Aboveground and belowground biodiversity responses to seed mixtures and mowing in a long-term set-aside experiment

Set-asides sown to wildflowers are introduced to promote agrobiodiversity. Often, sown seed mixtures have been used to enhance aboveground biodiversity while belowground biodiversity has received less attention. We studied in a 15-year-old set-aside experiment, the impact of seed mixtures (grass vs. meadow) and mowing (mown vs. unmown) on the abundance and diversity of plants, two aboveground invertebrate groups (pollinators and natural enemies of pest species) and one key belowground group, earthworms. We also studied the treatment effects on selected soil chemical properties. In general, a sown meadow seed mixture mostly benefitted aboveground biodiversity, whereas the impacts of late-season mowing were most discernible for earthworms. Plant species richness or coverage did not differ between seed mixtures, but plant coverage showed a positive response to mowing, while the impact on species richness was minor. The meadow seed mixture was dominated by Centaurea jacea. A non-mown meadow treatment supported the highest earthworm density and species richness, while the values were lowest in the mown meadow, where earthworm total mass was also at its lowest. In a grass seed mixture of intermediate density and richness, the mowing had no measurable effect on the earthworm community. Topsoil carbon, nitrogen and soluble phosphorus concentrations showed a typical gradient for non-inverted soils, with the highest concentrations in the 0–2.5 cm layer. The two uppermost soil layers (to 5 cm depth) had higher soluble phosphorus content under grass than under meadow seed mixture. Abundance of natural enemies of pests was not significantly different between the treatments, but for carabids there was a marginally significant positive response to mowing. Bumblebee species richness and the abundance of bumblebees and honeybees were highest in the mown meadow seed mixture plots. Butterfly species richness was marginally significantly higher in mown than in non-mown treatments in both seed mixtures. The results showed the potential of set-asides to promote both aboveground and belowground biodiversity through increased plant diversity.