The purpose of the Biosoil soil inventory (ICP Forests - level I - test plots) was to assess the chemical state of the soil and its changes, and secondly to assess the soil properties that affect the functioning of the forest land. In addition to the fact that the Biosoil data provides soil information for the study of the effects of atmospheric deposition on a larger scale, the soil inventory serves other purposes as it provides data for both research diciplines, climate change mitigation (e.g. the amount of carbon and its changes) and sustainable forest management (e.g., in addition to acidification, also, for example, studies on nutrient cycles and imbalances). Soil profile data was collected as part of the BioSoil project, Natural Resources Institute Finland, Luke (former Finnish Forest Research Institute, METLA). The BioSoil project was implemented as part of ICP Forests (International Cooperative Program on the Assessment and Monitoring of Air Pollution Effects on Forests; icp-forests.net/) and the EU Forest Focus program (Tamminen and Ilvesniemi 2013). The aim of the project was to collect information on soil properties, composition and changes in carbon, nutrients, acidity and heavy metals in Finnish forest lands. The material was collected in the so-called From the network of ICP Forests Forest Focus Level I experimental area in 2006-2007, which were first sampled for soil samples in 1986-1995 (Tamminen and Starr 1994, Tamminen 2009). This network is part of the so-called 3,000 permanent sample plot data. In Finland, the BioSoil database contains 631 experimental plots, while the total number of the plots in Europe is around 6000 (icp-forests.net/). A detailed description of all the collected data and variables is given in BioSoil Kuvio- ja puusto-ohjeet (2006). In the laboratory, the particle size categories (clay, silt and sand) of the soil profile samples were determined. Approximate content of soil organic matter for the mineral soils was determined for different layers based on loss on ignition at a temperature of 550 °C for about two hours. In addition, the concentrations of several elements were determined from the samples. The basic data of the experimental area include: latitude, longitude, altitude from sea level, soil moisture, slope, topographical location, bare rock cover (%), soil type, parent material, coarse particle content (%), soil texture, site type and Sphagnum cover (%). Information was collected from the mineral soils by the EU funded Biosoil project during 2006-2007 (Tamminen, 2006; Tamminen ja Ilvesniemi, 2013). Data have been used in several research projects (Heiskanen ym. 2018, Heiskanen ym. 2020, Launiainen ym. 2022, Lehtonen ym. 2016, Mäkelä ym. 2022, Rantakari ym. 2012). ## 3,000 permanent sample plots established 1985-86 In connection with the 8th national forest inventory in 1985-1986, approximately 3,000 permanent experimental plots were established for the purpose of monitoring the nature of the forest and the state of the environment. The plots were located in a network of forest inventory covering the entire country. The distances between the clusters of plots in southern Finland are 16 km both in the south-north direction and in the east-west direction. In northern Finland, the distances are 24 km in the south-north direction and 32 km in the east-west direction. Each cluster has a group of plots in the direction from south to north including 4 plots every 400 meters in Southern Finland and 3 plots every 600 meters in Northern Finland. For the BioSoil project, one plot was selected and measured from each cluster. The data collection of the forest inventories and the assessment of surface areas are based on the division of each plot on the forestry land into management units. Management units have been formed based on the properties of the tree stand. A management unit is a homogeneous area in terms of soil type and trees, as well as the forest management measures that have been carried out in the area. The 4-acre permanent circular sample plot area used in the inventory can be divided into several forest management units. For example, if a part of the test area belongs to a different land class, forest type, or sample plot locates to be on the border of a peatland and a forest or on the border of a tree development class, it is divided into different management units according to the stand properties. If necessary, the biologists refined the management unit borders assessed by the inventory team by refining management units borders by the cover drawing made from the sample plot. The refinement of the management unit was based on the site types. The smallest distinguishable management unit had to be at least 5% of the area of the of the sample plot (4 acres). In the inventory, the coverage of field and understory species was estimated from four 2 m2 sample squares systematically placed in the 4-acre experimental area. The abundance of each plant species was evaluated as projected coverage, i.e. the percentage covered by aerial shoots and mixed stems on the sample plot. Coverages were estimated with an accuracy of 1%, coverage below 1% with an accuracy of 0.1%. The biologists of the inventory groups received uniform training for the field work. A large number of tree stand and growth location codes have been inventoried from the plots. Soil samples have also been taken from the inventory plots.

2023