Hot spots and hot zones of soil organic matter in forests as a legacy of historical charcoal production

Human land use often leads to distinct modifications of soils within small, clearly delimited areas. Legacy effects of past land use are also abundant in recent forest areas, but are hardly considered in ecosystem assessment and soil mapping. The soils on relict charcoal hearths (RCHs) are a widespread example for such land use legacy soils in forests. Soils on RCHs differ clearly from surrounding forest soils in their stratigraphy and properties, and are most prominently characterized by a technogenic substrate layer with high contents of charcoal and thus soil organic matter (SOM). Although RCH soils can clearly contribute to the SOM stocks of forest soils, their relevance on the landscape scale has hardly been quantified. In this paper we analyze and discuss the distribution and effects of RCH soils across scales for forests in the state of Brandenburg, Germany, with a focus on SOM stocks. Our analysis is based on a large-scale mapping of RCHs from digital elevation models (DEMs), combined with a linear regression model of SOM stocks in RCH soils. The spatial distribution of RCH soils in the study region has a scale dependent heterogeneity. The large-scale variation in site densities is related to the concentration of charcoal production to specific forest areas while the small-scale variation is related to the irregular distribution of single RCHs within the charcoal production fields. Up to 1.5 % of the surface are covered by RCH soils in the forest areas within the study region. The results also show that RCHs can significantly contribute to the SOM stocks of forests, even where they cover only a small fraction of the soil surface, with up to 18 % additional SOM for forest areas. Because of the heterogeneous distribution of RCHs, the SOM stock additions depend on the scale of observation. Our results suggest that RCH SOM additions can be overestimated when sites in small areas (< 1 km(2)) are analyzed; and that characteristic spatial concentration patterns of RCHs are even observable when values are aggregated to large areas (25 km(2) and more). Aggregating effects to irregularly-sized forest administrative units resulted in maps that well represent the spatial distribution of RCH soils and SOM. The study highlights that considering land use legacy effects can be relevant for the results of soil mapping and inventories; and that prospecting and mapping land use legacies from DEMs can contribute to improving such approaches.

Automated summary

Human land use can lead to modifications of soils in small areas, and the legacy effects of past land use are abundant in forests. Relict charcoal hearth (RCH) soils are a widespread example of such legacy soils, with distinct differences from surrounding forest soils in stratigraphy and properties, characterized by a technogenic substrate layer with high charcoal content. RCH soils can significantly contribute to forest soil organic matter stocks, but their distribution and impacts vary across scales, emphasizing the importance of considering land use legacy effects in ecosystem assessment and soil mapping.