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Historic charcoal hearth remains provide a unique archive of the long-term interaction between biochar, soil development, and plant growth. Charcoal as raw material was crucial for production of iron in iron works, and hence numerous charcoal hearths can be found in the forests near historic iron works in Europe and in the eastern united States. Charcoal hearths are round to elliptical forms often around 10 m in diameter and consist of several-decimeter-thick layers that contain charcoal fragments, ash, and burnt soil. We studied the soil chemistry of 24 charcoal hearths and compared them with the surrounding "natural" soils in the northern Appalachians of northwestern Connecticut. The thickness of the topsoils on the charcoal hearths and their carbon content are remarkably higher than in the surrounding topsoils. The presence of residual products from charcoal production classifies the soils as Anthropic Udorthents (US Soil Taxonomy) or Spolic Technosols (Humic) according to the World Reference Base for Soil Resources. The widespread occurrence of charcoal hearth remains, and their high spatial density in different ecosystems underlines their importance for further pedological research. [ABSTRACT FROM AUTHOR]

• Soils from 18th–20th century charcoal hearths were analyzed across Connecticut, USA. • Available phosphorus is more than 200% lower in hearths soils than adjacent soils. • Hearth soils are enriched in organic carbon and extractable cations Ca2+, Mg2+, Na+. • Hearths with two charcoal layers indicate multiple episodes of charcoal production. • Charcoal dispersal increases the area of impact for individual hearths by 30×. 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 (RS adj), (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 RS adj (p < 0.01). Soil organic carbon (SOC), total C, and extractable Ca2+, Mg2+, Na+ were greater in RCH sites as compared to RS adj (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. [ABSTRACT FROM AUTHOR]