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 pasture soils was examined, From the disturbed soil, the top 10 cm of the profile was examined and the soil split into fractions based on particle size, Plant shoot and root material contained similar low enrichments in N-15, whereas recently deposited shoot residues were highly enriched, Differences between the soil fractions in observed total N did not reflect similar N-15 variation. However, the enrichment of humic material 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 the E(4)/E(6) ratio. Analysis of the profile from the undisturbed soil showed increasing N-15 enrichment with depth which corresponded well with 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 chemical complexity as shown by the E(4)/E(6) ratio.