Spatio-temporal variation of stable isotope ratios in earthworms under grassland and maize cropping systems

We investigated the specific diet and habitat of earthworms in relation to land use changes by integrating spatial and temporal scales and by using st able isotope (C-13 and N-15) techniques. The study involved two sites: Sant iago (Northwest Spain) and North Wyke (Southwest England), both consisting of long term grassland which was partly converted to a maize crop in 1997. In 1998, the maize crop in Spain was divided into two, and one half was re- planted with maize (2 years maize) and the other half reconverted to grassl and (1 year grassland); the same procedure was followed for the grassland r esulting in two treatments, 2 years grassland and 1 year maize. At the Engl ish site only 2 years maize and 2 years grassland were under investigation. Within each of the four treatments in Spain and the two in England, three replicate plots were established. Random soil samples from three different depths (0-10, 10-20 and 20-30 cm), and earthworm specimens belonging to four different ecological categories (epigeic, anecic, epi/anecic and endogeic), were taken from each plot, trea tment and site at the peak of the maize growth and after harvesting, Spanish soils were C-13-enriched and N-15-depleted when compared to the Eng lish ones, which was also reflected in the earthworm tissue, and allowed a direct relationship between the delta values of the animals and the croppin g treatments. The enrichment in the C-13 values of the worms feeding on the maize (C-4) P lots, when compared to those found under the grassland (C-3) plots, was gre ater than the difference detected in the C-4 vs C-3 soils. This result clea rly indicates selective feeding by earthworms with a preference for fresh C -4 residue over older native C-3. Different ecological and age groups appeared to consume organic material of differing quality, with endogeic species and mature worms showing the high est N isotope values as a result of preferential feeding in deeper soil pro files. This information proves that combined C and N isotope analysis const itutes a powerful tool in studying feeding ecology and emphasises the need for long-term studies which incorporate spatial and temporal scales to the experimental set-up. (C) 2001 Elsevier Science Ltd. All rights reserved.