Short- and long-term changes in soil physical properties following biochar addition to soils with different textures

In the context of global soil degradation, biochar is being promoted as a potential solution to improve soil quality in addition to its carbon sequestration potential. Burying biochar in soils has been shown to affect soil physical properties, but the intensity of this effect depends, among others, on soil properties and application rates. Moreover, the long-term effects of biochar remain largely undocumented yet they are important to quantify given biochar’s persistency in soils. The objective of this study was therefore to determine the short-term and long-term effects of biochar on soil physical properties in function of soil texture and biochar application rates. For this purpose, experiments were conducted in Wallonia (southern Belgium) at two sites with different soil texture (silt loam and sandy loam). Both sites are characterized by the presence of former kiln sites that were used for studying the long-term (> 150 years) effect of hardwood biochar. In addition, fresh biochar produced from Picea abies at 500°C by GreenPoch SA was buried in subplots at both sites at rates of 1% and 2% in mass in the top 10 cm just before the spring sowing in 2021. Samples were collected on September 2021 in kiln sites, in fresh biochar (1 & 2%) subplots and in biochar-free reference plots, in triplicate. The measured soil physical properties were the bulk density, the water retention curve and the hydraulic conductivity curve.

Reference subplots were characterized by higher bulk density and lower saturated water content than biochar subplots. The presence of biochar increased the water content for pF < 1.5. These biochar effects were higher for the sandy loam compared to the silt loam and in the short-term compared to the long-term. The plant available water capacity increased with the presence of biochar on the sandy loam, but was unaffected on the silt loam. The hydraulic conductivity was slightly higher for pF < 1.5 in the biochar subplots compared to the reference subplots. On the silt loam, this effect was more visible for fresh biochar whereas on the sandy loam, this effect was higher in the kiln sites. Therefore, the study highlighted a positive effect of biochar in the short-term and in the long-term on soil physical properties for the range of soils and biochar application rates investigated here. Further research may be needed to confirm the observed trends over a wider range of soil types, biochar types and biochar application rates as well as to better understand the underlying mechanisms.