Diffusion technique for ¹⁵N and inorganic N analysis of low‐N aqueous solutions and Kjeldahl digests

Abstract

Diffusion of ammonia is a common sample preparation method for the stable isotope analysis of inorganic nitrogen in aqueous solution. Classical diffusion methods usually require 6-12 days of diffusion and often focus on 15 N/ 14 N analysis only. More recent studies have discussed whether complete N recovery was necessary for the precise analysis of stable N isotope ratios. In this paper we present a newly revised diffusion technique that allows correct and simultaneous determination of total N and 15 N at% from aqueous solutions and Kjeldahl digests, with N concentrations down to sub-0.5-mg N L S1 levels, and it is tested under different conditions of 15 N isotope labelling. With the modification described, the diffusion time was reduced to 72 h, while the ratios of measured and expected 15 N at% were greater than 99% and the simultaneous recovery of total N was >95%. Analysis of soil microbial biomass N and its 15 N/ 14 N ratio is one of the most important applications of this diffusion technique. An experiment with soil extracts spiked with 15 N-labelled yeast showed that predigestion was necessary to prevent serious N loss during Kjeldahl digestion of aqueous samples (i.e. soil extracts). The whole method of soil microbial biomass N preparation for 15 N/ 14 N analysis included chloroform fumigation, predigestion, Kjeldahl digestion and diffusion. An experiment with soil spiked with 15 N-labelled yeast was carried out to evaluate the method. Results showed a highly significant correlation of recovered and added N, with the same recovery rate (0.21) of both total N and 15 N. A k N value of 0.25 was obtained based on the data. In conclusion, the diffusion method works for soil extracts and microbial biomass N determination and hence could be useful in many types of soil/water studies.

The fate of fertilizer nitrogen in a high nitrate accumulated agricultural soil

“…After distillation above, the remaining samples were digested with H 2 SO 4 and H 2 O 2 to convert all organic N to NH 4 + , and subsequently distilled with excess NaOH solution to determine extractable organic N (EON) concentration. Similarly, after conversion of EON (after diffusion above) to NH 4 + , the digested solution was transferred to plastic container (resistant to acid and alkaline solutions), kept frozen at −20 °C overnight, and then diffused with excess NaOH solution (4 °C) to prepare samples for analysis of 15 N abundance in the EON pool 51 . The 15 N abundances in the prepared samples were obtained by an elemental analyzer (Flash EA1112, Thermo Finnigan, USA) coupled with an Isotope-Ratio Mass Spectrometer (Delta plus XP, Thermo Finnigan, USA) (EA-IRMS).…”

The Influence of Altered Rainfall Regimes on Early Season N Partitioning Among Early Phenology Annual Plants, a Late Phenology Shrub, and Microbes in a Semi-arid Ecosystem

“…Following digestion, NO 3 was converted to NH 3 using Devarda's alloy and diffused onto acidified disks for 15 N analysis (procedure modified from Stark and Hart 1996;Hagedorn and Schleppi 2000;Chen and Dittert 2008). For persulfate digestion, 2.5 ml of soil extract was added to 10 ml of persulfate buffer; because N concentrations were below detection limits, unlabeled KNO 3 was added to increase N to at least 20 mg N/l.…”