Historical Charcoal Production Research Articles

Historical charcoal production as a factor in soil cover heterogeneity in a fluvioglacial landscape − A case study from northern Poland

Although in situ wood thermal processing, as a common practice in European forests in historical times, has been confirmed in many studies, the environmental implications of that process remain poorly explored. In this study, we focused on evaluating the impact of that process on forest vegetation and soil cover heterogeneity in a fluvioglacial landscape dominated by Brunic Arenosols. This study covered three 70–95-year-old stands of Scots pine in northern Poland. Three soil profiles, including from the relict charcoal hearth (RCH), from the surrounding ditch and a control, were examined from each stand, as well as associated vegetation. The studies showed that 19th century charcoal production platforms were overgrown with 20–60% fewer plant species than control stands. The effect on soils varied according to their characteristics. The presence of anthropogenic horizons consisting of a mixture of burnt soil material, ashes, charcoal particles, and other products of wood pyrolysis was the most distinguishing feature of RCH soils classified as Spolic Technosols (Arenic, Dystric, Pyric, Protospodic, Transportic). Typically, these horizons were characterised by lower bulk density, and higher porosity, as well as higher contents of total organic carbon, nitrogen, and exchange cations than the control soils. The differences between the RCH and control soils were highlighted by a comparison between the pools of the components in a soil column (pedon). Clear tendencies were not recorded for pH, the phosphorus content and stocks, the forms of iron and aluminium, the saturation of the soil sorption complex with bases or the trophic index. This may be due to the long-term neutralising effect of forest vegetation, dominated by Scots pine. Moreover, the results of our study may indicate the problem of toxicity of the RCH soils, which should be undertaken, considering the utilitarian importance of the issue.

A multidisciplinary investigation of historical charcoal production in the 18–19th centuries (Czech Republic)

Starting in the Middle Ages, charcoal production was essential for industrial development, in the manufacture of metals, glass and gunpowder. However, the ancient craft of charcoal burning and other commercial forest activities remain little understood. In this study, we used data from aerial laser scanning (ALS) to identify relic charcoal kilns in deciduous and coniferous forests at three different sites in the Drahany Highlands of the Czech Republic. We applied anatomical, archaeological, dendrochronological and radiocarbon methods to describe and analyse six selected kilns (two from each site) in detail. The ALS method revealed 245 potential relic charcoal kilns, of which 116 were confirmed during a field survey. Five of the six selected kilns dated from the 18th and 19th centuries, which corresponds to the period of greatest charcoal consumption by the region's metallurgical industries. While oak and birch charcoal dominated at one of the three sites under study, fir, oak and beech prevailed at another, while only fir and beech were identified at the third. These results from the anatomical identification of charcoal samples reflect historical as well as potential forest species composition but do not match the current composition. The results of our multidisciplinary research coincide with the peak and decline of charcoal production in the region. Thanks to the clear identification and dating of relic charcoal kilns in this study, an appropriate degree of protection from forest management practices that contribute to their gradual disappearance can be proposed.

Historical charcoal-burning mounds in the Engure area, Latvia, and their significance for the history of forest exploitation

The application of airborne laser scanning (LiDAR) has led to the discovery of numerous man-made mounds encircled by pits in the forests near Engure, north-western Latvia. Field survey revealed these to be historical features remaining from charcoal-burning in mound kilns. Archaeological excavation was undertaken at one of the features, along with analysis of recovered wood charcoal, dendrochronological and radiocarbon (14C) dating and archival research into documents relating to historical charcoal production in the area. All of the analysed charcoal samples are from Scots pine (Pinus sylvestris L.), the dominant tree species in this area. It is concluded that the large-scale charcoal production served the needs of the ironworks operating at Engure in the 17th–18th century. The study of the charcoal-burning mounds has permitted estimation of the source area and volume of the wood supplied for charcoal production, and the period during which it was undertaken. Wood harvesting on such a scale had a significant impact on the surrounding forests and also had some effect on subsequent forest growth in the area. These findings demonstrate that features of this kind deserve to be regarded as archaeological remains of considerable research potential. Keywords: Duchy of Courland and Semigallia, forest history, Scots pine, charcoal-burning mounds, charcoal kilns, wood charcoal

Quaternary and geoarcheology in Lower Lusatia

Abstract. Open-cast lignite mines in Lower Lusatia provide unique insights into human-induced environmental changes in the Northern European Lowland and improve our understanding of Quaternary landscape dynamics. The excursion will focus on soils and sediments of Late Pleistocene and Holocene age that have been modified by land use since the Slavic Middle Ages. In the vicinity of the open-cast mine Jänschwalde and in the Tauer forest north of Cottbus characteristic remains of historical charcoal production, a ridge-and-furrow system and late Quaternary rubification in soils are presented.

