Diversity of fungivorous mites in Dutch forests affected by nitrogen deposition and soil acidification

The old acidophilous oak forest Natura 2000 habitat (H9190) in the Netherlands is rapidly deteriorating in quality due to excess nitrogen deposition, evidenced by soil acidification, leaching of base cations, and an accumulation of soil organic matter. The accumulation of organic matter suggests disruption of the decomposition process, which is driven by the interplay between fungi and microarthropods. To better understand the current role of microarthropods for decomposition and changes therein during the process of deterioration, we analyzed microarthropod communities in old oak forests and compared them to data from other forest sites from the same geographical area and a similar habitat, collected in 1994 and resampled in 2023. Relative to the reference data, we observed reductions in both abundance and species richness of microarthropods. When analyzing the data by feeding guild we found that these reductions are especially pronounced for fungivorous mites. Similar declines were found during the resampling of the reference sites. We hypothesize that nitrogen-driven declines in ectomycorrhizal fungal diversity, which are known from literature, negatively affect fungivorous mite communities, which is expected to contribute to slower decomposition and the accumulation of organic matter. Soil conditions in these oak forests are characterized by low soil pH, while oak leaves are characterized by imbalanced nutrient ratios, contributing to strong reductions in oak vitality. Restoration of these forests starts at the root of the problem, improving soil conditions and fungal diversity and abundance. Our research shows that effective restoration will also require enhancing fungivorous mite populations, potentially through soil inoculation.