Spatial distribution of ammonium and nitrate fluxes along roots of wetland plants

Abstract

Nothing is known about nutrient fluxes along the roots of floating and submerged macrophytes that may be used for removing nutrients from eutrophicated water systems. We have used ion-sensitive microelectrodes to measure fluxes of NH 4 +, NO 3 − and H + along the root apices. One floating ( Azolla spp.) and three submerged plant species ( Vallisneria natans Lour. Hara; Bacopa monnieri L. Pennell; and Ludwigia repens J.R. Forst) were tested. Ion fluxes showed a specific pattern linked to root zones in all four species. The highest influx of all three ions was found in the meristem zone of B. monnieri, V. natans and L. repens. B. monnieri had the greatest capacity to acidify the surrounding medium. In the four species studied, there was a consistent negative relationship between the fluxes of NO 3 − and H + measured simultaneously along the root and a positive relationship between H + and NH 4 + fluxes. When NH 4 + and NO 3 − were both present in the bathing medium, the meristem zone had the largest capacity for net NH 4 + uptake, whereas the elongation zone showed the highest net NO 3 − uptake. In the short-term experiments, Azolla spp. had preference for NO 3 − uptake when NO 3 − was supplied as a sole source of nitrogen, whereas L. repens required both nitrogen forms in the medium for net nitrogen uptake. When NO 3 − and NH 4 + fluxes were summed, L. repens had the largest and V. repens the smallest nitrogen accumulation capacity. Therefore, for industrial purposes, when plants are used for removing N from eutrophicated water, plants species should be selected according to their preferences for different N forms.