1018 SSSAJ: Volume 74: Number 3 • May–June 2010 Soil Sci. Soc. Am. J. 74:1018–1027 Published online 30 Mar. 2010 doi:10.2136/sssaj2009.0389 Received 14 Oct. 2009. *Corresponding author (wolfgang.wanek@univie.ac.at). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Alternative Methods for Measuring Inorganic, Organic, and Total Dissolved Nitrogen in Soil Nutrient Management & Soil & Plant Analysis P lant-available N is one of the main constraints to productivity in agroecosystems and an important parameter in deining the structure and function of natural ecosystems. In the soil there are many soluble N pools (e.g., dissolved organic N, NH 4 + , NO 3 - , and NO 2 - ), which are interdependent on the size of and luxes from other N pools (total soil N and microbial biomass N), as well as environmental and physicochemical factors. Studies of isolated pools or processes rarely reveal the subtle interactions that drive globally important ecosystem functions and processes, such as the loss of greenhouse gases, NO 3 - leaching, or the terrestrial C cycle. he complex nature of the N cycle therefore calls for a holistic approach to the understanding of the system, combining measurements of the sizes and dynamics of multiple soil N pools and of net and gross N transformation processes. One of the constraints of a holistic approach is the range and number of pa- rameters that must be simultaneously measured. Many of these measurements are laborious, time consuming, or require expensive specialized equipment. his is an issue in remote ield settings without state-of-the-art laboratories and in develop- ing countries. Particularly in developing countries, however, a better understand- ing and management of the N cycle could yield the greatest beneits, not only from an agroecological perspective, but also through the generation of validated experi- mental data for predictive N cycle models. Rebecca Hood-Nowotny Dep. of Chemical Ecology and Ecosystem Research Univ. of Vienna Althanstrasse 14 1090 Vienna, Austria Nina Hinko-Najera Umana Dep. of Forest and Ecosystem Science Univ. of Melbourne Water St. Creswick, VIC 3363, Australia Erich Inselbacher Dep. of Forest Ecology and Management Swedish Univ. of Agricultural Sciences SE-901 83 Umeå, Sweden Petra Oswald- Lachouani Wolfgang Wanek* Dep. of Chemical Ecology and Ecosystem Research Univ. of Vienna Althanstrasse 14 1090 Vienna, Austria here are numerous methods for measuring inorganic, dissolved organic, and microbial N in soils, although many of these are complex or require expensive equipment. We have modiied methods for the measurement of NH 4 + , NO 3 − , total dissolved N (TDN), and soil microbial biomass N (SMBN) in soils. he methods are based on a microtiter plate format and are rapid and simple to perform. Ammonium is quantiied by a colorimetric method based on the Berthelot reaction. Total dissolved N and SMBN (by CH 3 Cl fumigation-extraction) are quantiied as NO 3 − ater alkaline persulfate oxidation. Nitrate is estimated directly or ater persulfate oxidation by reduction of NO 3 − to NO 2 − by VCl 3 and subsequent colorimetric determination of NO 2 − by acidic Griess reaction. he new suite of methods was compared with conventional methods such as high-performance anion- exchange chromatography for NO 3 − and high-temperature catalytic oxidation for TDN. Our methods produced comparable detection limits, linearities, and precisions compared with the conventional methods. Limits of quantiication were 7 μg NH 4 + –N L −1 , 55 μg NO 3 − –N L −1 , and 0.275 mg TDN L −1 . he accuracy of the proposed methods was excellent, with recoveries of added NH 4 + , NO 3 − , and glycine ranging between 96 and 99%. Linearities of the respective calibrations were high (R 2 > 0.99), and precisions for NH 4 + (CV = 2.1%), NO 3 − (CV = 3.5%), and TDN (CV = 3.9%) were comparable to the reference methods. he simplicity, rapidity, and low cost of the proposed methods therefore allow an expansion of the scope and range of N cycle studies where sophisticated instrumentation is not available. Abbreviations: DON, dissolved organic nitrogen; HPAEC, high-performance anion-exchange chromatography; HTCO, high-temperature catalytic oxidation; LOD, limit of detection; SMBN, soil microbial biomass nitrogen; TDN, total dissolved nitrogen.