Effects of soil fauna on decomposition and nutrient dynamics in coniferous forest soil

Abstract

The influence of soil fauna on decomposition and nutrient cycles in coniferous forest soil was studied at the Department of Biology, University of Jyväskylä. Experiments were conducted in the laboratory using homogenized soil, needle litter of spruce and leaf litter of birch as test materials. The study included four steps: First, litter and humus were tested separately in "microcosms"; then the complexity of the system was increased stepwise while controlled laboratory conditions were maintained (="macrocosm experiments"). Finally, birch seedlings were planted in experimental chambers with reconstructed forest floor. In an additional experiment, one animal group was present at a time, or two groups in different combinations. Depending on the experiment, the incubations lasted 20 - 98 weeks. Before the experiments were initiated the materials were defaunated by freezing-thawing or microwave treatment and re-inoculated with microbes from the soil suspension. Soil fauna was introduced into half of the replicates. During the experiments evolution of CO₂ was monitored, and concentrations of N and P were analyzed in the water leachates drained through the test materials. At destructive samplings several soil variables were measured. The animal populations established themselves relatively well in the experimental chambers; in the structurally most complex systems the densities approached those found in similar soil in the field. It became evident, however, that the absence of predatory fauna leads to overabundance of their potential prey, thus emphasizing the importance of biotic relationships in modifying the community structure of soil fauna. In most cases the soil fauna increased the decomposition rate of dead organic matter. Moreover, a complex faunal community with various feeding guilds evidently enhances decomposition more than a simple system composed only of microbial feeding fauna does. In the later phases of incubation, however, the soil fauna began to influence the decomposition rate "negatively". As a general rule, the fauna also increased the mobilization of N and P in the systems. This was generally true throughout the whole period of incubation. There were also differences in mobilization of nutrients between structurally different faunal communities: the more diverse a system was, the greater the amounts of nutrients released. According to the hypothesis formed in previous experiments (enhanced nutrient cycling –> more nutrients available momentarily), the birch seedlings grew faster in the presence of soil fauna. After two growing seasons the growth of leaf, stem and root biomass was 70 %, 53 % and 38 % higher, respectively, in the refaunated systems. In addition, the nitrogen content in the leaves was many times greater than that of the control with microbes only. New techniques were developed to simulate the complexity of forest soil in the laboratory. Environmental variables can thus be controlled, allowing 1) repetition of the experiment without changes in the experimental conditions, and 2) manipulation of the desired variables at the same time. The design also renders possible the establishment of a "natural" decomposer community. This is a clear improvement on earlier microcosm studies, in which decomposition processes have been examined in systems with a radically oversimplified assemblage of soil organisms. These results confirm the general hypothesis that the soil fauna plays a considerable role in nutrient cycling and decomposition of dead organic matter. This conclusion is also valid in systems simulating Finnish coniferous forest floor. In addition, it was shown. that interrelations between faunal groups and species may exert a substantial influence on the below-ground processes, which may, in turn, reflect changes in other ecosystem levels as well. The enhanced plant growth caused by soil fauna has previously been shown by earthworms in cultivated or grassland soils, but not in other kinds of soils, nor with faunal groups other than earthworms.