Soil Restoration and Monitoring Guidelines

Here we propose to measure total soil carbon, total soil nitrogen and soil pH. Other properties that could be interesting to evaluate the soil’s chemical condition are cation exchange capacity (CEC), as it reflects the soil’s ability to retain and supply nutrients, electrical conductivity (EC), as it indicates soil salinity, and the soils status for specific nutrients that plants need to grow (e.g. NO3-, NH4+, P, K, base cation contents).

Measuring soil carbon helps to track how much carbon forests are sequestering and how environmental changes affect this role. Additionally, soil carbon is closely linked to soil fertility, structure, water retention and biological activity. Withing SUPERB, sampling for carbon was through the same method as for bulk density (FunDivEUROPE, 2011). Soil carbon occurs in both organic molecules and inorganic carbonates. Different analysing techniques are used depending on the type of carbon being measured. Methods for determining total carbon are through dry combustion or wet oxidation (Vogt et al., 2015). Within SUPERB, total carbon was analyzed. Soil samples were dried and crushed to obtain finely ground soil and analysed with the Flash 2000 Organic Elemental. This analysis works via dry combustion (T = 950 °C) followed by gas chromatography (Robertson, 1999). When carbonates were present in the soil, first hydrochloric acid was added to the soil sample to only obtain the soil organic carbon.

Soil pH is perhaps the most important factor in soil fertility and a critical indicator of soil quality, significantly influencing forest health and ecosystem functioning by affecting various biological and chemical processes. It also plays a crucial role in the activity and diversity of soil bacteria and fungi and is essential for the specific pH preferences of different tree species. (O’Neill et al., 2005; Singh et al., 2011; Thomas, 1996; Vogt et al., 2015). The soil pH was sampled using the same method as for soil carbon and pH (FunDivEUROPE, 2011). Regarding lab analysis, there is no standard procedure for measuring pH and can vary from laboratory to laboratory. pH can be measured using rapid field methods or more precise laboratory techniques: pH-H2O and pH-KCL. pH-H₂O is determined by adding soil with water and measures only the free hydrogen ions (H⁺) in the soil solution. It does not account for hydrogen ions that are attached to clay and organic matter. The pH-KCl is determined by mixing soil with a potassium chloride solution. The potassium ions (K⁺) replace the hydrogen ions bound to clay and humus, so pH-KCl measures both the free and the exchangeable hydrogen ions in the soil (Van Ranst et al., 1999). Within SUPERB, pH-H2O was measured on a 1g:10mL soil:liquid extract for the forest floor samples and a 1g:5mL soil:liquid extract for the mineral soil samples (Van Ranst et al., 1999). These ratios also apply when using the pH-KCl method with KCl 1M.