Mixing a small amount of biochar in green waste and chicken manure compost changes the P availability

Manure and bio-waste have a great potential for conversion into bio-energy or as an alternative to chemical fertilizers. The FP7 project FERTIPLUS assists the EU in the design and implementation of innovative strategies and technologies for the recycling of waste to both compost and biochar for use in agriculture. Biochar can be added to mature compost or can be mixed in the feedstock mixture. The activities of the FP7 project FERTIPLUS include a full scale trial on the effect of biochar on the performance of the composting process, and a study on the effects of biochar on compost storage, i.e., when biochar is added to stable compost. The aim of the research presented here is to assess the effect on P availability when a smaller amount of biochar and additional P is added to the compost. In the incubation trial two composts (green waste compost and compost based on chicken manure) were mixed with 2 types of biochar, with the pure compost as control treatment. Mixing these composts and biochar types resulted in mixtures with variable P concentrations and C/P ratio. Additional P is supplied as KH2PO4 solution. Blending compost with biochar results in an increase in carbon content and may affect the C/P ratio of the compost. Even when a small dose of biochar was applied, mixing biochar in mature compost significantly reduced the easily available P concentration. This effect was found on the longer term as well. The rate of reduction of the P availability is affected by both the compost and the biochar characteristics.