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

Variability in the natural enrichment of 15N in total nitrogen (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 to 6.79¿(expressed as ä15N 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 ä15N over a 12 m2 grid were within the range of 5.01 to 7.95¿. The ä15N 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 ä15N than the less humified organic N in sand and silt-sized fractions. One soil was separated into four depth layers in which ä15N in the inorganic N, mineralizable N, plant-extractable N and total soil N were determined. The first three parameters all had lower ä15N values than the total soil N. The ä15N of total N increased with soil depth, whereas there was no change with depth in the other three parameters.