We investigated the use of biochar (10% on a dry weight basis) to improve the composting process and/or
the compost quality by adding it to either the feedstock mixture or the mature compost. The addition of
biochar to the feedstocks was essayed in a full scale trial using a mixture of green waste and the organic
fraction of municipal solid waste. Addition of biochar to mature compost was performed in a medium
scale experiment. The use of biochar, even in small amounts, changed the composting process and the
properties of the end products. However these effects depended on the time of application. We observed
a faster decomposition in the bio-oxidative phase and lower greenhouse gas emissions when biochar
was added at the beginning of the composting process, and a reduction in readily available P when
biochar was applied during compost storage. Biochar as a means to increase the C content of the compost
was only effective during compost storage. The P fertilizer replacement value of the compost with and
without biochar was tested in a plant trial with annual ryegrass. While there was a clear effect on readily
available P concentrations in the compost, adding biochar to the feedstock or the compost did not affect
the P fertilizer replacement value.