Changes in the soil microbiome following Si-rich rock powder amendment

1. Elevated atmospheric acid deposition across Europe has led to the acidification of poorly buffered heathland soils, correlating with a decline in plant and soil biodiversity.2. Liming with dolomite (CaMg(CO3)2), the traditional method to counteract soil acidification and restore base saturation (BS), can lead to undesirable outcomes due to the rapid spike in pH and the release of large quantities of calcium (Ca2+) and magnesium (Mg2+). Therefore, novel alternatives, such as slow-release silicate rock powder (SRP), are currently under investigation.3. Here, we compared effects of SRP on soil chemistry, the soil microbiome (prokaryotes and fungi), and vegetation with traditional liming five years after application. We established 40 large research plots, equally divided over a wet and another dry acidified heathland. Four treatments were setup in a randomized block design: the application of Dolokal (liming with ground dolomite), Biolit (SRP from chlorite/muscovite concentrate), Soilfeed (SRP from K-feldspar/biotite concentrate), or no application (control).4. Liming had the largest effects on soil chemistry, triggering a relatively large increase in soil pH (by about 1 unit) and BS (by 49% and 65%), with the largest effects in soils with lowest cation exchange capacity (CEC). Although Biolit and Soilfeed increased BS by 10%-20% and 5%-8%, respectively, the increase was only significant for Biolit. The SRP treatments led to subtle increases in pHNaCl by 0.1-0.3 units. All treatments significantly reduced exchangeable Al3+ concentrations thereby soil Al:Ca ratios. In the wet site, treatment application led to a decrease in the average relative abundance of all functional groups in favor of saprotrophs, which increased by up to 20%. The latter trend, however, did not result in lower quantities of soil organic matter (SOM), nor did it lead to changes in soil C:N ratios. In the dry site, Dolokal and Biolit application facilitated the colonization of ruderal, non-heathland, plant species such as Geranium molle L. and Jacobaea vulgaris Gaertn.5. Overall, despite subtle shifts in soil chemistry and biotic communities, the effects of SRP application seem limited within an intermediate time span. However, these subtle changes could prove beneficial in the context of heathland restoration and indicate that the effects of SRP operate on a much longer time span.