Vertical SOC distribution and aromatic carbon in centuries old charcoal‐rich Technosols

AbstractCharcoal‐rich Technosols on century‐old relict charcoal hearths (RCHs) are the subject of ongoing research regarding potential legacy effects that result from historic charcoal production and subsequent charcoal amendments on forest soil properties and forest ecosystems today. RCHs consist mostly of Auh horizons that are substantially enriched in soil organic carbon (SOC), of which the largest part seems to be of pyrogenic origin (PyC). However, the reported range of SOC and PyC contents in RCH soil also suggests that they are enriched in nonpyrogenic SOC. RCH soils are discussed as potential benchmarks for the long‐term influence of biochar amendment and the post‐wildfire influences on soil properties. In this study, we utilised a large soil sample dataset (n = 1245) from 52 RCH sites in north‐western Connecticut, USA, to quantify SOC contents by total element analysis. The contents of condensed highly aromatic carbon as a proxy for black carbon (BC) were predicted by using a modified benzene polycarboxylated acid (BPCA) marker method in combination with diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy‐based partial least square regression (r2 = 0.89). A high vertical spatial sampling resolution allowed the identification of soil organic matter (SOM) enrichment and translocation processes. The results show an average 75% and 1862% increase in TOC and BPCA‐derived carbon, respectively, for technogenic Auh horizons compared to reference soils. In addition to an increase in aromatic properties, increased carboxylic properties of the RCH SOC suggest self‐humification effects of degrading charcoal and thereby the continuing formation of leachable aromatic carbon compounds, which could have effects on pedogenic processes in buried soils. Indeed, we show BPCA‐derived carbon concentrations in intermediate technogenic Cu horizons and buried top/subsoils that suggest vertical translocation of highly aromatic carbon originating in RCH Auh horizons. Topmost Auh horizons showed a gradual decrease in total organic carbon (TOC) contents with increasing depth, suggesting accumulation of recent, non‐pyrogenic SOM. Lower aliphatic absorptions in RCH soil spectra suggest different SOM turnover dynamics compared to reference soils. Furthermore, studied RCH soils featured additional TOC enrichment, which cannot be fully explained now.Highlights BC to TOC ratio and high resolution vertical SOC distribution in 52 RCH sites were studied. RCH soils non‐BC pool was potentially different to reference soils. RCH soils feature TOC accumulation in the topmost horizon. There is BC translocation into buried soils on RCH sites.

C:N:P stoichiometry associated with biochar in forest soils at historical charcoal production sites in Poland

Biochar amendment leads to change of physical, chemical and biochemical properties of soil. The aim of our research was to determine C/N/P stoichiometry of soils with biochar in pine forest in places of historical charcoal production. The research was carried out in the Manowo Forest District, northern Poland (54°08′52″N; 16°27′53″E). The research was carried out in an area where relict charcoal hearths are abundant. The burning of charcoal in this area took place about 200–250 years ago. Pine wood was used in relict charcoal hearths. The samples for analysis were taken from relict charcoal hearths and in the immediate vicinity (background). We determined the enzymatic activity and we recorded the content of C, N and P in testing samples. We also calculated the C/N/P stoichiometry for samples and we used soil enzyme activity stoichiometry for reflects the nutrient availability. Our research confirmed the role of biochar in shaping the C/N/P stoichiometry in forest soils. Soils with biochar were characterized by wider stoichiometry and significantly higher C and P content compared to the control soils. The enrichment of the soil with biochar resulted in a significant increase in the activity of enzymes involved in the carbon cycle (β-glucosidase and β-D-cellobiosidase), nitrogen (N-acetyl-β-D-glucosaminidase) and phosphorus (phosphatase). The enzyme stoichiometry confirmed limited availability of nutrients, especially nitrogen. Biochar in historical charcoal production sites increased total carbon stocks in temperate pine forests. This implied that relict charcoal hearths can significantly promote the carbon sequestration capacity.

