Hydrologic properties of biochars produced at different temperatures

Adding charcoal to soil (biochar soil amendment) can sequester carbon and improve soil performance, although the extent and exact mechanisms of soil improvement are not clear. Additionally, biochar properties can vary significantly with production conditions. Here we characterize the impact of pyrolysis temperature on two important soil hydrologic properties: field capacity and hydrophobicity. We show that pure biochar exhibits a wide range in both properties depending on feedstock and pyrolysis conditions. We find that both properties can be controlled by choice of pyrolysis temperature; 400 °C–600 °C produced biochars with the most desirable hydrological properties (peak field capacity and minimum hydrophobicity). Further, we show that hydrophobicity is strongly correlated (R2 = 0.87; p < 0.001) to the presence of alkyl functionalities in FTIR spectra, suggesting that this property derives from aliphatic domains on the surface of low-temperature biochars. Although we could relate hydrophobicity to biochar chemistry, our chemical characterization techniques were insufficient to describe variation in field capacity of soil–biochar mixtures. Field capacity may be related to large biochar pores, suggesting the need for quantitative techniques to characterize large (greater than 0.1 μm) pores within biochar particles.