EVALUATION OF METHODS FOR N-15 ANALYSIS OF INORGANIC NITROGEN IN SOIL EXTRACTS .2. DIFFUSION METHODS

Diffusion of inorganic nitrogen (N) onto acidified filters is a rapid method of preparing soil extracts for N-15 analysis by automated mass spectrometers (ANCA-MS); however, recent studies indicate variable recovery of N-15 added labels from some soils. A study was conducted to evaluate this method for recovery of N and N-15 by diffusion of 2 M KCl extracts from 10 diverse soils. The extracts were treated with: (i) ((NH4)-N-15)(2)SO4 and KNO3 (ii) (NH4)(2)SO4 and (KNO3)-N-15, or (iii) KNO3 and (NaNO2)-N-15. 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.