A new approach for removing iron interference from soil nitrate analysis

The accurate measurement of soil nitrate (NO3-) is critical for determining rates of nitrogen (N) cycling and potential N losses from ecosystems. Iron (Fe) can interfere with the colorimetric NO3- analysis of soil extracts to cause the appearance of anomalously low NO concentrations. To resolve the interference, imidazole or NH4Cl-DTPA has been recommended to replace NH4Cl-EDTA as the buffer in the analysis. Here we show that phosphate (PO43-) filtration can completely remove Fe interference whereas any of these buffers alone may not. Regardless of which buffer was used, 5.5-55 mg Fe L-1 ferrous iron (Fe2+) interfered with NO3- determination in 0.3 mg N-NO3- L-1 2 M KCl solutions. Phosphate filtration removed all detectable Fe2+ in 2 M KCl anaerobic soil slurry extracts with high Fe2+ concentrations (25.9 +/- 1.7 mg Fe2+ L-1). With each of the three buffers tested, the measured NO3- concentrations in the anaerobic soil slurry extracts were significantly higher with PO43- filtration compared to without filtration. After filtration, the measured NO concentrations were similar across all three buffers, suggesting that NO3- concentrations were accurately measured in PO43- filtered soil extracts regardless of the buffer used. The Fe:N ratio of Fe interference with NO3- determination depended on Fe concentration, NO3- concentration, buffer, and cadmium column age, so that the amount of Fe interference that could occur can be difficult to predict. We suggest comparing measured NO3- concentrations for unfiltered and PO43- filtered soil extracts to determine the potential for Fe interference in colorimetric NO3- determination as standard additions may not detect all forms of Fe interference. (C) 2011 Elsevier Ltd. All rights reserved.