Spatial distribution of ammonium and nitrate fluxes along roots of wetland plants Yun Ying Fang a,b , Olga Babourina b, * , Zed Rengel b , Xiao E Yang a , Pei Min Pu c a College of Environmental and Resource Sciences, Zhejiang University, 310029 Hangzhou, China b Soil Science and Plant Nutrition, School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia c Nanjing Institute of Geography & Limnology, CAS, Nanjing 210008, China Received 12 February 2007; received in revised form 15 May 2007; accepted 21 May 2007 Available online 29 May 2007 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 À 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 À and H + measured simultaneously along the root and a positive relationship between H + and NH 4 + fluxes. When NH 4 + and NO 3 À 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 À uptake. In the short-term experiments, Azolla spp. had preference for NO 3 À uptake when NO 3 À 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 À 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. # 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Bacopa monnieri; Vallisneria natans; Azolla spp.; Ludwigia repens; Ammonium; Nitrate 1. Introduction Ecological engineering offers a simple, cheap and energy- efficient method of treating polluted water and wastewater. Aquatic macrophytes can take up excessive nutrients and also play a crucial role in creating a favourable environment for a variety of chemical, biological and physical processes that contribute to the nutrient removal and degradation of organic compounds [1,2]. The main research on aquatic plants has been focused on comparing actual nutrient uptake and improving the nutrient uptake potential from polluted water and wastewater by different species across time and space [3]. However, there is little knowledge about the capacity of roots of aquatic plants to absorb ammonium (NH 4 + ) and nitrate (NO 3 À ). For efficient nitrogen (N) removal in ecological engineering, it is important to quantify the preferences of aquatic plants for various N forms and to characterise the relationship between uptake rates of the two mineral forms of N (NH 4 + and NO 3 À ). Given that ion uptake by various parts of the root is variable [4,5], the profiles of H + , NH 4 + and NO 3 À fluxes were measured non-invasively along roots of four aquatic plants by the Microelectrode Ion F lux Estimation (MIFE) technique. To date the root profiles of both NH 4 + and NO 3 À fluxes were reported for only four plant species: barley [6], rice [7], maize [7,8] and Eucalyptus nitens [4]. In all earlier studies ion fluxes demonstrated high spatial and temporal variability, especially in the elongation zone of the root. Moreover, NH 4 + fluxes www.elsevier.com/locate/plantsci Plant Science 173 (2007) 240–246 Abbreviation: N, nitrogen * Corresponding author. Tel.: +61 8 6488 2846; fax: +61 8 6488 1050. E-mail address: olgab@cyllene.uwa.edu.au (O. Babourina). 0168-9452/$ – see front matter # 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2007.05.006