Bewertung von befahrungsbedingten Bodenveränderungen mittels Bakterienpopulationen | Evaluation of machine-induced soil compaction in forest soils by means of changes in soil bacteria populations

The use of heavy logging machinery in the Swiss forest most often leads to the formation of ruts in which the soil suffers profound and long-lasting damage. In Switzerland the establishment of legal guidelines concerning the measurement of parameters in forest soil physics is being devised, although the necessary scientific bases are still largely lacking. The central question is what degree of structural change in the soil is to be regarded as soil damage. The purpose of this study is to investigate the functional relationship between the physical properties of the soil (compaction), the way these influence vital soil functions and the composition of the bacterial population in the deep ruts. Wheel tracks were divided into three categories, depending on the degree of compaction, by an optical evaluation of easily recognizable morphological characteristics. Soil samples from places showing typical characteristics were investigated for their physical parameters, and genetic analyses of the soil bacteria were carried out. First results show that mechanical pressure on soil by heavy harvesting machines interferes with the balance of gases in the soil of the ruts, and this leads to a shift in the species of soil bacteria present. Anaerobic conditions in the compacted wheel tracks encourage those bacteria adapted to conditions of low oxygen, and increase the production of trace gases (methane, nitrous oxide). The study shows that bacterial populations are sensitive to mechanical pressures on the soil and react to changing habitats. They can thereby be used in an evaluation of the degree of structural damage in the forest soil after the passage of machines.