The use of remote sensing tools for accurate charcoal kilns’ inventory and distribution analysis: Comparative assessment and prospective

Historical charcoal production is one of the significant factors affecting today’s forest dynamics. A key challenge is to develop tools to investigate historical charcoal production over large areas, allowing a more comprehensive understanding of past impacts and history of charcoal production over a given landscape. In this study, high-resolution remote-sensing airborne LiDAR images over a large woodland area were used to compare manual on-screen versus algorithm-based automatic methods to inventory charcoal kilns with inputs of field-validated data. The results revealed that (1) the on-screen detection method provided less false-positives, (2) the automatic method detects a higher number of kilns and (3) kiln distribution seemed to be connected mostly to land ownership rather than to environmental variables. This study validates a new method of charcoal kilns’ inventory and spatial analysis that can be applied to other areas to better understand the effect of past biomass harvesting for charcoal production on forest dynamics.

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 km2) are analyzed; and that characteristic spatial concentration patterns of RCHs are even observable when values are aggregated to large areas (25 km2 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.

Legacies of historic charcoal production affect the forest flora in a Swedish mining district

Iron production was historically associated with major impacts on forests worldwide, as vast amounts of wood were harvested to produce the charcoal needed for heating the furnaces and reducing iron oxides in the ore to iron. This impact has left abundant legacies which potentially may remain in the present‐day vegetation. We investigated how remains of historic charcoal production, mainly from the 18th to the early 20th century, at still remaining charcoal kiln platforms (CKPs), affect the current species richness, species occurrences and cover of vascular plants in the ground vegetation in a Swedish mining district located in the boreo‐nemoral forest zone. CKPs have a significantly higher species richness than the surrounding forest, and they also affect cover (negatively) for ericaceous species typically dominating the forest ground vegetation. Several forest species are more frequent at CKPs, and these also harbor significantly more uncommon species, of which many are typical for traditionally managed grasslands. These latter species are likely to represent remnants in present‐day forests reflecting former land‐use such as livestock grazing. The soil chemistry at CKPs is strongly deviating from the surrounding forest, and this, together with a lower cover of ericaceous shrubs, are the most likely mechanisms behind the higher species richness. CKPs represent conspicuous and abundant historic anthropogenic habitats in the forest vegetation. As far as we are aware, the flora at CKPs in boreal and boreo‐nemoral forests has not previously been investigated in detail, and they deserve more attention, both from a biological and a cultural–historical perspective.

When Computers Dream of Charcoal

AbstractThis research employs machine learning (Mask Region-Based Convolutional Neural Networks [Mask R-CNN]) and cluster analysis (Density-based spatial clustering of applications with noise [DBSCAN]) to identify more than 20,000 relict charcoal hearths (RCHs) organized in large “fields” within and around State Game Lands (SGLs) in Pennsylvania. This research has two important threads that we hope will advance the archaeological study of landscapes. The first is the significant historical impact of charcoal production, a poorly understood industry of the late eighteenth to early twentieth century, on the historic and present landscape of the United States. Although this research focuses on charcoal production in Pennsylvania, it has broad application for both identifying and contextualizing historical charcoal production throughout the world and for better understanding modern charcoal production. The second thread is the use of open data, open source, and open access tools to conduct this analysis, as well as the open publication of the resultant data. Not only does this research demonstrate the significance of open access tools and data but the open publication of our code as well as our data allow others to replicate our work, to tweak our code and protocols for their own work, and reuse our results.

A modified Mask region‐based convolutional neural network approach for the automated detection of archaeological sites on high‐resolution light detection and ranging‐derived digital elevation models in the North German Lowland

AbstractDue to complicated backgrounds and unclear target orientation, automated object detection is difficult in the field of archaeology. Most of the current convolutional neural network (CNN) object‐oriented detection techniques are based on a faster region‐based CNN (R‐CNN) and other one‐stage detectors that often lack adequate processing speeds and detection accuracies. Recently, the two‐stage detector Mask R‐CNN technique achieved impressive results in object detection and instance segmentation problems and was successfully applied in the analysis of archaeological airborne laser scanning (ALS) data. In this study, we outline a modified Mask R‐CNN technique that reliably and efficiently detects relict charcoal hearth (RCH) sites on light detection and ranging (LiDAR) data‐based digital elevation models (DEMs). Using image augmentation and image preprocessing steps combined with the deep learning‐based adaptive gradient method with a dynamic bound on the learning rate (AdaBound) optimization technique, we could improve the model's accuracy and significantly reduce its training time. We use DEMs based on high‐resolution LiDAR data and the visualization for archaeological topography (VAT) technique that give images with a very strong contrast of the terrain and the outline of the sites of interest in the North German Lowland. Therefore, the model can identify RCH sites with an average recall of 83% and an average precision of 87%. Techniques such as the modified Mask R‐CNN method outlined here will help to greatly improve our knowledge about archaeological site densities in the realm of historical charcoal production and past human‐landscape interactions. This method provides an accurate, time‐efficient and bias‐free large‐scale site mapping option not only for the North German Lowland but potentially for other landscapes as well.

