Carnivory and nitrogen supply affect the growth of the bladderwort Utricularia uliginosa

Abstract

A glasshouse experiment was designed to determine whether the terrestrial bladderwort species Utricularia uliginosa Vahl. gained any growth advantage from carnivory at different levels of nitrogen enrichment of the substrate. Three organism treatments were used: a control, a protist-only treatment (Euglena spp.) and a protist + meiofauna treatment (Euglena spp. plus nematodes, copepods, ostracods, cladocerans and Acarina). The organism treatments were factorially combined with three nitrogen levels (N0, N1, N2). Populations of Euglena were successfully established in the substrate of pots in the protist, and meiofauna + protist treatments, and were found in the traps of plants growing in these treatments. Populations of the meiofauna from the inoculating cultures were successfully established in the meiofauna + protist treatment, and were trapped by plants in these treatments. A background contamination of the control and protist-only treatments by Acarina occurred over the course of the experiment; however, numbers of Acarina in these treatments were at least an order of magnitude lower than numbers of meiofauna in the meiofauna + protist treatment. Non-acarinid meiofauna were restricted solely to the meiofauna + protist treatment. Organism treatment interacted significantly with the nitrogen level of the substrate to affect growth of Utricularia. Plants trapping Euglena (+ Acarina) had significantly less dry weight than control plants at the N0 base level of nitrogen; this negative effect of trapping Euglena on plant growth disappeared at the two higher nitrogen levels. The dry weight of plants trapping the full range of meiofauna at the N0 level was comparable with control plants. Plants trapping the full range of meiofauna + Euglena at the N1 level had significantly more dry weight than plants trapping Euglena only, and the highest dry weight of any treatment; the benefit of trapping the full range of meiofauna + Euglena was non-significant at the N2 level. Dry weight of plants was not significantly affected by nitrogen in any organism treatment. These results suggest that carnivory was overcoming a nitrogen deficiency induced by Euglena at the N0 level, but was overcoming deficiencies of other nutrients at the N1 level.