Black carbon and soil properties at historical charcoal production sites in Germany

- The legacy of historical charcoal production profoundly affected soil properties.
- Added charcoal stabilized SOC following a logarithmic saturation curve.
- Historical charcoal additions hardly affected soil fertility and plant biodiversity.

The use of charcoal as a soil amendment is currently of great interest to sequester carbon and improve soil fertility, however, studies of sites where charcoal amendments to the soil have been made many years ago are lacking at the moment. In this study we investigated historical charcoal production sites in Germany that have not been in use for > 60 years, and evaluated the effects of the former charcoal inputs on soil and vegetation parameters relative to those of adjacent, unamended areas. Surface soil samples (0-5, 5-20 cm) were taken from five sites located on extremely acidic (Siegerland, pH 3.8-4.1) and base rich soils (Eifel, pH 4.8-5.3) in species poor (Luzulo-Fagetum) and species rich (Hordelymo-Fagetum) beech forests, respectively. We determined stocks of black carbon (BC) and natural soil organic carbon (SOC = total C minus BC) as well as of soil nutrient stocks (NO3-N, P, K, Mg), cation exchange capacity and water holding capacity, and we mapped plant composition to calculate richness and evenness. The results showed that historical charcoal production sites were enriched with BC and also exhibited increased stocks of natural SOC and total N possibly due to enhanced stabilization of natural SOC by the charcoal. The availability of nitrate-nitrogen, phosphate and potassium was increased when the charcoal was added to the base rich soils and less so when charcoal was added to the extremely acidic soils. Plant biodiversity was not different between the sites of historical charcoal addition and the reference sites. We conclude that charcoal additions may increase soil carbon storage capacity over prolonged periods of time without negatively affecting plant ecological interactions over the long term.