Impact of biochar addition on the physical and hydraulic properties of a clay soil

Biochar represents a soil conditioner that can change the physical and hydraulic properties of the soil. To date, little information is available about the biochar-induced changes on physical and hydraulic properties of fine textured soils. Therefore, an evaluation of its effects before the field use is advisable. The main objective of this investigation was to evaluate the impact of biochar addition on saturated (K<inf>fs</inf>) and unsaturated (K(h)) hydraulic conductivity, water retention, capacitive indicators such as macroporosity (P<inf>mac</inf>), air capacity (AC), plant available water (PAWC) and relative field capacity (RFC, equal to the ratio between field capacity and saturated soil water content), dry bulk density (ρ<inf>b</inf>) of a repacked clay soil. Biochar effects on simulated wheat yields were also evaluated using the DSSAT model. Five levels of amendments (0-5-10-20-30g biochar per kg^-1 soil) were used and the soil columns remained in the field for about 30 months until undisturbed soil conditions were reached. No significant differences of the K<inf>fs</inf> values were detected between amended and unamended soils and the ratio between K<inf>fs</inf> values was, on average, equal to a factor of 1.01-0.93-0.98-1.25 (respectively for C5-C10-C20-C30). In the same way, biochar did not affect appreciably the K(h) values. Depending on the applied pressure head or the biochar concentration, the differences were within a factor of 0.83-0.39. On the contrary, significant increases of soil water retention were detected close to water saturation (0<h<-10cm) for the highest biochar concentration. This behavior resulted in an imbalance between the liquid and gaseous phases of the soil. The relative field capacity, that gives an account of the optimal balance between water capacity and air capacity of the soil, was lower than the critical limit of 0.6, suggesting excessive soil aeration. Since these results are coupled to non-significant differences in ρ<inf>b</inf> values, our results confirm that small decreases in bulk density (on average, 0.014gcm^-3) may result in appreciable modifications in soil water retention close to water saturation. The simulations carried out with DSSAT suggested that a moderate addition of biochar to a clay soil (not higher than 10gkg^-1) has the potential to increase the production of durum wheat (mean increase±standard deviation, 236±126kgha^-1). These findings will have to be verified under field conditions.