Superior nutrient recovery and release by chicken manure-derived biochar over hydrochar and compost for soil fertilization

Abstract

Recycling nutrient-rich organic waste materials in agriculture is increasingly recognized for its potential to establish sustainable systems, especially considering the diminishing availability of chemical fertilizers. However, appropriate pretreatment methods are essential for safe and efficient utilization in soil systems. In this study, we compared conventional composting (compost) with low-temperature pyrolysis (biochar) and hydrothermal carbonization (hydrochar) for conserving nutrients from chicken manure (CM) and producing stable organic fertilizer. Biochar exhibited superior nutrient recovery, with 76.4 % of N retrieved from CM compared to 37.4 % for hydrochar and 36.2 % for compost. The escape of NH₃ gas from deaminated hydrochar slurry and volatilization loss during composting might have caused such low N recovery in hydrochar and compost, respectively. When applied to soil, biochar released nearly 100 % of its nitrogen in phyto-available form and may satisfy 88.5 % of the N requirements for optimum sweetcorn production. Decomposition of pyridinic- and pyrrolic-N in low-temperature CM biochar might have contributed to this high N release. In contrast, compost released 85 % of its N, and hydrochar released only 33 %. Biochar also preserved the highest amount of carbon (67.6 %) during production, with a significantly higher fixed carbon content (19.2 %). Over a 140-day incubation period, while 26.6 % of compost‑carbon was emitted as CO₂ from soil, biochar remained significantly stable (11.5 %). These findings underscore manure biochar's potential as an effective tool for soil fertilization and carbon sequestration, surpassing compost and hydrochar in nitrogen recovery, nitrogen release, and carbon stability in soil.