A new approach for removing iron interference from soil nitrate analysis

The accurate measurement of soil nitrate ( NO 3 − ) is critical for determining rates of nitrogen (N) cycling and potential N losses from ecosystems. Iron (Fe) can interfere with the colorimetric NO 3 − analysis of soil extracts to cause the appearance of anomalously low NO 3 − 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 ( PO 4 3 − ) 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 NO 3 − determination in 0.3 mg N– NO 3 −  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 NO 3 − concentrations in the anaerobic soil slurry extracts were significantly higher with PO 4 3 − filtration compared to without filtration. After filtration, the measured NO 3 − concentrations were similar across all three buffers, suggesting that NO 3 − concentrations were accurately measured in PO 4 3 − filtered soil extracts regardless of the buffer used. The Fe:N ratio of Fe interference with NO 3 − determination depended on Fe concentration, NO 3 − 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 NO 3 − concentrations for unfiltered and PO 4 3 − filtered soil extracts to determine the potential for Fe interference in colorimetric NO 3 − determination as standard additions may not detect all forms of Fe interference.