Exploring the minimum light availability to support plant biomass and soil life in solar parks

Solar parks are a rapidly expanding form of land use, primarily aimed at producing renewable energy. However, the aim is to make them multifunctional, and limit negative impacts on soils and biodiversity which requires light availability for plant growth. Previous research has shown a significant decline in plant biomass and soil biota beneath solar panels across different solar parks in the Netherlands that have a dense packing of relatively large, relatively flat solar panel arrays. The ground irradiance under these solar panel arrays is well below 10% of the open field irradiance, and the ecological decline was related to very low light availability beneath the panels. However, plant growth, soil biota and soil organic carbon may still be well-supported at intermediate levels of light availability and by favourable microclimatic conditions. In this research an experimental solar park was constructed with varying solar panel density to test the effect of a light gradient on the vegetation, CO₂-fluxes, and on soil biota. The experiment consisted of 6 different light treatments (8%, 20%, 30%, 40%, 65% and 100% of annual open field irradiance) and 3 vegetation treatments (non-sowing, sowing of shade tolerant plant species and sowing of a “standard” species diverse grassland mixture). Plant biomass and species composition, nematode and earthworm abundance and net ecosystem exchange (NEE) were measured. Plant biomass, and the abundance of earthworms and nematodes all significantly increased with increasing levels of light, with the strongest increases between 8% and 20% light. At low light levels, shade tolerant plant species’ biomass was higher compared to the other vegetation treatments. These results show with a relatively small increase in light availability can lead to large benefits to soil health and biodiversity.