Recovery of soil nitrate by Kjeldahl analysis

Recovery of 15N‐labeled and non‐labeled NO3 ‐‐N (100 μg) during total N determination by a semimicro‐Kjeldahl procedure, not modified to include NO3 ‐‐N quantitatively, was studied in the presence and absence of soils varying in organic C. Recovery of NO3 ‐‐N was negligible in non‐soil systems, irrespective of whether water was present or not, unless an oxidizable C source, octyl alcohol (0.04 g), was added to the digestion mixture; addition of octyl alcohol resulted in a recovery of 78 and 87 μg NO3 ‐‐N in the presence and absence of water, respectively. Recovery of 100 μg NO3 ‐‐N added to soils containing from 0.1 to 3.8 g of C/cg of soil ranged from 34 to 90 pg NO3 ‐‐N in the absence of water. The recovery of the added NO3 ‐‐N was in the same order, but not proportional to, the organic C content of these soils. Addition of soil NO3 ‐‐N, determined by a separate method of analysis, to a regular Kjeldahl‐N value is not a satisfactory method for determining total soil N.