Microbial dinitrogen (N₂) fixation has been reported as an additional N source for terrestrial carnivorous plant species. Sirová et al. assess the importance of fixed N in the nutrition of rootless, aquatic, carnivorous Utricularia species (bladderworts), and find that although traps harbour abundant N₂-fixing micro-organisms and provide conditions highly suitable for N₂-fixation, the fixation rates are low and plant uptake of fixed N is negligible. High mineral N concentrations in the traps, resulting from rapid turnover of organic matter, seem to inhibit the N_2-fixation process, and thus they conclude that it is unlikely to contribute significantly to the N nutrition of aquatic carnivorous plants under their typical growth conditions.