The effect of pre‐industrial charcoal kilns on chemical properties of forest soil of Wallonia, Belgium

Abstract

Summary In Wallonia, Belgium, intensive in situ charcoal production that was linked closely to pre‐industrial smelting and steel‐making affected a large part of the forested area in the late eighteenth century. Charcoal kiln relics can be detected under forest as domes of about 10 m in diameter, with the topsoil greatly enriched with charcoal residues. We sampled 19 charcoal kiln sites and the adjacent reference soil by soil horizon on four different soil types (Arenosols, Luvisols, Cambisols and Podzols). Data were analysed with linear mixed models to assess the effect of the charcoal kiln site on soil properties in relation to depth and soil conditions. We also addressed the evolution of soil properties over time by a comparison of the soil characteristics at a currently active kiln site. The charcoal‐rich topsoil has a larger C:N ratio and cation exchange capacity (CEC) per unit of organic carbon than the reference soil. The largest CECs per unit of carbon were observed on soil with coarser textures. On acidic soil, the increase in base saturation in the subsoil reflects the past liming effect of ash produced by wood charring, whereas the topsoil is re‐acidified. The acidity of carbonate‐rich Cambisols, however, is not reduced. Regardless of soil type, the kiln topsoil is greatly depleted in exchangeable K+ and available P, which may be attributed to the small affinity of the exchange complex of charcoal for K+ and a decrease in P availability with time. Therefore, we recommend further research on the long‐term effects of biochar on the dynamics of plant nutrients.

Soils on Historic Charcoal Hearths: Terminology and Chemical Properties

“…The lower nitrogen concentrations of the charcoal hearths in Connecticut also result in a more variable C/N ratio. These findings agree with charcoal hearths at forestry sites in Belgium (Hardy et al, 2016), which also have lower nitrogen concentrations in the substrate from the charcoal hearths and a scattered C/N ratio. For topsoils from charcoal hearths under agricultural use in Belgium, higher nitrogen concentrations than in the surrounding soils have been reported (Hardy et al, 2017).…”

Long term change in chemical properties of preindustrial charcoal particles aged in forest and agricultural temperate soil

“…11a and b), which supports the idea that Ca is strongly associated with the carbonaceous structure of charcoal. The strong affinity of Ca 2+ for carboxylate groups is well known (Kalinichev and Kirkpatrick, 2007) and the consequent high affinity of Ca for aged charcoal was highlighted in earlier studies (Hardy et al, 2016(Hardy et al, , 2017. Intriguingly, the content of carbonate in soil from which charcoal was extracted is systematically < 0.10% (data not shown), whereas charcoal from cultivated plots contain up to 5.73% of carbonate (Fig.…”

“…For comparison with charcoal aged in forest soil (never converted to cropland since the last production of charcoal) we refer to four sites on Luvisols from the weathering of quaternary loess described and characterized by Hardy et al (2016). The sites are the forest equivalent of those deforested for cultivation.…”

“…We also refer to the topsoil of a currently active charcoal kiln close to Dole (France) and characterized previously (Hardy et al, 2016), as a reference subjected to limited ageing (the site was sampled 6 months after the last pyrolysis). Except for the currently active kiln, the sites here result from the same episode of charcoal production, related to preindustrial iron smelting and steel making (Hardy et al, 2016). It is likely that they date to the late 18th century, when charcoal production reached a peak in Wallonia.…”

“…1% of Ca, the content decreases through ageing in acidic forest soils (Fig. 1), due to the leaching of ''base" cations through natural re-acidification occurring over time after pyrolysis (Hardy et al, 2016). In contrast, Ca content in charcoal increases over time of cultivation in agricultural soils (Fig.…”