VARIATION IN N-15 NATURAL-ABUNDANCE OF SOIL, HUMIC FRACTIONS AND PLANT MATERIALS IN A DISTURBED AND AN UNDISTURBED GRASSLAND

The natural abundance of delta(15)N in disturbed and undisturbed pastu re soils was examined, From the disturbed soil, the top 10 cm of the p rofile was examined and the soil split into fractions based on particl e size, Plant shoot and root material contained similar low enrichment s in N-15, whereas recently deposited shoot residues were highly enric hed, Differences between the soil fractions in observed total N did no t reflect similar N-15 variation. However, the enrichment of humic mat erial extracted from the largest soil fraction was considerably lower in N-15 relative to that from the smaller fractions. The complexity of the humic material from the larger fractions was less according to th e E(4)/E(6) ratio. Analysis of the profile from the undisturbed soil s howed increasing N-15 enrichment with depth which corresponded well wi th visible soil horizons and showed an inverse relationship with total soil N. This N-15 enrichment was mirrored by the enrichment in humic materials down the profile and also corresponded with an increasing ch emical complexity as shown by the E(4)/E(6) ratio.