Circular nutrient management through slurry separation and pyrolysis: a field study

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.