Lower carbon but higher mesofauna β-diversity under bracken in open habitats not indicative of woodland soil: case study

Bracken (Pteridium aquilinum) is of major concern to land managers due to its ability to invade grassland and heathland systems, but it is believed to indicate where woodland once was or where conditions may be favourable for woodland in the future. The effects of bracken on the soil system are also not well understood despite the key role of soils in carbon and nutrient cycling and as one of the most biodiverse habitats on the planet. Whilst soil mesofauna are amongst the most abundant animals and present at multiple trophic levels in the soil food web, the importance of their diversity remains underappreciated and overlooked in policy and land management. Due to their role in litter decomposition, mesofauna should be explored in light of the conspicuous shift in litter form and quantity when bracken is present. In this study, soil mites, springtails, nematodes and soil properties in grassland and heathland were compared to those in woodland in the presence and absence of bracken to ascertain whether these communities could provide useful detail for potential woodland restoration. Under bracken soils, total and water-extractable organic carbon, water-extractable nitrogen, C:N ratios, Zn2+ concentrations and mesofauna abundances were lower. Reductions in microinvertebrate α-diversity under bracken were specific to heathland only, whilst β-diversity in grassland and heathland was enhanced under bracken by increased morphotype turnover. These results indicate that soil nutrients and communities under bracken do not represent a shift towards those found in woodland soils and are more strongly driven by ecosystem type than plant species composition or the presence of bracken. Future work should investigate whether bracken is causing the observed differences in soil nutrients and community structure, focusing on possible mechanistic drivers such as changes in bacterial-fungal ratios and microbial activity.