Pyrolysis-Driven Trade-Offs Between Carbon Stabilization and Micronutrient Partitioning in Poultry Waste-Derived Biochars in Galicia (NW Spain)

The conversion of livestock manure, including poultry waste (PW), into biochar represents a sustainable strategy to recycle nutrients while reducing environmental risks. This study evaluated how pyrolysis temperature regulates physicochemical properties, carbon structure, and nutrient dynamics in biochars produced from PW. Raw PW and biochars generated at 300 and 600 °C were characterized through proximate and elemental analyses, Fourrier Transform Infrared spectroscopy (FTIR), soil nutrient assessment, and germination bioassays. A multivariate approach was used to analyze the experimental data sets. Increasing pyrolysis temperature significantly reduced biochar yield (83.62% to 64.36%), while promoting carbon condensation and mineral enrichment, as indicated by the decline in H/C ratio from 1.02 to 0.22 and the increase in ash content from 41.47% to 56.77%. FTIR analysis revealed a progressive attenuation of O–H and aliphatic C–H functional groups and a relative increase in aromatic structures with increasing temperature, indicating structural reorganization of the carbon matrix. Total concentrations of macro- and micronutrients generally increased with temperature; for example, total Cu increased from 78.62 to 114.17 mg kg−1, while Zn increased from 557.03 to 819.66 mg kg−1 between 300 and 600 °C. In contrast, the bioavailable fractions of Fe, Cu, and Zn determined using the chelating agent DTPA declined, although not significantly (p < 0.05), with increasing pyrolysis temperature. Principal component analysis clearly distinguished raw PW from pyrolyzed materials, confirming pyrolysis temperature as the main factor dictating biochar properties. PW exhibited severe phytotoxicity, which was partially mitigated with increasing pyrolysis temperature. Overall, pyrolysis enhanced carbon stabilization and micronutrient immobilization, highlighting PW-derived biochars as promising soil amendments for improving nutrient management and reducing the environmental risks associated with raw PW application.