Microflora, Protozoa and Nematoda in Lumbricus terrestris burrow walls: a laboratory experiment

Abstract

Anecic earthworms Lumbricus terrestris L. were kept in laboratory microcosms containing soil and litter from a lime (Tilia cordata) and beech (Fagus sylvatica) forest. Nutrient (mineral nitrogen and phosphorus) contents, microbial activity and densities of protozoa and nematodes were determined in burrow walls and control soils after 165 days. Well-developed burrow linings consisting of earthworm faeces were formed in the “Tilia” treatment, but not in the “Fagus” treatment, presumably because beech litter was not an adequate food resource for L. terrestris. Consequently, increases in microbial biomass, basal respiration and microbial volume in burrow walls compared to surrounding soil were significant in the “Tilia” treatment only. However, in both treatments burrow walls were strongly enriched in mineral nitrogen and phosphorus. The density and biomass of protozoa were significantly greater in burrow walls compared to the control soil. The numbers of naked amoebae increased by similar factors of 4 and 3.5 in burrow walls of the “Tilia” and “Fagus” treatment, respectively. Flagellate density increased more than tenfold in burrow walls of the “Tilia” treatment but only twofold in the “Fagus” treatment. In addition, a comparatively large ciliate population was present in burrow walls in the “Tilia” treatment. The total abundance of protozoa was significantly correlated with the contents of inorganic N and P in the samples (r = 0.68 and 0.63 respectively, P< 0.03) suggesting that protozoan grazing participated in the mobilisation of nutrients from microbial biomass. The total nematode density was increased in burrow walls by a factor of 3 in the “Tilia” treatment (mostly due to bacterivorous nematodes), but was not affected in the “Fagus” treatment. In both treatments density of fungivorous nematodes increased, while that of plant parasites decreased in burrow walls compared to the control soil. It is concluded that the grazing pressure of protozoa and nematodes may control the dynamics of the microbial succession in earthworm burrow walls, strongly affecting nutrient cycling processes in these microhabitats.