The influence of dietary nitrogen and phosphorus on Cd accumulation in the woodlouse Porcellio scaber Latr.

Bioaccumulation of potentially toxic metals by litter-feeding invertebrates is determined not only by the concentration of metal in the diet, but also by the flux of food through the body. Nitrogen and phosphorus are the main elements regulating food consumption and so are expected to affect the bioaccumulation of trace elements such as cadmium. To test this idea, we applied a three-factor orthogonal experimental design to estimate the effects of nitrogen, phosphorus and cadmium additions to the food on cadmium accumulation by the terrestrial isopod Porcellio scaber. Cd, N and P were added to milled poplar litter in concentrations of 0, 10 and 20 μg

Agar as a medium for removing soil from earthworm guts.

Earthworms were kept on a water-agar gel for 96 h at 20°C, after that time all soil had been voided from their guts. Earthworms treated in this way may be used for soil-free chemical analysis, as required in biomonitoring programmes for soil contamination. (C) 2000 Elsevier Science Ltd

Microbial links and element flows in nested detrital food-webs.

Microbial links are obligate in any food chain in soil, because detritivorous animals derive essential amino acids and other resources from microorganisms. To degrade recalcitrant substrates such as cellulose, soil animals do not produce their own cellulase, but they use cellulases derived from gut microorganisms. We argue that in detrital food-webs, carbon (energy) is usually not a limiting factor. Other elements, for example nitrogen and phosphorus, are present in ratios (relative to carbon) which are lower in the food than in the animal itself, and are more likely to be limiting. This implies that we need to consider the dynamic stoichiometry of N, P and C in the food-web and we cannot assume a fixed ratio between the three elements. In addition, detrital food-webs consist of communities at three different scales. The bacteria-algae-Protozoa compartment is nested inside the fungi-microarthropod compartment and this is in turn is contained within the earthworm-rhizosphere compartment. Animals of the higher levels consume communities of the lower levels as a whole. Present approaches for the structure of detrital food-webs do not take this nested structure into account. Our hierarchical concept of food-web structure may explain why soil pollutants that are not directly toxic to animals, may still affect the functioning of soil animals, either through deterioration of their food resources or through effects on internal food-chains