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.

Research on biochar has increased, but its long‐term effect on the fertility of temperate agricultural soil remains unclear. In Wallonia, Belgium, pre‐industrial charcoal production affected former forested areas that were cleared for cultivation in the nineteenth century. The sites of traditional charcoal kilns, largely enriched in charcoal residues, are similar to soil amended with hardwood biochar more than 150 years ago. We sampled 17 charcoal kiln sites to characterize their effect on soil properties compared with adjacent reference soils. Charcoal‐C content was estimated by differential scanning calorimetry. The kiln soil contains from 1.8 to 33.1 g kg−1 of charcoal‐C, which markedly increases organic C:N and C:P ratios. It also contains slightly more uncharred soil organic carbon (SOC) than the reference soil, which accords with larger total N content. We measured a small increase in nitrates in the kiln soil that might relate to greater mineralization and nitrification of organic N. Frequent application of lime raised the pH to values close to neutral, which offset the residual effect of charcoal production on soil acidity. A cation exchange capacity (CEC) of 414 cmolc kg−1 was estimated for charcoal‐C, whereas that of uncharred SOC was 213 cmolc kg−1. Despite the large CEC of the kiln soil, exchangeable K+ content was no different from the adjacent soil, whereas exchangeable Ca2+ and Mg2+ contents were considerably larger. Charcoal enrichment has little effect on available, inorganic and total P, but it can form strong complexes with Cu, which reduces the availability of the metal. Biochar is very persistent in soil; therefore, long‐term implications should not be overlooked. Highlights Charcoal kiln soil contains from 1.8 to 33.1 g kg−1 of charcoal‐C, which raises C:N and C:P ratios. Charcoal‐C content was estimated by differential scanning calorimetry. We estimated a CEC of 414 cmolc kg−1 for charcoal‐C and 213 cmolc kg−1 for uncharred SOC. Retention of exchangeable K+ remained unaffected by charcoal but that of Ca2+ and Mg2+ increased.

This study was set up to identify the long-term effect of biochar on soil C sequestration of recent carbon inputs. Arable fields (n = 5) were found in Belgium with charcoal-enriched black spots (>50 m 2 ; n = 14) dating >150 years ago from historical charcoal production mound kilns. Topsoils from these 'black spots' had a higher organic C concentration [3.6 AE 0.9% organic carbon (OC)] than adjacent soils outside these black spots (2.1 AE 0.2% OC). The soils had been cropped with maize for at least 12 years which provided a continuous input of C with a C isotope signature (d 13 C) À13.1, distinct from the d 13 C of soil organic carbon (À27.4 &) and charcoal (À25.7 &) collected in the surrounding area. The isotope signatures in the soil revealed that maizederived C concentration was significantly higher in charcoal-amended samples ('black spots') than in adjacent unamended ones (0.44% vs. 0.31%; P = 0.02). Topsoils were subsequently collected as a gradient across two 'black spots' along with corresponding adjacent soils outside these black spots and soil respiration, and physical soil fractionation was conducted. Total soil respiration (130 days) was unaffected by charcoal, but the maizederived C respiration per unit maize-derived OC in soil significantly decreased about half (P < 0.02) with increasing charcoal-derived C in soil. Maize-derived C was proportionally present more in protected soil aggregates in the presence of charcoal. The lower specific mineralization and increased C sequestration of recent C with charcoal are attributed to a combination of physical protection, C saturation of microbial communities and, potentially, slightly higher annual primary production. Overall, this study provides evidence of the capacity of biochar to enhance C sequestration in soils through reduced C turnover on the long term.

Keywords:Preindustrial charcoal kiln Land-use change Biochar X-ray photoelectron spectroscopy (XPS) Differential scanning calorimetry (DSC) Fourier Transform Infrared Spectroscopy (FTIR) 13 C nuclear magnetic resonance ( 13 C NMR) Dichromate oxidation a b s t r a c t Black carbon (BC) plays an important role in terrestrial carbon storage. Nevertheless, the effect of cultivation on long term dynamics of BC in soil has been poorly addressed. To fill this gap, we studied the chemical properties of charcoal particles extracted from preindustrial kilns in Wallonia, Belgium, along a chronosequence of land use change from forest to agricultural soil, up to 200 years of cultivation. Preindustrial charcoal samples were compared with charcoal subjected to short term ageing in a currently active kiln.Cultivation increased the association of charcoal with soil minerals, which is favored by deprotonation of carboxylic acids under liming, thereby enhancing the reactivity of charcoal toward mineral surfaces. The large specific surface area of charcoal, related to its porosity, promotes the precipitation of 2:1 phyllosilicates and CaCO 3 . Both ageing and cultivation decreased the resistance of charcoal to dichromate oxidation, related to an increase in the H/C of charcoal. Differential scanning calorimetry revealed the presence of three fractions of distinct thermal stability. Saturation of carboxylate groups with Ca 2+ under liming decreased the thermal stability of the O-rich, less thermally stable fraction of charcoal. This fraction decreased over time of cultivation, leading to a relative accumulation of the thermally most stable fraction of charcoal. This might result from the preferential loss of the O-rich fraction or the slowdown of charcoal from oxidation via association with minerals. Our results highlight the idea that land use significantly affects the properties of BC through the modification of soil conditions, which might influence the kinetics of BC loss from soil.

Soil amendment with biochar can modify soil microbial abundance, activity and community structure. Nevertheless, the long-term evolution of these effects is unknown and of critical importance because biochar persists in soil for centuries. We selected nine charcoal kiln sites (CKS) from forests (four sites) and croplands (five sites) and determined the microbial properties of their topsoil, largely enriched with charcoal for >150 years. Adjacent soils were used as references unaffected by charcoal production. Soils were incubated in controlled conditions and emissions of CO 2 were measured for 138 days. At day 68, an aliquot was sampled from each soil to determine microbial abundance and community structure by phospholipid fatty acid (PLFA) analysis. Before the extraction, one standard PLFA (C21:0 PC) was added to the soil to test the influence of charcoal on PLFAs recovery. The content of uncharred SOC and pH explained a main part of the variance of soil CO 2 emissions, which supports the view that charcoal had a limited effect on soil respiration. The recovery of C21:0 PC was increased in presence of aged charcoal, which contrasts with the decreased recovery recorded shortly after biochar application. This underlines that properties of charcoal evolve dramatically over time, and that a long-term vision is critical in the perspective of amending soils with biochar. Land-use had an overriding control on the microbial community structure, surpassing the effect of a vast amount of charcoal present in the soil. In forests, 10 PLFAs from gram positive and general bacteria were significantly different between CKS and adjacent reference soils, whereas in croplands only four PLFAs from fungi, gram negative bacteria and actinomycetes were significantly affected. These results suggest that the long-term effect of charcoal on soil microbiota is overwritten by management practices. Biochar properties must therefore be regarded altogether with soil conditions to correctly design a successful soil amendment with biochar. Additionally, the absence of a relationship between individual PLFAs and charcoal-C supports the idea that the long-term effect of charcoal is related to a modification of soil ecological niche (e.g., nutrient availability, pH) rather than to an alteration of the source of organic C available to biota.