Biochar improves N cycling during composting of olive mill wastes and sheep manure.

The use of biochar has been revealed to have beneficial effects during the composting of manures and other N-rich materials by reducing N losses and enhancing the rate of the process. However, the impact of biochar has not been explored in other complex organic matrices with low N nitrogen that may hinder the composting process. The main novelty of this work was to study the impact of a small amount of biochar (4%) on the composting process of olive mill wastes, which are characterised by a recalcitrant lignocellulosic composition with reduced nitrogen (N) availability. Two treatments: (i) control (olive mill waste 46%+sheep manure 54%, dry weight) and (ii) the same mixture treated with biochar (4%), were composted during 31 weeks. The incorporation of a small amount of biochar improved N cycling by increasing NO3(-)-N content, indicating a higher nitrifying activity, and reducing N losses by 15% without affecting the amount of N2O released. The use of biochar as an additive for composting could improve the value of olive mill waste composts by reducing N losses and increasing N availability in lignocellulosic and N-poor materials.