Tracing nitrogen derived from slurry in earthworms using 15N/14N stable isotope ratios at natural abundances

We used natural abundance measurements of nitrogen stable isotope ratios (N-15/N-14, expressed as delta(15)N parts per thousand) to trace nitrogen derived from dairy cattle slurry into two common earthworm species under field conditions. Three annual spring applications of N-15-rich slurry totalling 450 kg N ha(-1) resulted in significantly elevated tissue delta(15)N values in Aporrectodea caliginosa and A. longa collected in the autumn. This suggests that both earthworm species assimilated slurry-derived nitrogen. Earthworm delta(15)N values were unchanged after a single slurry application and in plots receiving inorganic N fertilizer compared to those from control plots. A. caliginosa, an endogeic species, had consistently higher delta(15)N values than the anecic A. longa. Preservation of earthworms in formalin for 58 days did not significantly alter their tissue delta(15)N signatures but affected delta(13)C values. This study demonstrates the potential of the natural abundance stable isotope approach for tracing anthropogenic N inputs through soil food webs in situ, on a field scale. (C) 1999 Elsevier Science B.V. All rights reserved.