Biochar can improve soil fertility and biomass production. However, biochar derived from different processes and different feedstocks differ with respect to their physicochemical properties, which leads to diverse effects on soil functions after application. Moreover, the prerequisite for an amendment of biochar to soils is the exclusion of negative effects on soil functions and the environment. Therefore this study investigates effects of three biochars differing strongly in their physical and chemical properties on soil functions of two temperate soils in a field trial, a flash pyrolysis coke from spruce wood (PC), a gasification coke from beech wood (GC) and conventional charcoal produced from beech wood (CC). CC and GC exhibited clearly higher pH values (CC: 8,4; GC: 10,6; PC: 4,2), ash and nutrient contents, specific surface areas (CC: 122 m2g-1; GC: 191 m2g-1; PC: <0,4 m2g-1) and micro porosities (CC: 450 m2g-1; GC: 449 m2g-1; PC: 262 m2g-1) than PC. The degree of condensation increased in the order PC (H/C = 0.58) < CC (H/C = 0.45) < GC (H/C = 0.20) and the degree of oxidation decreased in the order PC (O/C = 0.13) > CC (O/C = 0.05) > GC (O/C = 0.04). The different physicochemical properties of the biochars are reflected in the results of the field trial. Clear differences between soils mixed with different biochars could be shown. However, significant positive effect of the added biochars (1.5% weight) on soil functions was only detected for GC. Applied PC significantly decreased the soil pH, whereas the application of GC increased pH and soil water content. GC also significantly increased cation exchange capacity and available nutrients, especially phosphorous in the sandy soil. Concentrations of inorganic and organic pollutants were found in the biochars, but did not exceed German environmental standards. Our results indicate that biochar may have positive effects on soil functions in temperate soils, but it can also have negative effects, which might outweigh potential merits such as carbon sequestration. Properties of different biochars and their effects on soil functions such as fertility must be considered and regulated prior to the widespread use of biochar as a soil conditioner.