Soil microbial community assembly precedes vegetation development after drastic techniques to mitigate effects of nitrogen deposition

Oligotrophic semi-natural systems are threatened by high levels of nitrogen deposition. To mitigate these effects, drastic techniques such as sod-cutting and topsoil removal are applied to reduce nitrogen loads in existing systems and expand their area on former agricultural fields. We assessed the effects of these techniques along with the influence of previous land-use, isolation and vegetation development on subsequent microbial community assembly in restored agricultural areas. Microbial community phenotypic structure was measured using PLFA-analysis, along with soil chemistry and vegetation development. Differences in soil nitrogen pools due to restoration techniques were the most differentiating factor for both microbial community assembly and vegetation development. Only after topsoil removal was resemblance of both below- and above-ground communities to well-developed heathlands increased within 10–15 years. After sod-cutting both microbial community and vegetation composition remained more similar to agricultural sites. The relative contribution of agricultural sites and heathlands in the direct vicinity had more pronounced effects on local microbial community composition than current land-use in all study sites including agricultural areas and heathlands. Vegetation development was apparently of minor importance for microbial community assembly, since characteristic belowground assembly preceded that of aboveground development in both restoration contexts

Facilitating ecosystem assembly: plant-soil interactions as a restoration tool

Although plant-soil interactions are increasingly recognized as an important factor in ecosystem restoration, their effects on community assembly during de novo ecosystem establishment are largely unknown. In a heathland restoration trial after topsoil removal we introduced either only aboveground heathland species with fresh herbage or both above- and belowground heathland species with sods to facilitate community assembly. Sod inoculation increased resemblance of the microbial community to the reference system, with a higher fungal and lower bacterial proportion to the community structure. Also densities of bacteriophagous and phytophagous nematodes, Acari and Collembola increased after sod inoculation. The cover of heathland plant species increased by 49% after sod inoculation. The introduction of solely aboveground heathland species increased the cover of these species by only 13%, and did not affect soil community assembly. Additionally, the increase in cover of heathland species over time was inversely correlated to the cover of mesotrophic grassland species. Inverse correlations were also observed between changes in fungal and bacterial abundances. Simultaneous introduction of key species of both above- and below-ground communities had a critical effect on the establishment of both communities, providing a potential shortcut for successful restoration of target ecosystems on disturbed soils