Biochar of animal origin: a sustainable solution to the global problem of high-grade rock phosphate scarcity?

Phosphorus (P) is an essential element for all living organisms. However, in soil-plant systems, this nutrient is the most limiting, leading to frequent applications of soluble P fertilisers. Their excessive use provokes alterations in the natural P cycle, soil biodiversity and ecological equilibrium and is the main reason for the eutrophication of water, with consequences on food safety. Biotechnology offers a number of sustainable solutions that can mitigate these problems by using various waste materials as a source of P and, on the other hand, their solubilisation by selected micro-organisms. This review present results on the solubilisation of animal bone char with high phosphate content by micro-organisms to produce organic acids such as lactic acid, citric acid and itaconic acid. All experiments were performed under conditions of liquid submerged and solid state fermentation processes. Freely suspended and immobilised cells of the corresponding microbial cultures were employed using substrates characterised by low cost and abundance. Other alternative technologies are discussed as well in order to stimulate further studies in this field, bearing in mind the progressive increase in P fertiliser prices based on high global P consumption and the scarcity of rock phosphate reserves.