Although biochar has been intensively investigated in recent years, thereis still a lack of knowledge due to the variability of soil and biochar properties, and the wide variety of experimentalconditions used in these investigations. The properties of biochar strongly vary depending on the feed materialand the production process. Therefore, it is of great interest to have methods which allow the characterization andlong-term in-situ monitoring of biochar properties at different scales ranging from small laboratory columns to fieldsites. In this study, measurements on the complex electrical conductivity have been performed by spectral inducedpolarization (SIP). The method has been found to be a valuable tool for distinguishing different types of biocharand for monitoring the release of ions from biochar. SIP uses sinusoidal alternative current in the frequency rangebetween 1 mHz and 45 kHz and provides information on the ohmic conductivity and on the electrical polarizationof soil materials with added biochars. Whereas the release of ions leads to an increase of the ohmic conductivity,information on the chemical structure of biochars can be obtained from the polarization. Five types of biochar havebeen investigated in this study. The magnitude and the frequency dependence of the polarization can be used todistinguish the different types of biochar. Biochars with a larger degree of carbonization showed higher electronicconductivity and yielded higher polarization. The frequency dependence of the polarization further depended onthe amount and the size of the biochar particles. The measurements also confirmed that the release of ions frombiochar can be monitored using the ohmic conductivity. Whereas SIP is suitable for the characterization of biocharin the laboratory, electrical impedance tomography (EIT) can be used to obtain the spatial distribution of the complexelectrical conductivity in laboratory columns and in the field. This method is an imaging technique based onthe determination of SIP signals with a larger set of electrode positions. It opens up promising research avenuesfor in-situ monitoring of biochar in field soils
