Long‐term effect of biochar on physical properties of agricultural soils with different textures at pre‐industrial charcoal kiln sites in Wallonia (Belgium)

Besides its carbon (C) sequestration potential, biochar is being promoted as an amendment to improve soil quality. Burying biochar in soils is known to affect soil physical quality in the short‐term (<5 years), although the intensity of these effects depends on soil texture and biochar concentration especially. However, the long‐term effects of biochar remain largely unknown, yet are important to quantify given biochar's persistency in soils. The objective of this study was therefore to assess the long‐term effect of biochar on physical properties as a function of biochar concentration for soils with differing textures. For this purpose, soil physical properties were measured in the topsoil of three fields (silt loam, loam and sandy loam textures) in Wallonia (southern Belgium) presenting former kiln sites containing charcoal more than 150 years old. Particle density and bulk density slightly decreased as a function of charcoal‐C content but the presence of charcoal in kiln sites did not affect total porosity. The water retention curve measurements revealed that water content was mostly affected in the mesopore range by the century‐old charcoal‐C content. This effect was strongest for the sandy loam. On the other hand, the presence of century‐old charcoal increased the hydraulic conductivity for pF values between 1.5 and 2 for the silt loam, whereas no effect of charcoal was observed for the loamy and sandy loam soils. Results are discussed with respect to biochar characteristics and the various ways in which biochar particles could be distributed within soils. Overall, the present study highlights a limited effect of century‐old charcoal on the pore size distribution (at constant porosity) and on the resulting soil physical properties. HIGHLIGHTS: The effect of century‐old biochar on physical properties was investigated for 3 different soil textures. Water retention was affected mostly in the mesopore range by biochar. Small effect of biochar on hydraulic conductivity was observed for silt loam soil only. Overall, there was a limited effect of century‐old biochar on soil physical properties.