Evaluation of methods for nitrogen‐15 analysis of inorganic nitrogen in soil extracts. II. Diffusion methods

Abstract

Diffusion of inorganic nitrogen (N) onto acidified filters is a rapid method of preparing soil extracts for ¹⁵N analysis by automated mass spectrometers (ANCA‐MS); however, recent studies indicate variable recovery of ¹⁵N added labels from some soils. A study was conducted to evaluate this method for recovery of N and ¹⁵N by diffusion of 2 M KC1 extracts from 10 diverse soils. The extracts were treated with: (i) (¹⁵NH₄)₂SO₄ and KNO₃, (ii) (NH₄)₂SO₄ and K¹⁵NO₃, or (iii) KNO₃ and Na¹⁵NO₂. Diffusions of ammonium‐nitrogen (NH₄ ⁺‐N) and nitrate‐nitrogen (NO₃ ^‐‐N) were either done sequentially or combined; a sulfamic acid pretreatment was employed for diffusion of NO₃ ^‐‐N in the presence of NO₂ ^‐‐N. Nitrogen‐15 analyses of diffused NH₄ ⁺‐N by ANCA‐MS yielded 99 to 100% recovery. The range for diffused NO₃ ^‐‐N was 97 to 102%, using a surfactant to improve recovery and minimize variance. Isotopic carryover of NH₄ ⁺ into NO₃ ^‐ diffusions was observed to occur, but could be minimized with pretreatment and analytical calibrations.

Notes

Visiting from the Nuclear Institute for Agriculture & Biology, Faisalabad, Pakistan.