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, CO 2 , 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. CO 2 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 . CO 2 -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. CO 2 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 CO 2 -neutralized material, are recommended pending field-scale validation.