Long term anthropogenic enrichment of soil organic matter stocks in forest soils - Detecting a legacy of historical charcoal production

Recent findings of several thousand relict charcoal hearths (RCHs) in the Tauer Forest (Brandenburg, Germany) raise questions about the legacy effects of historical charcoal production on soil properties. RCH sites are characterized by the admixture of charcoal fragments to the natural forest soil, constituting an anthropogenic addition of carbon. Dendrochronological dating of large charcoal pieces has identified RCH ages ranging from 1709 CE to 1823 CE for a subarea in the Tauer Forest, which allows for assessing century old effects of charcoal application on forest soils. We measured the geometry and soil organic matter (SOM) content of 20 RCH sites in a selected part of the forest. The results provide a basis for estimating RCH SOM stocks on a 40-ha forest plot with 120 RCH sites. We also assessed the accuracy of RCH mapping based on digital elevation models (DEMs) with results from a ground survey. Over 50 % of sites were not detected by the DEM-based mapping. In the range of statistical uncertainties, RCHs add between 18 % and 32 % of SOM to the stock of the natural forest soil in the study area. This equals an additional 4.9-8.9 Mg of carbon per hectare of forest soil. These substantial amounts of SOM are so far not recognized by any survey or forest soil SOM inventories. Considering recent studies, which uncovered tens- and hundreds- of thousands of RCHs throughout Europe and the USA, we conclude that further knowledge of RCH SOM is important for quantifying anthropogenic SOM stocks on a global scale.