Comparison of methodologies for nitrate determination in plants and soils

Several procedures have been described for the determination of nitrate by biological reduction, cadmium reduction, and salicylic acid electrophilic substitution. However, all are subject to interference by constituents of soil and plants. The aim of this work is to compare the results obtained using these methods with those obtained by ion chromatography. Because most of the interferences to which these methods are subject are due to organic matter, three pretreatments to reduce the presence of organic matter in the sample extracts were compared: 1 - adding activated charcoal; 2 - filtering through a 0.45 mum filter; and 3 - ultra-filtering through a 10 kDa membrane. A method and respective pretreatment were considered acceptable for measurement of nitrate in a particular soil or plant extract if. (i) nitrate added to the extraction medium used (without the soil or the plant material) was quantitatively recovered, and an acceptable reference curve could be established; (ii) between 95% and 105% of the nitrate added to the sample was recovered; and (iii) replicates agreed within 10%. Method recommendation for future analysis was based on: 1 - simplicity; 2 - accuracy; 3 - interferences 4 - equipment needed; and 5 - cost of analysis. Application of these criteria to the results indicated that the biological reduction of nitrate with nitrate reductase (NR) from E. coli is the most suitable method for nitrate determination in laboratories that are not equipped with an ion chromatograph.