Alternative Methods for Measuring Inorganic, Organic, and Total Dissolved Nitrogen in Soil

There 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 modified methods for the measurement of NH 4 + , NO 3 − , total dissolved N (TDN), and soil microbial biomass N (SMBN) in soils. The methods are based on a microtiter plate format and are rapid and simple to perform. Ammonium is quantified by a colorimetric method based on the Berthelot reaction. Total dissolved N and SMBN (by CH 3 Cl fumigation‐extraction) are quantified as NO 3 − after alkaline persulfate oxidation. Nitrate is estimated directly or after persulfate oxidation by reduction of NO 3 − to NO 2 − by VCl 3 and subsequent colorimetric determination of NO 2 − by acidic Griess reaction. The 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 quantification were 7 μg NH 4 + –N L −1 , 55 μg NO 3 − –N L −1 , and 0.275 mg TDN L −1 The 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. The 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.