Pre-industrial charcoal kiln sites in Wallonia, Belgium : spatial distribution, effects on soil properties and long-term fate of charcoal in soil

Black carbon (BC), the solid residue of the incomplete combustion of biomass and fossil fuels, is pervasive in soil and contributes to critical soil functions. Therefore, soil amendment with artificial BC (biochar) is more and more regarded as a means to mitigate greenhouse gas emissions while improving sustainably soil fertility. Nevertheless, the long-term fate of BC in soil is still poorly understood. In Wallonia, Belgium, intensive in situ charcoal production affected a main fraction of the forested area in the late 18th century. The topsoil of pre-industrial charcoal kiln sites is largely enriched with charcoal residues, which offers the opportunity to evaluate the long-term effect of BC on temperate soil properties. We sampled the soil of 40 pre-industrial charcoal kiln sites from forest and cropland. Soil properties were compared to that of adjacent reference soils, unaffected by charcoal production, and to soil characteristics at a currently active kiln site to trace the evolution of soil properties over time. We also studied the properties of charcoal particles extracted from soil along a chronosequence of land use change from forest to cropland. Dynamic thermal analysis and chemical oxidation were used to investigate the stability of aged charcoal particles and to discriminate between charcoal and uncharred soil organic matter (SOM) in the soil of charcoal kiln sites. The content of charcoal and uncharred SOM was related to the availability of nutrients, acidity, cation exchange capacity and microbial activity, biomass and community structure of soil. Overall, we highlighted that the features of charcoal and its effect on soil properties evolve over time and depend on soil conditions. BC is very persistent in soil, therefore, long-term implications of soil amendment with biochar should be further addressed before allowing its large-scale application.