Reductions in microbial biomass along disturbance gradients in a semi-natural grassland

Abstract

Soil disturbance is a common phenomenon in semi-natural sandy grasslands, caused by management practices, livestock trampling and erosion. To better understand ecosystem processes when the surface layers of the soil are disturbed, we investigated the effects of disturbance on different functional microbial groups by the use of signature fatty acid analysis. Four levels of disturbance were investigated in local disturbance gradients in an area with semi-natural sandy grassland. Festuca brevipila Tracey (Poaceae) was used as the study plant to investigate the response of plant and microbial biomass to disturbance. The plant was chosen since it dominated the site and was present at all levels of disturbance. The amount of arbuscular mycorrhizal (AM) fungi in the roots of the study plant was not affected by soil disturbance. The amount of AM fungi, saprotrophic fungi and bacteria in the soil was significantly reduced by increasing level of soil disturbance, coinciding with a reduced vegetation cover and decreasing soil organic matter content. Increasing levels of soil disturbance resulted in elevated soil pH and concentration of inorganic N, whereas the water content and organic matter content was reduced. The shoot N:P ratios of the grass investigated increased with the level of disturbance. We conclude that soil disturbance disfavours fungi due to mechanical interruption of hyphae and through reduced plant biomass. We also conclude that soil disturbance disfavours bacteria in the long run due to reduced organic matter content.