Changes in nutrient content and availability during the slow pyrolysis of animal wastes

Although a large number of reports are available on the total and soluble phosphorus, potassium, and nitrogen (and other nutrient elements) content of manure biochars, information is lacking to understand the changes in chemical speciation of different elements during pyrolysis. Manure is intrinsically heterogeneous, and is composed of feces, urine, bedding materials, waste feed (and their degradation products formed during storage) and in some cases soils. Molecular level understandings in thermochemical transformation pathways are necessary to predict the utility of manure biochars as a sterile, renewable, organic fertilizer for different soil types. This critical review will utilize micro/spectroscopic characterization of manure biochars from slow pyrolysis and gasification as well as manure ash to understand the phosphorus speciation. Inorganic (ash) component is enriched with phosphorus and potassium, and can comprise over 50 wt% (on a dry weight basis) of manure biochar. Ash is a complex mixture of amorphous, semi-crystalline, and crystalline inorganic phases as well as organic (char) components. These ash components may be originally present in the manure feedstock or thermochemically produced (e.g., apatite). Organic carbon fraction of biochar likely stabilizes ash by various organo-mineral interactions, e.g., by serving as a template for the formation of nano-crystals during pyrolysis. Depending on the thermochemical conversion procedure, stable minerals like hydroxyapatite and whitlockite may form. Much like phosphate rock, acidulation and other pretreatments such as blending with more soluble fertilizer and compost may be necessary to fully utilize less soluble P of manure biochars under alkaline conditions.