Effects of inorganic nitrogen forms on growth, morphology, nitrogen uptake capacity and nutrient allocation of four tropical aquatic macrophytes ( Salvinia cucullata, Ipomoea aquatica, Cyperus involucratus and Vetiveria zizanioides)

Abstract

This study assesses the growth and morphological responses, nitrogen uptake and nutrient allocation in four aquatic macrophytes when supplied with different inorganic nitrogen treatments (1) NH 4 +, (2) NO 3 −, or (3) both NH 4 + and NO 3 −. Two free-floating species ( Salvinia cucullata Roxb. ex Bory and Ipomoea aquatica Forssk.) and two emergent species ( Cyperus involucratus Rottb. and Vetiveria zizanioides (L.) Nash ex Small) were grown with these N treatments at equimolar concentrations (500 μM). Overall, the plants responded well to NH 4 +. Growth as RGR was highest in S. cucullata (0.12 ± 0.003 d −1) followed by I. aquatica (0.035 ± 0.002 d −1), C. involucratus (0.03 ± 0.002 d −1) and V. zizanioides (0.02 ± 0.003 d −1). The NH 4 + uptake rate was significantly higher than the NO 3 − uptake rate. The free-floating species had higher nitrogen uptake rates than the emergent species. The N-uptake rate differed between plant species and seemed to be correlated to growth rate. All species had a high NO 3 − uptake rate when supplied with only NO 3 −. It seems that the NO 3 − transporters in the plasma membrane of the root cells and nitrate reductase activity were induced by external NO 3 −. Tissue mineral contents varied with species and tissue, but differences between treatments were generally small. We conclude, that the free-floating S. cucullata and I. aquatica are good candidate species for use in constructed wetland systems to remove N from polluted water. The rooted emergent plants can be used in subsurface flow constructed wetland systems as they grow well on any form of nitrogen and as they can develop a deep and dense root system.