added, while pyrochar promotes plant growth despite its equally wide C-to-N-ratio. Microorganisms, on the other hand are favoured in the presence of materials which are easily degradable either due to the absence of aromatic structures (corn silage) or their lower level of structural organization (hydrochar). In this context,hydrochar takes an intermediate position e despite containing also aromatic structures, its narrow C-to-N-ratio enables microbes to mineralize the material to a similar degree as uncharred corn silage without immobilising the contained N, thus sustaining a relatively large amount of microbial biomass while simultaneously guaranteeing a strong plant growth. A study by Bai et al. (2013) confirms
that hydrochar can be similar in its degradability to uncharred material who produced both types of char from Miscanthus and found that the half-life of hydrochar was comparable to that of the educt and much lower than for pyrochar. However, the application of additional N fertilizer has to be evaluated carefully, since plants are obviously not able to take up the added N but use the NH4 set free via the mineralization of the char material instead. The situation is different with pyrochar: A 15N tracer experiment with pyrochar produced from corn revealed that pyrochar caused a transfer of N from a stabile to a more labile pool and thus increased plant available N. This is in agreement with Zheng et al. (2013) who argued that biochar might increase the bioavailability of N and thus lead to reduced N fertilizer requirements in corn plants. Likewise, in our experiment plants effectively use added NO3 fertilizer in the presence of pyrochar,which might help to stabilize NO3 in soil, as suggested by the fact that different fertilizer levels are well reflected by soil NO3
concentrations.Both types of char thus have the potential to reduce the required fertilizer amounts e hydrochar because N might be made available due to the mineralization of the char material, pyrochar because it stabilizes N and increases its bioavailability for plants.