Seasonality of the soil biota of grazed and ungrazed hill grasslands

Abstract

Temporal and spatial measurements of soil microbial biomass, activity and community structure and nematode abundance were made in grazed and ungrazed Agrostis-Festuca and Nardus dominated hill grasslands, with brown earth and podzolic soils, respectively. Microbial biomass and activity were significantly higher in podzolic soils with Nardus dominated vegetation, than in brown earth soils with Agrostis-Festuca vegetation. Phospholipid fatty acid analysis (PLFA) revealed differences in microbial community structure between soils. The ratio of fungal-to-bacterial fatty acids was higher in the brown earth than in the podzolic soil, whereas the diversity (Shannon-Weaver index) of PLFAs was greater in the podzolic soil. A large proportion of the microbial biomass (50%) and activity (40–70%) was within the surface 0–5 cm soil, with reduced amounts at lower depths of 5–10 cm and 10–15 cm. Microbial biomass and activity and nematode abundance showed pronounced summer maxima and winter minima. The long-term removal of sheep grazing from both grassland types resulted in significant reductions in microbial biomass and activity in the surface soil. The abundance of active soil fungi, measured as the fungal fatty acid 18:2ω6, was significantly reduced by the removal of sheep grazing, as was the ratio of fungal-to-bacterial fatty acids. Bacterial fatty acids were unaffected by the removal of sheep grazing. Possible mechanisms for these changes are discussed. Numbers of soil nematodes were significantly lower in the ungrazed sites. The effects of removing sheep grazing on nematodes and microbial community structure were most pronounced in brown earth soils. Factors responsible for these changes are discussed.