The weathering of a trioctahedral mica (phlogopite, 2–100‐µm particle size) was investigated in the rhizosphere of ‘Turilo’ Italian ryegrass (Lolium multiflorum Lam.). A special culture vessel made it possible to simulate a one‐dimensional macroscopic rhizosphere by developing a root mat on a polyamide net. This root mat was laid down on a mica‐agar substrate that was cut into slices 0.5 mm thick (parallel to the root mat) after 1, 2, 3, and 4 d of contact. The phlogopite mica was the sole source of both K and Mg for the plants. Whereas there was no significant release of Mg as a consequence of the roots' activity, K was released and concomitantly the mica lattice expanded after 3 d of continuous exposure to the root mat, in response to K uptake by plants. After 4 d, the vermiculitization of the phlogopite was detectable up to 1.5 mm from the root surface (i.e., the root mat) by x‐ray diffractometry. Such rapid mineralogical changes induced by roots support the view that primary minerals such as trioctahedral micas might contribute significantly to the supply of K to the plants, at least around the most active parts of the roots.

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.