Ibera (Argentina) are here presented. The budgets were estimated from the differences in the total amounts of N and P in the aboveground biomass in February (summer) and August (winter), and the total amount of N and P in the floating soils sediments. Samples of vegetation – for aboveground biomass estimation – and of sediments were collected. The sediment samples were picked with a corer, and included roots, rhyzomes, detritus and inorganic material. The total P and total N concentrations were determined for the vegetation samples and the bioavailable P and total N concentrations were determined in the sediments. The results show that the aboveground biomass production gets less than 10% of the N content of the sediments. There is a decrease in the total amount of N in the sediments during the growing period. This observation can be partly explained by lacking data from non-estimated compartments, like the grazed aboveground biomass and the increment of the belowground biomass. Other possible fates for N would be losses by denitrification and lixiviation to the lagoon. The N:P ratio in the biomass indicates a deficiency of N, which is consistent with the negative N balance estimated for the embalsado . With respect to bioavailable P, a negative balance between winter and summer was also found. This was not the case for total P. The significant decrease of bioavailable P in depth in the embalsado suggests a net flow towards the lagoon along the growing season.