Ludwig Beck

Structure and function of soil fauna communities in Amazonian anthropogenic and natural ecosystems

The soil biological conditions of two 5-year-old polyculture tree plantations in Amazonia were studied comparatively to a 13-year-old secondary forest and a nearby undisturbed primary forest. The polycultures had been planted to regenerate the soil degraded by land preparation and a former rubber tree monoculture. Abundance and biomass of functional groups of soil meso- and macrofauna were measured at three-months-intervals over 2 years and litterbag experiments with fauna exclusion were carried out. This paper concentrates on the description of the structure of the soil fauna communities, forming the background for an evaluation of the decomposition processes in polyculture plantations. Decomposition rates were strongly determined by the macrofauna particularly in primary forest, where large earthworms, termites and ants dominated the soil fauna. In the plantations, where litter originated predominantly from the non-planted, adventitious vegetation, an abundant decomposer fauna was found, in which however other groups or species dominated. Although decomposition rates in the plantations were about 60 % lower and soil biological variables like organic matter-, nitrogen-content and water holding capacity were slightly lower than in the primary forest, conditions seem favourable for a manipulation of the soil fauna by management of secondary vegetation and litter quantities.

Litter decomposition, microbial biomass and activity of soil organisms in three agroforestry sites in central Amazonia

Soil organisms play a central role in the decomposition of organic matter. The activity of soil organisms was comparatively examined in three experimental sites in central Amazonia (Brazil): a peach palm monoculture (Bactris gasipaes) a, rubber tree plantation (Hevea sp.), and an agroforestry system (four tree species planted in rows, the space between covered by upcoming secondary vegetation). The overall decomposition rates in the systems and the role of different groups of soil organisms (macrofauna, mesofauna, microflora) were studied with leaf litter (Vismia guianensis) enclosed in litter bags. Microbial respiration and biomass (SIR method) in litter and soil were measured (IRGA). Microbial respiration in all sites decreased in the gradient litter > topsoil (0–5 cm) > soil at 5–15 cm. The highest decomposition rate was always observed in the litter bags of coarse mesh size, pointing to the crucial role of the macrofauna in maintaining a high decomposition rate of the organic material in all systems. The Hevea (k = 3.4) and the Bactris plantation (k=3.1) both showed the highest decomposition rates, followed by the polyculture system (k=1.9). The Bactris plantation also had the highest level of microbial respiration and biomass in litter and soil. We discuss these findings in the light of data on rainfall, pH and canopy closure. They suggest that microclimate is a more important factor than biomass in determining litter decomposition rates and activity of soil organisms at these sites.

Resistance to disturbance is a diverse phenomenon and does not increase with abundance:The case of oribatid mites

Abstract The resistance of a population to a disturbance can be described by different proxies, such as abundance, long-term abundance trend, and relative abundance in the more disturbed part of the habitat. Each proxy reflects a different aspect of resistance. Here we investigated oribatid mite species and asked: (i) Are the species generally less resistant with respect to one of these aspects than another? (ii) Is there a correlation between different aspects of resistance across species? (iii) Are abundant species more resistant than rare species? We disturbed a forest soil by a single application of diflubenzuron pesticide onto the litter layer and followed 14 taxa from 10 months before to 18 months after application. In the analysis we adopted the concepts of effect size and of phylogenetically independent contrasts. We found large interspecific differences in the degree of resistance. Most species were less resistant with respect to their relative abundance in the more disturbed part of the habitat (the litter layer) than with respect to their abundance or abundance trend. We found no correlation between any two aspects of resistance, or between any aspect of resistance and abundance. We explain interspecific differences in resistance by interspecific differences in life strategies, and by a relaxation of interspecific competition due to the disturbance.