Variations in natural enrichment of 15N in the profiles of some Australian pasture soils

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Abstract
Variability in the natural enrichment of 15N in total N and several N fractions was examined for profiles of pasture soils differing in parent material and period under Trifolium subterraneum. The natural enrichment of 15N in the total N of surface soils (0-5 cm) within a 400 km2 catchment ranged from 2.55-6.79.permill. (expressed as .delta.15N with respect to atmospheric N2). There was no relationship with parent material, period under T. subterraneum or concentration of total N in the soil. In one soil, lateral variations in .delta.15N over a 12 m2 grid were within the range of 5.01-7.95.permill.. The .delta.15N of total soil N always increased with depth in the soil profiles. This was associated with an increase in the stable clay-sized humus fraction, which had a higher .delta.15N than the less humified organic N in sand and silt-sized fractions. One soil was separated into 4 depth layers in which .delta.15N in the inorganic N, mineralizable N, plant-extractable N and total soil N were determined. The first 3 parameters all had lower .delta.15N values than the total soil N. The .delta.15N of total N increased with soil depth, whereas there was no change with depth in the other 3 parameters.