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Circular nutrient management through slurry separation and pyrolysis: a field study
Summary
Scientists tested a new way to recycle farm waste by separating liquid and solid parts, then turning the solids into charcoal-like material called biochar. The liquid parts worked well as fertilizer and produced less harmful greenhouse gas emissions than regular animal waste. This method could help farmers grow food more sustainably while reducing environmental pollution that can affect air and water quality in farming communities.
Abstract Mechanical separation of biowastes into liquid and solid fractions, coupled with pyrolysis of the solid fractions into biochar, offers a promising strategy for sustainable nutrient recycling in agriculture. This approach enables localized nitrogen (N) supply through liquid fractions and long-distance phosphorus (P) transport via biochar. We compared biogas digestate (BD) and pig manure slurries (PS) with their respective liquid fractions (LFD, LFS) as N sources, and biochars produced from their corresponding solid fractions (BcD, BcS) as P source applied in combination with mineral N. All treatments were evaluated against a mineral fertilizer control (MinF). Spring barley and winter wheat were cultivated in Year 1 and 2, respectively, of this 2-year field trial. Treatments were evaluated based on agronomic performance (grain yield and apparent N recovery, ANR) and environmental impact (cumulative and yield-scaled N 2 O emissions). Liquid fractions demonstrated superior plant N recovery compared to raw slurries in Year 2 (when applied as standalone N sources), with LFD achieving 28% ANR in wheat versus 18% for BD, while maintaining yields at 92–97% of MinF levels. Although LFS had 26% lower cumulative N 2 O than PS (1.49 vs 2.01 kg N 2 O-N ha −1 , Year 2), its yield-scaled emissions remained 60% higher than MinF (0.16 vs 0.10 kg N 2 O-N t −1 grain). Biochars applied at rates of 8–11 t ha −1 showed no immediate agronomic benefits or N 2 O mitigation. Notably, BcS reduced barley yield by 61% relative to MinF, while BcD performed comparably, indicating feedstock-dependent biochar effects. These findings support mechanical separation as a viable approach for circular nutrient management, with liquid fractions demonstrating advantages as effective N sources. The strategy requires integrated emission mitigation approaches to optimize both agronomic and environmental outcomes.