Nitrogen and Phosphorus Availability of Biologically and Thermochemically Decomposed Human Wastes and Urine in Soils with Different Texture and pH

Human waste contains ample plant-available nitrogen (N) and phosphorus (P), which can be used to address soil nutrient deficiencies. Biological and thermochemical sanitization methods have been proposed for safely treating the solid fraction of human waste (called human solid waste [HSW] in this study) but have rarely been compared directly. Furthermore, interactions between amendments and soil properties are also unclear, as pH, texture, and the presence of oxides all affect N and P availability. We therefore evaluated the effect of three HSW sanitization methods, thermophilic composting (60°C), torrefaction (200°C), and pyrolysis (300°C, 500°C), on N and P availability following incubation in four soils with different pH (4.6–7.4), textures (clay loam to loamy fine sand), and mineralogy. We also evaluated the effect of added urine with pyrolyzed HSW (500°C) on N and P availability in soils. A trade-off between N and P availability was observed with highest heating temperature of HSW, the magnitude of which varied with soil type. Compared with pyrolyzed HSW (500°C), composted HSW increased available soil N by 15% to 25%, whereas torrefied HSW increased available soil N by 40% to 57%. The change in P availability over time increased with pyrolyzed HSW (500°C) by 41% to 106% and 84% to 165% compared with composted and torrefied HSW, respectively. Multivariate linear regression highlighted the negative correlation between P availability and both soil oxides and amendment dissolved organic carbon. Combined fertilization with pyrolyzed HSW (500°C) and urine ameliorated low N, while preserving high soil P availability, which was 44% to 147% greater than soils fertilized with urine alone. Pyrolyzed HSW (500°C) lowered pH by 0.06 to 0.38 units compared with urine-amended soils with no pyrolyzed HSW. In conclusion, low N and P availability was observed with composted HSW (29–46 kg N ha⁻¹, 6–15 kg P ha⁻¹); ample N and moderate P were available with torrefied HSW (71–107 kg N ha⁻¹, 12–30 kg P ha⁻¹), whereas excess mineral N and ample P were provided in soils amended with pyrolyzed HSW (500°C) together with urine (404–486 kg N ha⁻¹, 33–81 kg P ha⁻¹) albeit at the expense of mild soil acidification.