Potential of Chicken Manure-Derived Hydrochar as Soil Amendment or Organic Fertilizer and the Assessment of Its Phytotoxicity

To test hydrochar from chicken manure (CM) produced with the hydrothermal carbonization (HTC) process as a potential soil amendment, the fertilization potential and phytotoxicity of the product was assessed to ensure both effectiveness and safety for crops. The hydrochar produced by heating the feedstock at 250 °C for 0.5 h showed low nutrient and organic carbon (Corg) recovery but a high relative abundance of aromatic and alkyl C. Its favorable available-to-recalcitrant nitrogen (N) ratio, and low heavy metal (HM) concentrations suggest low phytotoxicity and high short- to medium-term N fertilization potential of the amendment. Pot experiments with lettuce, sunflower, and tomato plants confirmed species- and dosage-dependent effects. A dosage equivalent to 3.25 t ha⁻¹ (2.7% w/w) improved lettuce and sunflower yields, whereas higher dosage (6.5 t ha⁻¹; 5.4% w/w) did not provide additional growth benefits. Tomato plants, however, exhibited phytotoxic sensitivity to the higher dosage, which was expressed in a negative impact on germination and yield. Overall, these results highlight the need for a thorough chemical characterization of HTC-derived hydrochar before its use as an organic fertilizer amendment, considering dosage, crop species, and their specific sensitivities when applying it.