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

Abstract

We investigated the specific diet and habitat of earthworms in relation to land use changes by integrating spatial and temporal scales and by using stable isotope ( 13C and 15N) techniques. The study involved two sites: Santiago (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 grassland (1 year grassland); the same procedure was followed for the grassland resulting in two treatments, 2 years grassland and 1 year maize. At the English 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, treatment and site at the peak of the maize growth and after harvesting. Spanish soils were 13C-enriched and 15N-depleted when compared to the English ones, which was also reflected in the earthworm tissue, and allowed a direct relationship between the delta values of the animals and the cropping treatments. The enrichment in the 13C values of the worms feeding on the maize (C 4) plots, when compared to those found under the grassland (C 3) plots, was greater than the difference detected in the C 4 vs C 3 soils. This result clearly 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 highest N isotope values as a result of preferential feeding in deeper soil profiles. This information proves that combined C and N isotope analysis constitutes 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.