Effects of earthworms on nutrient dynamics, carbon turnover and microorganisms in soils from cool temperate forests of the Canadian Rocky Mountains — laboratory studies

Abstract

Two experiments were set up to study the effect of invasion of forest soils of the Canadian Rocky Mountains by earthworms on nutrient dynamics, carbon turnover and soil microorganisms. In the first experiment material from four soil layers of an aspen ( Populus tremuloides Michx.) forest (L, F, H, A h) and from F/H layers of a pine ( Pinus contorta Loud.) forest was placed separately in laboratory systems. In this experiment the effect of Dendrobaena octaedra (Savigny) on nutrient leaching (N and P) and on carbon mineralization was monitored at regular intervals for 84 days. In addition, microbial biomass (substrate induced respiration (SIR) was determined at the end of the experiment. In the second experiment the effect of two earthworm species ( D. octaedra and Octolasion lacteum (Oerley)) on nutrient leaching, carbon turnover, microbial biomass and bacterial/fungal ratio in reconstructed aspen forest floor was determined at destructive samplings after 28, 56 and 84 days of incubation. In addition, the effect of the earthworms on the composition of the fungal community in L/F layer was studied after 56 days of incubation. D. octaedra caused a strong increase in leaching of mineral N and P from organic soil materials but not from aspen A h. The effect of the earthworms increased during the incubation in most of the layers. In aspen L, A h and pine F/H D. octaedra caused a significant increase in NH 4 +/NO 3 − ratio in leachates; in aspen F and H leaching of mineral nitrogen occurred almost exclusively as NO 3 − irrespective of earthworms. D. octaedra generally caused a reduction in microbial biomass. In the second experiment the effect of D. octaedra on nutrient leaching was assumed to be masked by internal nutrient immobilization. Microbial biomass in L/F layer was reduced by both earthworm species but enhanced in H and A h layer which was assumed to be caused by mixing of soil layer materials. Bacterial/fungal ratio was generally increased by both earthworm species in L/F and A h material but no effect was found in H layer. The dominance structure of the fungal community was modified by the earthworms but the effect was small. Carbon turnover was generally not significantly affected by earthworms in both experiments.