Distillation and Microdiffusion Modifications for total Nh_4f Quantification and ¹⁵N Recovery

Abstract

Problem statement: Applying ¹⁵N techniques to accurately determine the fate of fixed ammonium (NH_4f) in the strongly and weakly held pools require modifications to existing methodologies. Modifications are necessary for measurement of total NH_4f in soils by direct digestion with 5 M HF: 1 M HCl, excluding alkali pretreatment, followed by distillation and quantification of NH₄. Quantification by distillation was used as a precursor to optimize microdiffusion protocols for continuous flow-isotope ratio mass spectrometry (CF-IRMS). This paper reports on the modifications applied to these procedures since the direct 5 M HF: 1 M HCl digestion of soil samples may also dissolve some organic N fractions. Approach: Distillation followed by ¹⁵N microdiffusion trials were conducted on soil digests amended with rice husks, manure, compost or glycine, using different molarities (2, 5, and 10 M) and volumes (5, 10, 15, 25, 32.5, and 40 mL) of KOH. Results: The distillation study identified 32.5 mL of 2 M KOH to be the optimum volume and molarity of KOH that must be combined with 10-mL aliquots of direct 5 M HF: 1 M HCl digests of each of seven soils to ensure that only NH4 in the digest is recovered and none of the organic N is hydrolyzed during the process. Results also showed that a minimum incubation time of 192 h was needed to trap approximately 100 µg ¹⁵N on the disks for subsequent accurate analysis by CF-IRMS, with minimal recovery of organic N. Conclusions/Recommendations: These findings support the use of a direct-digestion/distillation method to quantify total NH4f and thereby provide opportunity to distinguish between strongly and weakly held NH_4f in soils.