Nutrient characteristics of biochar prepared from animal manures at different temperature with limited oxygen

Compared with traditional manure composting, the pyrolysis of livestock manures is one of the thermochemical technologies for converting biomass into nutrient-rich biochars with potential agronomic uses, and reduces the environmental problems caused by fecal accumulation. Studies are needed to clarify biochar nutrients properties across manure varieties under similar controlled conditions. Of all parameters affected the biomass pyrolysis process, temperature is one of the most important factors. In this study, three types of biochars derived from chicken manure (CM), solid-liquid separated pig manure (PR) and dairy manure (DM), were prepared by pyrolyzing at different temperatures (350, 450, 550, 650 and 750 ℃), under an oxygen-limited condition in muffle furnace. The effects of temperature on the ash content, total carbon content, macro to micro nutrients contents and their remaining rate of biochars were analyzed. Meanwhile, the C/N ratio, the correlation of nutrient contents between raw materials and carbonized products, and the relationship between pyrolysis temperature and nutrient characteristics of biochars were also discussed. The results showed, as pyrolysis temperature increased, the biochar carbon (C) and nitrogen (N) contents decreased, while the contents of ash, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn) in biochars gradually increased. When pyrolysis temperature reached 750 ℃, the C and N contents decreased by 8.25%-19.86% and 59.62%-70.71%, respectively. Due to the decrease of volatile matter contents, the increased ash contents ranged from 43.28% to 57.65% at this temperature. The contents of P, K, Ca, Mg, Fe, Mn in biochars increased by 85.84%-128.54%, 85.03%-123.26%, 68.66%-169.03%, 33.02%-123.30%, 63.92%-103.62% and 56.92%-75.19% at 750 ℃, respectively. All nutrient differences reached a significant level (P<0.05). Although high temperature pyrolysis increased the total nutrient content and C/N ratio of manure-derived biochars, it also reduced the remaining rate of nutrients. When the pyrolysis temperature increased from 350 to 750 ℃, the residue rates of C, N, P and K in biochars decreased significantly by 58.25%-76.07%, 85.01%-88.23%, 33.31%-69.54% and 40.27%-71.29% (P<0.05), respectively. Those of Ca, Mg, Fe and Mn also reduced significantly by 45.18%-71.21%, 47.22%-73.52%, 40.84%-70.91% and 39.19%-68.11% (P<0.05). In general, the nutrient contents and their remaining rates in biochars were closely related to material source and pyrolysis temperature, and the correlation between nutrient characteristics and pyrolysis temperature was significant (P<0.05). Therefore, the selection of high C and high nutrient contents of animal manure, was the basis for improving the biochar nutrient contents, and suitable temperature was the key to retain the high nutrient residue rates of biochars. When the temperature increased from 450 to 550 ℃, the nutrients loss rates ranged from 20.12% to 71.23%, and the decrease was most obvious, indicating that the suitable temperature for manure pyrolysis was 450 ℃. Overall characteristics of biochar nutrients at this temperature, the selection of livestock manures for nutrient-rich biochars production was in order of DM > PR > CM in this study. Future studies focusing on other factors and the persistence of biochar fertility in the field must explicitly take into account additional factors to transfer this technology.