Influence of biochar incorporation on TDR ‐based soil water content measurements

Summary The incorporation of biochar ( BC ) into agricultural soil changes the soil's physical properties, which leads to changes in the soil's hydraulic properties, such as water retention and permeability, and alters the soil moisture environment in agricultural fields. To elucidate the effects of the incorporation of BC on the soil moisture environment, measurements of the soil water in biochar‐amended agricultural fields are needed. Time domain reflectometry ( TDR ) is a widely used and established technique for the continuous measurement of the soil water content ( SWC ) in agricultural fields. However, TDR measurements are affected by the conductivity of soils. Biochar formed at higher pyrolysis temperatures is known to be very conductive. Therefore, we investigated the influence of the incorporation of BC on TDR ‐based SWC measurements. We examined calcaric dark red soil and the BC produced by pyrolysis of sugarcane bagasse at 400, 600 and 800°C. The apparent relative permittivity ( ε a ) of the BC (800°C)‐amended soil was greater than that of the non‐amended soil at a given water content, whereas the ε a values of the soils amended with the BC (400°C) and BC (600°C) were the same as that of the non‐amended soil at a given water content. We concluded that when a calibration curve obtained from a non‐amended soil is used, TDR ‐based measurements tend to over‐estimate the SWC containing the BC formed at higher pyrolysis temperatures because of conductive and dielectric losses. Therefore, the use of the real component ( ) of the soil's complex relative permittivity instead of ε a is effective when making TDR ‐based water content measurements of soils that contain BC formed at higher pyrolysis temperatures.