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

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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.