Eliminating amino acid interference during spectrophotometric NH4+ analysis

Amino acids can interfere with NH4+ in spectrophotometric NH4+ determination hampering accurate quantification of the fate of NH4+ and dissolved organic N in soils. Serious interference has been reported for soils rich in organic matter, and for soils that have been fumigated, oven-dried or fertilized where between 5 and 60% of the NH4+ detected could be attributed to amino acid interference. We investigated whether a combination of gas diffusion and the classical Berthelot method can eliminate this interference, increasing the selectivity of the NH4+ analysis. We tested this approach using synthetic amino acid solutions and a large set of terrestrial samples (n = 353), including pore water samples, (fumigated or dried) soil extracts, and extracts of manures, composts and crop residues. The evaluated method produced accurate (recovery > 99.7%) and reproducible (standard error = 2.2%) NH4+ concentrations, eliminating any interference between amino acids and NH4+ (interference <0.3%). Interference from K2SO4 was also eliminated. The method is robust, broadly applicable and will improve our understanding of nitrogen cycling in various ecosystems, in particular those where high levels of amino acids occur. © 2011 Elsevier Ltd.