Effect of fast pyrolysis biochar on physical and chemical properties of a sandy soil

Biochar is the solid product of biomass pyrolysis, which is one of the technologies available for bioenergy production. Biochar additions to soils have shown a wide range of benefits, which are strongly influenced by the nature of the feedstock and pyrolysis conditions used for the biochar production. The present study was focused on assessing the potential of a hardwood (red oak) fast pyrolysis (500°C) biochar to increase water holding capacity of a sandy loam soil and its influence on nutrient availability. We hypothesized that biochar increases the water holding capacity of the soil. Moreover, we hypothesized that the depth of biochar incorporation influences the rate of biochar surface oxidation and hence moisture retention, effective cation exchange capacity (ECEC) and nutrient availability in biochar amended soils. Biochar (0, 3 and 6% w/w) mixed with soil was placed into columns in either the bottom 11.4 cm or the top 11.4 cm of the columns to simulate deep-banding within rows (DBR) and uniform topsoil mixing (UTM) applications, respectively. Columns were incubated at 28°C and 80% relative humidity. Every 7 days 150 mL of a 0.001M calcium chloride solution was added to the columns to produce leaching. Addition of biochar increased by 23% the gravity drained water content relative to the control soils. Biochar did not affect the ECEC of the soil. Soil bulk density of the controls increased with incubation time (from 1.41 to 1.45 g/cm3), while bulk density of biochar treated soils was 9% less than the control and remained constant through the incubation period. After 91 days of incubation, soil pH increased 0.51 units (from 7.10 to 7.61) in biochar treatments relative to the controls. Soil treatments with 6% (w/w) biochar (UTM_6 and DBR_6) showed an increase of 185% in exchangeable potassium (K) compared to the control soils (0% biochar). Exchangeable Ca and Mg decreased in biochar treated soils compared to the control. Overall, these results suggest that addition of hardwood (red oak) fast pyrolysis (500°C) biochar has the potential to increase water holding capacity of sandy loam soils, and that it increases availability of some nutrients. The insights of this study indicate that biochar amendments have the potential to enhance the quality of sandy soils, and therefore should be considered as a management option to enhance the sustainability of biomass harvesting for bioenergy production.