Gradients of geochemical change in relic charcoal hearth soils, Northwestern Connecticut, USA

Relic charcoal hearths (RCHs) have produced distinct legacy effects in forest soils around the world. Recently, LiDAR imagery has revealed thousands of 18th–early 20th century RCHs in Litchfield County, Connecticut, USA; however, the effects of RCHs on a landscape-scale are not well-documented, particularly fine-scale heterogeneity within RCHs and surrounding soils. This study examines the long-term impacts of charcoal production by measuring RCH soil chemical and physical properties from three perspectives: (1) compared to adjacent reference sites (RSadj), (2) laterally at systematic distances away from the RCH center, and (3) vertically within the RCH soil profile. Mean charcoal abundance was greater in RCH sites than RSadj (p < 0.01). Soil organic carbon (SOC), total C, and extractable Ca2+, Mg2+, Na+ were greater in RCH sites as compared to RSadj (p < 0.01), and available phosphorus (p < 0.01), K+, and trace elements (Mo, Ag, Hg, and Se) were lower (p < 0.05). In vertical profiles, many RCHs had 2 charcoal-rich layers within the anthropic epipedon, demonstrating multiple episodes of charcoal production. Peaks in SOC, C:N, Ca2+, Mg2+ corresponded with charcoal-rich layers. Systematic transect sampling across the RCH boundary identified charcoal fragments in soils at distances up to 25 m beyond the RCH boundary, increasing the surface-level (0–15 cm) area of impact for an individual RCH by more than 30×, from a 5-m radius (RCH area = 78.5 m2) to a 30-m radius (total area of impact = 2826 m2). These findings capture fine-scale variations within and among RCH and reference sites and contribute to estimating the total area of forest soils impacted by historical charcoal production.

2500 years of charcoal production in the Low Countries: The chronology and typology of charcoal kilns and their relation with early iron production

The study of archaeological remains of charcoal kilns is a fast growing field of research. The history and evolution of different types of (pre)historical charcoal production features is still poorly understood however. Based on a large number of radiocarbon dated charcoal kilns from Belgium and the Netherlands, covering c. 2500 years, a typology and chronology of charcoal kilns is presented. The oldest charcoal production features date to the late Iron Age, though many of these dates seem to suffer from an old-wood effect and might actually date to the Early Roman period. Both Iron Age and Roman period charcoal production is done in rectangular pit kilns. During the Early Middle Ages, there is a shift to circular pit kilns which become the dominant kiln type during the High Middle Ages. From c.1300 CE onwards, charcoal is no longer produced in pit kilns, but a new kiln type, i.e. the mound kiln is used. The size of the charcoal (pit) kilns decreases from the Roman period till c. 1300 CE when, with the use of mound kilns the volume of produced charcoal dramatically increases. These changes in kiln size most likely reflect changes in both the organisation, scale and technology of iron production in the Low Countries.

Characteristics of small anthropogenic landforms resulting from historical charcoal production in western Connecticut, USA

Relict charcoal hearths (RCHs) are anthropogenic geomorphic features with an average diameter of 12 m found in many forests of Central Europe and in the eastern USA wherever pre-coal iron production took place or other industries demanded the production of charcoal. To expand the knowledge about their geoarchaeological significance and their legacy effect on soil properties and forest ecosystems, we propose a method for a generalized description of soil stratigraphy on RCHs. We studied 154 soil profiles at 52 RCH sites alongside two 1 km transects in Litchfield County, Connecticut, USA. The sites can be classified based on the slope inclination, with sites on <4° mostly having a single-layered stratigraphy and an elevated circular shape, while sites on slopes >4° mostly are built as levelled and multilayered platforms. The latter have two or more charcoal rich technogenic Auh-layers separated by intermediate Auh-layers mostly consisting of mineral substrate. Based on average layer thicknesses and their dependence on the sites slope inclinations, we propose a model with two idealized RCH shapes with slope controlled properties that allow for an easy computation of site diameters and elemental stocks. With ongoing advances in remote sensing of RCH sites, our proposed model can help to further understand the effects of historic land use on a landscape scale.

