Probing the Activities of Soil Invertebrates Using Stable Isotope Approaches

Soil dwelling invertebrates play a vital role in determining the physical properties and nutrient cycling in soil. Their diverse behaviours influence organic litter, water and gas transport. They impact on other soil biota, e.g. microbes, plants, other invertebrates, etc. via their various grazing and predatory activities, and their role in the comminution of litter influences the activities of other decomposer organisms. However, major challenges exist in the study of the activities of such invertebrates due to the small sizes of many of the key organisms and the opaque nature of soil. This paper will provide an overview of a number of new approaches that have been developed to investigate the behaviours of soil invertebrates. The techniques we employ are based on the use of stable isotopes, exploiting both natural abundance labelling and artificially isotopically enriched tracers. Experiments range from simple feeding and choice experiments in laboratory arenas to pot-based microcosm studies, and field experiments (Chamberlain et al., 2004; Black et al. in press). The philosophy underpinning this research is to exploit fundamental biochemical information to determine the activities of organisms. Thus, compound-specific stable isotope determinations are one of our major goals since these yield high specificity stable isotopic information, often at the biochemical building block level. Compound-specific approaches also have the virtue of enhancing analytical sensitivity, such that the δ ¹³C values of the biochemical components of individual specimens of low microgram-sized organisms, i.e. mesoinvertebrates, can be recorded their behaviours investigated (Evans et al., 2003; Black et al. in press).