From site to state – Quantifying multi-scale legacy effects of historic landforms from charcoal production on soils in Connecticut, USA

Abstract

Relict charcoal hearth (RCH) landforms are relatively small (average diameters of 11 m) and circular microrelief features found in many forests of the North-Eastern USA and Central Europe. Soils on RCHs are special in that they are significantly enriched in organic- (OC) and pyrogenic carbon (PyC), caused by the admixture of charcoal. Many studies have shown that this results in changed soil chemical and physical properties, making RCHs unique soil microhabitats. However, questions about their larger impact on a soil landscape’s carbon storage have hardly been studied so far; an aspect that could become more relevant with growing RCH site location databases. Here we show that RCHs can substantially add to a landscape soil organic- and pyrogenic carbon storage. This effect is scale dependent, i.e. a larger scale of observation (1:20,000) will result in significant additions for areas with high site densities (+22.6 % OC, +128.6 % PyC), contrary to smaller scales of observation (>1:650,000) where the effect is diminished (+0.2 % OC, +1.2 % PyC). A comparison with a nationwide soil database (gSSURGO) shows that in areas with high site densities, individual soil unit OC contents are increased by up to 91.6 %. An exemplary comparison to historic terrace/lynchet systems shows that RCHs store vastly more carbon when pedons of the same size are compared. However, if the actual landform surface areas are taken into account, RCHs store a lot less. This is showcasing the spot-like distribution of RCHs on a landscape scale. Furthermore, we broaden the knowledge about RCH site specific properties in Connecticut by a first analysis of pedogenic (hydr-)oxide and exchangeable cation concentrations, with the result indicating increased weathering rates and enrichment of exchangeable Ca2+, Mg2+ and Mn2+ in RCH soils.