Effect of biochar amendment during composting and compost storage on compost quality, N losses and P availability

We tested the effect of adding 10% biochar (on a dry weight base) in a mixture of green waste and the organic fraction of municipal solid waste before processing in a composting plant. Composting was executed in lanes with automated mechanical turning and forced aeration. Both the compost and the biochar-blended compost were characterised chemically and physically in order to quantify the effect of biochar on compost quality and nutrient losses during the process. Temperature in the reference treatment was significantly higher than in the biochar-blended compost throughout the whole bio-oxidative phase (1-27 days). Mixing biochar in the compost feedstock clearly affected the process and compost characteristics, i.e. the oxygen uptake rate (OUR) and nutrient availability during this phase. A higher organic matter decomposition and dry matter content and lower P and NH4+-N concentrations were observed for the fraction <10 mm of the biochar-blended compost. After 27 days a clearly lower OUR was detected for the biochar-blended compost. Higher water-extractable NH4+-N-concentrations coincided with a longer lag-phase for OUR in the reference compost, these effects were absent in the case biochar was added to the compost. We measured a higher NH4+-N retention for the biochar-blended feedstock in a laboratory scale experiment on compost storage. The effect of mixing biochar in the compost feedstock was further monitored during the maturation phase (28-90 days). A mass balance calculation indicates lower N losses for the biochar-amended compost. The experiment will be repeated in spring 2014 and the results will be presented at ORBIT2014.