Distillation and Microdiffusion Modifications for total Nh4f Quantification and 15N Recovery

Abstract

Applying 15 N 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 NH 4. 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 15 N 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 NH 4 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 15 N 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 NH 4f and thereby provide opportunity to distinguish between strongly and weakly held NH 4f in soils.

Ball milling pretreatment affects the content of fixed ammonium in soils in response to the content of exchangeable ammonium

“…1) in order to measure the concentration of total fixed NH 4 + directly (Matsuoka and Moritsuka 2011). This modification is expected to minimize the possible artifacts caused by the indirect determination of total fixed NH 4 + using separate samples (Eudoxie and Gouveia 2008), in which the concentration of exchangeable NH 4 + is subtracted from that of NH 4 + extracted by HF-HCl. To extract exchangeable NH 4 + , we added 10.0 mL of 2 mol L −1 KCl to 2.00 g of soil in a 50-mL centrifuge tube.…”

Increase in the Capacity of Interlayer Sites in a Paddy Soil as a Possible Factor Affecting the Dynamics of Fixed Ammonium at the Initial Stage of Anaerobic Incubation

“…In the preliminary experiment, grinding of the original soil was found to increase the concentration of nonexchangeable K (a cumulative value from step 2 to steps 3-5) from An experimental procedure to determine total and strongly fixed NH 4 + and the capacity of interlayer sites. Weakly fixed NH 4 + was calculated by subtraction of strongly fixed NH 4 + measured by Silva and Bremner (1966) from total fixed NH 4 + measured by Eudoxie and Gouveia (2008).…”

“…After the acid digestion, the supernatant was obtained by centrifugation and stored in a refrigerator for no longer than 2 days. The supernatant (7 mL) was steam distilled for 3 min with 4.55 mL of 10 mol L −1 potassium hydroxide (KOH) and 18.2 mL of distilled water (equivalent to 32.5 mL of 2 mol L −1 KOH per 10 mL of supernatant in the original method) to minimize recovery of alkali labile organic nitrogen during the distillation, because Eudoxie and Gouveia (2008) reported that minimal amounts of organic N from spikes were recovered after approximately 3 min of distillation at this KOH concentration (less than 0.2 percent of added N), whereas more than 97 percent of the spiked NH 4 + was recovered. The amount of NH 4 + liberated during the distillation (total fixed NH 4 + ) was measured by the indophenol method.…”

“…Total fixed NH 4 + was measured using the method described by Eudoxie and Gouveia (2008). In this method, 0.5 g of the finely ground soil, with and without anaerobic incubation, was digested with 5 mol L −1 hydrofluoric acid (HF):1 mol L −1 HCl at a soil/solution ratio of 1:20 for 24 h on a reciprocal shaker.…”

“…Ammonium ions (NH 4 + ) trapped in the 2:1-type clay-mineral interlayers (clay-fixed NH 4 + ) are classified into weakly and strongly held fractions (Paramasivam and Breitenbeck 2000;Eudoxie and Gouveia 2008). Weakly fixed NH 4 + is regarded as more reactive than strongly fixed NH 4 + (Gouveia and Eudoxie 2007;Matsuoka and Moritsuka 2011).…”