ELSEVIER 0960-8524(95)00062-3 Bioresource Technology 53 (1995) 113-123 © 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0960-8524/95/$9.50 HIGH TEMPORAL RESOLUTION MEASUREMENT OF NITRATE UPTAKE FROM FLOWING SOLUTIONS D. Raj Raman The University of Tennessee, Department of Agricultural Engineering, PO Box 1071, Knoxville, TN 37901-1071, USA Roger M. Spanswick Cornell University, Division of Biological Sciences, Section of Plant Biology, Plant Science Building, Ithaca, N Y 14853-5908, USA & Larry P. Walker Cornell University, Department of Agricultural and Biological Engineering, Riley-Robb Hall, Ithaca, N Y 14853-1701, USA (Received 6 December 1994; revised version received 24 April 1995; accepted 25 April 1995) Abstract The nitrate (NO j-) uptake rates of intact, 23 day old rice plants were measured to determine the relationship between the plant's NOA- nutrition history and the NO j- uptake rate. A system for measuring NO j- uptake was designed, built and tested. Specific design goals, which were met, include: low handling shock to the plants, high measurement accuracy (4%), high temporal resolution (10 min) and minimal mass-trans- fer limitations to uptake. Important design factors were identtfied and the overall uncertainties in the reported measurements were computed. The observed uptake rates were dependent on the NOA- concentration ([NO j-]) to which the plants were exposed for the 24 h prior to testing; plants pretreated at higher [NO j-] had lower uptake rates from 200 pu NO j- solutions than plants pretreated at lower [NO j-]. Key words: Nitrate, ion-uptake, measurement error, pretreatment, recirculating system. INTRODUCTION Constructed wetlands are gaining popularity as a wastewater treatment technique, due to their low cost, low energy requirements and simplicity. In these systems, removal of organic carbon (typically measured as biochemical oxygen demand, or BOD) occurs through sedimentation and through the activ- ity of aerobic and anaerobic microorganisms affiliated with plant roots, while nitrogen (N) removal occurs through microbial nitrification-deni- trification, through plant uptake of N and through volatilization of ammonia (Brix, 1993). The design of these systems has been hampered by a lack of 113 understanding of the biological and physiochemical mechanisms underlying the treatment processes. Without this understanding, design guidelines have been vague, as is evident in the large scatter of hydraulic- and BOD-loading rates of existing wet- land treatment systems (Reed & Brown, 1992). A rational design method is needed to make construc- ted wetland treatment systems more cost effective and reliable. Such a method would express pollutant removal rates as a function of environmental condi- tions and waste strengths and would require a knowledge of the process kinetics. Since Rogers et al. (1991) have demonstrated that, under certain cir- cumstances, over 90% of the N removal can be accounted for by plant uptake mechanisms, a knowl- edge of the kinetics of N uptake will probably be one part of a rational design method. In wastewater, N occurs primarily in the mineral forms of ammon- ium (NH4 +) and nitrate (NO3). Of this pair, the uptake kinetics of NO3 are of particular interest because NO3 is a pollutant of ground water and because microbiological processes on plant roots may convert NH2- to NO3 (Hammer & Bastian, 1989). Numerous methods for measuring NO3 uptake have been reported (Blom-Zandstra & Jupijn, 1987; Bloom & Chapin, 1981; Clement et al., 1974; Dod- dema & Telkamp, 1979; Glass et al., 1987; Goyal & Huffaker, 1986; Henriksen et al., 1990; Ingemarsson et al., 1987; Ingestad &Lund, 1979; Lee & Drew, 1986; Mattsson et al., 1991; review by Wild et aL, 1987; Youngdahl et aL, 1982). They vary in their accuracy, temporal resolution, spatial resolution and invasiveness, the latter being of interest because mechanical disturbance of plants has been shown to