Phytotoxicity of alkaline hydrolysate from poultry mortalities before and after anaerobic digestion: neutralization strategy and implications for land application.

Ambient alkaline hydrolysis (AAH) offers a decentralized strategy for biosecure poultry mortality management, producing a nutrient-rich but highly alkaline and saline hydrolysate. This study provides the first systematic characterization of the phytotoxicity of AAH poultry mortality hydrolysate and its anaerobic digestate, using a tomato seed germination bioassay across two alkali sources (KOH, NaOH), four neutralization treatments (non-neutralized, HCl, CO2, and silage), and four dilution levels (10° - 10-3) in triplicate. In Experiment 1, complete germination inhibition was observed across all treatments at undiluted concentrations, with recovery dependent on neutralization and dilution. KOH-derived hydrolysate showed less persistent phytotoxicity than NaOH-derived hydrolysate at 10-1, consistent with the lower residual pH measured for KOH at equivalent dilutions. HCl and silage neutralization most effectively mitigated phytotoxicity, reaching 93.33 ± 5.77% and 83.33 ± 5.77% germination at 10-1, respectively. CO2 neutralization produced persistent phytotoxic effects that were not resolved until 10-2 dilution. In Experiment 2, anaerobic digestion substantially reduced phytotoxicity across all treatments, with most digestates statistically comparable to the inoculum-only control by 10-2. CO2-neutralized digestate was the exception, remaining significantly more phytotoxic at 10-2 (80.00 ± 0.00% germination versus 96.67 - 100%) before recovering at 10-3. These findings support land application of digestate within the integrated AAH-anaerobic digestion pathway, which additionally enables biosecure mortality stabilization and renewable energy recovery. CO2 neutralization via engine exhaust sparging is cautioned against when land application is intended. Minimum dilution thresholds of 10-1 for HCl- or silage-neutralized material, and 10-2 for CO2-neutralized material, are recommended pending field-scale validation.