Biochar is currently of emerging interest, because it is a by-product of the increasing energetic use of organic waste and energy crops via pyrolysis. It is discussed as a modifier for soil to improve fertility and store carbon for long-term. The benefit of carbon for fertility is well known from terra preta (anthropogenic "black soil") in Amazonia since centuries, but many questions are still open, e.g. concerning the influence of charcoal on the hydrology in different soils. The large variety of feed material and pyrolysis techniques may result in different effects concerning the properties of soil. Spectral induced polarization (SIP) was considered as a method which can contribute to answer some of the questions. Therefore, a series of studies was performed on mixtures of 4 different biochars and commercial active carbon with sand. The results obtained provide additional information on the characterization of different types of charcoal and on the fertilizing effect of biochar. Considerable differences were found for biochars depending on the pyrolysis technique for both the real and the imaginary part of complex conductivity. The time dependence of the release of ions from the carbonaceous material could also be monitored. Based on the results of SIP, a 2D experiment was performed with electrical impedance tomography (EIT) which proved the suitability of the method to locate biochar and active carbon in sand. Since EIT can be applied at different scales, it has high potential for studying the influence of biochar on the hydraulic properties of soil and on its fertilizing effect in the laboratory and in the field.


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