Bypassing the Suess-effect: Age determination of charcoal kiln remains using OSL dating

Ancient forests all over Europe often preserve remains of (pre)historic charcoal production (kilns), informing on past forest composition and anthropogenic woodland exploitation. So far, the chronology of these features has been entirely derived from 14C-dating of associated charcoal. Though generally successful, 14C-dating cannot provide meaningful age information for post-1650 CE features, while most of the archaeological remains of charcoal kilns date to this period. Here, we investigate the potential of quartz-based optically stimulated luminescence (OSL) dating of sediments that were heated during charcoal production as a more precise alternative tool for age determination of these kilns.Using a set of 14 samples collected from both relict charcoal kilns and the underlying parent material, we first document the quartz OSL characteristics in terms of the procedural tests commonly used to assess the appropriateness of the single-aliquot regenerative-dose (SAR) protocol. The results suggest that the measurement procedure allows determining equivalent doses both accurately and precisely. We then demonstrate how the linear part of the dose-response curve can be exploited to examine completeness of resetting and/or post-depositional disturbance without compromising instrument time, precision and laboratory accuracy. Optical ages derived from these distributions are consistent with the available independent (14C) age information for the charcoal kilns. We conclude that OSL can provide an accurate and viable alternative to 14C and will be especially useful to constrain post-1650 CE features in time. Our results also suggest that the approach may allow establishing relative chronologies with a time resolution of 2–4%.

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.

Large‐scale mapping of anthropogenic relief features—legacies of past forest use in two historical charcoal production areas in Germany

AbstractWith the increasing availability of high‐resolution digital elevation models (DEMs), large‐scale mapping of anthropogenic relief features has become feasible for more areas. However, the landscape‐scale distribution patterns of anthropogenic landforms and the quality of DEM‐based mapping can be highly heterogeneous. In this study, we mapped relict charcoal hearths (RCHs) in two study regions with differing environmental backgrounds, covering forest areas of more than 15,000 km2, analyzed the RCH distributions and evaluated possibilities for predictive modeling of RCH occurrence with respect to natural and cultural landscape structures. More than 45,000 RCHs were recorded in each region, with high site densities even in areas remote from charcoal‐consuming industries. Variations in the quality of DEM‐based mapping were related to small‐scale differences in the DEM quality and larger‐scale substrate heterogeneity. A clear association between RCHs and historical industrial sites was found in the Northern European Lowland; while the density of mapped RCHs was predominantly related to geology and morphology in the lower mountain ranges. The results show that variations in mapping quality across scales and the natural and cultural background of a region need to be considered so that the mapping of anthropogenic relief features can contribute to an improved understanding of land‐use history.

Long-Term Effect of Charcoal Accumulation in Hearth Soils on Tree Growth and Nutrient Cycling

There is a lack of long-term field approach investigating biochar impact on soil properties and vegetation, particularly in forest ecosystems. Relic charcoal hearths (RCHs), the result of the historical charcoal production in the forests, preserve a charcoal-enriched topsoil horizon, thus representing a suitable proxy for studying the long-term effect of biochar addition to soil. In this study, we analyzed the chemical properties of a soil as impacted by charcoal accumulation in three RCH plots in southern Wallonia (Belgium) compared to the soil outside RCHs. We further evaluated the effects of RCHs soil properties on the growth performances of silver birch and European beech as well as the leaves’ nutrient concentration of the latter. RCHs soil stored much more carbon and nitrogen than the reference ones. Most of the C in RCHs derived from charcoal (70‒94% of total organic carbon), which would correspond to a total input of 342 tons of biochar per hectare in these soils. Such an accumulation of charcoal still affects nutrient status of soil even after 150 years since charcoal hearths abandonment: CEC and K, Ca, Mg, Na, Mn and Zn concentration remained higher in RCHs soil compared to the reference one. In spite of a seemingly higher fertility of RCHs soil, elemental concentrations of European beech leaves grown in RCHs did not show any significant difference compared to the reference plots, except for C and Mn concentration, higher and lower, respectively, in the leaves of European beech trees grown inside than outside RCHs. Overall, RCHs soil chemical properties were not a decisive factor in significantly improving tree growth. On the contrary, tree ring width average values of both tree species was slightly lower in RCH plots, suggesting to better investigate the potential long-term detrimental effect of a large biochar addition to soil on forest trees.

