Is there food competition between Hormogaster elisae (Oligochaeta, Hormogastridae) and soil microarthropods at El Molar (Madrid)?

Abstract In a previous paper [M. Gutierrez, M. Ramajo, J.B. Jesus, D.J. Diaz, The effect of Hormogaster elisae (Hormogastridae) on the abundance of soil Collembola and Acari in laboratory cultures, Biol. Fertil. Soils 37 (2003) 231–236] we reported the negative effect of the earthworm Hormogaster elisae on microarthropods at El Molar (Madrid, Spain). This paper examines the possibility of food competition existing between them in laboratory cultures. Microcosms were constructed from plastic boxes and a cage made from 2 mm mesh, with two earthworms inside, was placed at the centre of each. The soil of the cage was enriched with defaunated soil sieved to 0.2 mm or with defaunated, homogenised topsoil (first 3 cm) to increase the quantity of organic matter. In the controls, earthworms were absent from both microcosm compartments. The microcosms were kept at 15 °C for 21 days before being dismantled and the microarthropods extracted using the Berlese–Tulgren method, identified and counted. The numbers inside and outside the cages were then compared, and the data obtained were subjected to analysis of variance (ANOVA). In the microcosms enriched with the ≤0.2 mm soil fraction, nearly all the microarthropods decreased in number inside the cages, when earthworms were present despite containing extra organic matter. In the controls, no significant differences were seen between the compartments, suggesting that these particles are not used as a food source by microarthropods. In the microcosms enriched with homogenised topsoil, the microarthropods were just as numerous in both compartments or indeed even more numerous inside the cage. This shows that this material nullifies the negative effect of earthworms on microarthropod numbers. The soil horizons closest to the surface might provide food resources used by both groups, for which they compete.