**4.2 C sequestration and priming as function of biochar amount**

Biochar could cause a positive priming effect due to its high surface area providing habitat for microorganisms and due to input of partly labile C substrate (condensates). On the other hand, biochar is a stable compound which could stabilize labile compost OM thus providing a negative priming effect.

Composting of biochar could be successfully conducted over a wide biochar / organic material ratio covering up to 50% biochar by weight. During composting, a relative enrichment of biochar was observed which is obvious as biochar is much more stable than organic waste materials (Erben, 2011). However, biochar caused a significant positive priming effect on non-biochar composting materials at low (up to 1 weight%) biochar concentrations (Erben 2011) while at high (up to 50 weight%) biochar concentrations a significantly negative priming effect could be observed (Erben, 2011; Fig. 5). Therefore, a synergistic benefit for overall C sequestration could be observed when biochar was composted together with organic waste material (Erben, 2011). Further co-benefits might arise for soil microbial biomass and community structure composition and for biochar surface oxidation which still has to be proven scientifically.

Combining biochar addition and fermentation resulted in negative priming (Fig. 5), but the effect was weaker here than that of non-fermented treatments and hence ascribed rather to biochar alone than to its reinforcement of fermentation-induced negative priming.