Dye Tracer Visualization of Infiltration Patterns in Soils on Relict Charcoal Hearths

Anthropogenically modified soils are often characterized by a high heterogeneity of substrates and show unique patterns of water infiltration. Such effects are not limited to intensively used or disturbed agricultural and technogenic soils, but can also occur as legacies of former land use in forested areas. The remains of historic charcoal hearths represent a widespread legacy of historic land use. Soils at relict charcoal hearths (RCHs) are most prominently altered by the deposition of a layer of charcoal-rich substrate on top of the natural soil surface. The presence of such a technogenic layer can considerably influence infiltration and soil wetness patterns on the sites. This study describes the spatial patterns of infiltration and soil wetness at charcoal hearth sites compared with undisturbed sandy forest soils for a historic charcoal production area north of Cottbus, Germany. We characterized six plots on RCH and reference soils under pine, oak and mixed forest by visualizing preferential flow patterns of infiltrating water in dye tracer experiments. Additionally, we characterized bulk density, soil organic matter contents and water repellency, using water drop penetration time (wdpt) tests, of the RCH and reference soil horizons. The results reflect that the persistence of water repellency of both the technogenic substrates and the natural topsoils is extremely high under dry conditions, but is drastically reduced after wet antecedent conditions. The dye tracer experiments reflect increased preferential flow on the RCHs for dry soil conditions, for which infiltration is limited to very few flow paths in the technogenic substrate layer. Differences between RCH and reference soils are less clear for higher antecedent soil wetness, for which the results indicate more uniform wetting of the technogenic substrates. We conclude that the structural properties of the additional technogenic substrate layer of RCHs have characteristic effects on water infiltration, causing a high temporal variation of preferential flow in relation to antecedent soil moisture conditions. These effects can result in high heterogeneity of soil moisture for dry conditions, and generally in a high temporal variation of soil wetness in RCHs soils.

Formation, Classification, and Properties of Soils at Two Relict Charcoal Hearth Sites in Brandenburg, Germany

Historical charcoal production can have significant effects on soil properties. We studied soils at former charcoal production sites (relict charcoal hearths, RCHs) and compared these soils with undisturbed soil next to the charcoal hearths and four typical soils on similar parent material located at distances between 10 and 70 km from the RCHs. In a landscape typical of the northern German lowland, we found Podsolige Braunerde (WRB: Brunic Arenosols (Protospodic)) outside of the RCHs and soils with a clearly different stratigraphy within the RCHs. The main feature of the soils at both of the studied RCHs is a heterogeneous, charcoal-bearing deposit that is approximately 30 cm thick. No indications of translocation or mineral transformation processes, which form distinct soil horizons after the deposition of anthropogenic material on the RCHs, are present. Except for the differences in color and total carbon content, the soil chemistry of the RCHs hardly differs from that of the soil outside of the charcoal hearth sites. The soil colors and magnetic susceptibility values strongly suggest that the RCH substrates and the underlying topsoil were affected by thermally induced transformation of iron (hydr-)oxides. Although the charring procedure normally requires approximately two weeks, the heating effect only reaches to a maximum depth of 8 cm into the buried soil below the charcoal hearths. The presence of reddish soil and an abrupt increase in magnetic susceptibility in the upper 2 cm of the soil below the charcoal hearths indicate the heat-induced transformation of iron (hydr-)oxides into maghemite. Brighter soil color and an increase in soil organic matter in the lower parts of the buried topsoil demonstrate the combustion of soil organic matter up to 5 cm depth below the RCH. According to the German Guidelines for Soil Mapping, the soils in the RCHs are classified as Regosols above Braunerde (WRB: Spolic Technosols (Arenic)). However, because the anthropogenic features of these soil sediments are disregarded in the German Guidelines for Soil Mapping, we suggest adapting the ‘M’ horizon to permit a jM horizon. Thus, the soils in the RCHs could be classified as Kolluviale Braunerde.