Stable isotope natural abundances of soil, plants and soil invertebrates in an upland pasture

In an exploratory study of below-ground trophic relations, natural abundances of the stable isotope pairs C-13/C-12 and N-15/N-14 (delta(13)C and delta(15)N) were measured on samples of plant shoots, whole soil and soil invertebrates taken in 1994 from two contrasting treatments of a pre-existing experiment: (1) continued grazing by sheep, with N:P:K fertiliser additions from 1990 onward; and (2) no added fertiliser, but sheep grazing removed entirely. Stepwise trophic increases were documented better by seasonal averages of delta(13)C and delta(15)N and by seasonal trends, composed of data collected on several occasions, than by instantaneous values. Seasonal changes in plant monocot vs dicot differences for shoot delta(13)C and delta(15)N were detected from patterns over several individual sampling dates; instantaneous samples were neither statistically significant nor qualitatively interpretable. Significant isotopic differences between treatments were evident in invertebrates only as seasonal averages or trends. Seasonal variations of delta(13)C and delta(15)N in earthworms and slugs may reflect previously unsuspected invertebrate behaviour. Whole soil delta(13)C was static through time and across treatments. Whole soil delta(15)N changed seasonally, an effect consistent with N-15/N-14 fractionation, e.g. during denitrification. (C) 1998 Elsevier Science Ltd. All rights reserved.