Soil biota under future-proof forests

Nitrogen deposition and climate change lead to soil acidification and desiccation. As a result, the vitality of forests declines sharply. The Hoge Veluwe National Park is trying to revitalise forests. In a large trial, 43200 saplings will be planted in 432 experimental plots. The plots are distributed over eight sites throughout the park to include differences in soil conditions and also to include sufficient replications in the study. Sessile oak, silver birch, goat willow, common aspen, field maple and common hazel will be planted. These are "rich litter species" that bring minerals from the subsoil to the surface, where they are released back into the topsoil through the fallen leaves.

A key question in the study is how to improve the soil. Before planting, rock dust is applied in half of the plots. Part of it in the planting hole and part is spread broadly on the surface. This makes the soil a little less acidic. After planting, a bit of soil from a well-developed forest is applied to each sapling. This is also called "soil inoculation" or "soil transplantation". With this soil, soil organisms are also applied. This soil biodiversity includes bacteria, fungi, nematodes, springtails and mites. A handful of soil contains as many as 5,000 species and more than 10 billion individuals.

The soil life helps the forest ground develop and decompose leaves. Some of the decomposition products and also the soil organisms, after they die off, contribute to carbon sequestration in the soil, causing the soil to sequester greenhouse gases. This is good for mitigating climate change. In addition, soil life also contains species that cooperate with the trees. This allows the trees to survive better in times of drought. It also contains species that ensure that the composition of the forest remains diverse by restraining the development of fast-growing trees.

The research will take four years. During that time, we want to determine what the best soil treatment is: rock dust, soil inoculation, or both. We will then also know for which tree species this works the most effectively and under which soil conditions. After the four years, the forest stands will be monitored further, because it takes decades for the trees to grow. In that time, we can still learn a lot about wat the best recipe for a climate-smart forest soil is.