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 x , from a 5-m radius (RCH area = 78.5 m(2)) to a 30-m radius (total area of impact = 2826 m(2)). 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.

Automated summary

The study investigated the long-term impact of charcoal production on forest soils, finding higher organic carbon and nutrient content in RCH soils compared to reference sites, while phosphorus and some trace elements were lower. Multiple charcoal-rich layers were discovered in vertical profiles, indicating repeated charcoal production events. Systematic transect sampling revealed charcoal fragments beyond 25 meters from the RCH boundary, increasing the impact area for individual RCHs by over 30 times.