Effects of earthworms on soil processes in coniferous forest soil

Abstract

The effects of four earthworm species on decomposition, nutrient cycling and plant growth in coniferous forest soil were studied with microcosm experiments. The species were the epigeics Dendrobaena octaedra (Savigny) and Lumbricus rubellus (Hoffmeister), the endogeic Aporrectodea caliginosa tuberculata (Eisen), and an introduced anecic species A. velox (Bouché). Simulated forest soil was constructed into the microcosms, which were incubated in climate chambers for several months, including both summer and winter conditions. In two experiments, birch seedlings were also planted in the microcosms. In the experiments with Ac. tuberculata and A. velox the pH of the soil was manipulated by liming. In order to study the possibilities to introduce new deep burrowing species to northern latitudes, A. velox was also introduced to two different kinds of habitats in the field. In each experiment, the soil processes in the control microcosms (without earthworms) were compared to processes in identically prepared microcosms with earthworms. Evolution of C0₂ was monitored throughout the experiments, and concentrations of nitrogen and phosphorus were analysed from the leached water. At destructive samplings, numbers of animals, mass loss, pH, and KCl-extractable nutrients, as well as growth of the seedlings were determined. Generally, the earthworms increased the microbial activity in the soil, and they also enhanced the rate of nitrogen mineralization. Nitrification was also increased when nitrifying activity was present in the soil (in the presence of mineral soil layer, or after liming). The influence of earthworms on phosphorus mineralization was inconsistent being significant in some cases only. Earthworms clearly raised the soil pH when only the humus layer was included in the microcosms. The influence was opposite when mineral soil was under the organic horizon. The effect of earthworms on pH seemed to be related to the increased amounts of mineral nitrogen and changes in the ratio of ammonium to nitrate. In the presence of living plants, when concentrations of mineral nitrogen were very low, there were no differences in the pH between soils with or without earthworms. Both L. rubellus and A.c. tuberculata increased the growth of birch seedlings significantly. The nitrogen concentration of leaves was doubled and the shoot to root ratio increased in the presence of L. rubellus. Neither nitrogen fertilization, mechanical mixing with "artificial worms" nor the preworked soil by earthworms increased the growth of the seedlings. The capacity of earthworms for improving the structure and fertility of soils, which has previously been shown in temperate and tropical habitats by several authors, became herein confirmed for nortl1en1 coniferous forest soil. The results clearly showed that plants can derive benefit from enchanced decomposition and nutrient release from dead organic matter due to the activities of earthworms. Plants produced more biomass, and this biomass contained a higher concentration of nitrogen in the presence of earthworms. The results also encourage to study the possibilities to manipulate the earthworms in the field in order to increase the productivity of coniferous forests.