Long-term effect of biochar on soil physical properties of agricultural soils with different textures

In the context of global soil degradation, biochar is being promoted as a potential solution to improve soil quality, besides its carbon sequestration potential. Burying biochar in soils is known to effect soil physical quality in the short-term (<5 years), and the intensity of these effects depends on soil texture. However, the long-term effects of biochar remain largely unknown yet and 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 soil physical properties as a function of soil texture and biochar concentration.  For this purpose, soil physical properties (particle density, bulk density, porosity, water retention and hydraulic conductivity curves) were measured in the topsoil of three fields with former kiln sites containing charcoal more than 150 years old in Wallonia (southern Belgium).  The fields had a silt loam, loam and sandy loam texture.  Samples were collected along 3 transects in each field, from the center of the kiln sites outwards. 

Particle density and bulk density slightly decreased as a function of charcoal content. Because particle density and bulk density were affected to a similar extent by charcoal content, total porosity was not affected by the presence of century-old charcoal. Regarding the soil water retention curve, charcoal affected mostly water content in the mesopore range. This effect was strongest for the sandy loam. On the other hand, the presence of century-old charcoal increased significantly the hydraulic conductivity at pF between 1.5 and 2 for the silt loam, while no effect of charcoal was observed for the loamy soil.  The study highlights a limited effect of century-old charcoal on the pore size distribution (at constant porosity) and on the resulting soil physical properties for the range of soils and charcoal concentrations investigated here.  Further research may be needed to confirm the observed trends over a wider range of soil types.