Staged waste-based lactic acid broth redistributes multi-gas emission windows during swine manure composting

Composting recycles livestock manure, but temporally distinct ammonia, nitrous oxide and methane emissions can obscure trade-offs between nitrogen conservation and climate mitigation. We evaluated staged supplementation with waste-based lactic acid fermentation broth (LA) during 42-d forced-aeration rotary-drum composting of swine manure and maize straw, using three parallel reactors per treatment and equal-mass water addition (WA) as the control. LA showed a short initial heating/decomposition lag followed by thermophilic and carbon-transformation catch-up. Reactor-mean ammonia release during temperature-defined days 2-6 was 49.9% lower; the sampled peak shifted from day 3 to day 5 and was 17.3% lower. Later compensation limited the day-42 mean difference to 7.8%, indicating early-window attenuation and redistribution. Observed cumulative means of nitrous oxide and methane were 41.5% and 37.3% lower, respectively, producing a 38.5% lower direct non-CO₂ greenhouse-gas mean. Lower early pH was consistent with a lower estimated free-ammonia fraction and temporary maintenance of an ammoniacal pool. Higher early nosZ/(nirK + nirS) and nosZ/nirK ratios indicated greater annotated terminal nitrous-oxide-reduction potential relative to nitrite-reduction potential. After day 21, methane approached zero despite a higher sampled methane peak under LA, as late carbon dioxide release weakened and mcrA, mcrB and mcrG abundances remained low. These DNA profiles represent functional potential rather than gene activity. Both germination index (GI) means exceeded the 70% criterion by day 7; late temperature, carbon dioxide, total organic carbon (TOC) and humic-like dissolved organic matter (DOM) supported progressive stabilization. Strong early ammonia attenuation is promising for recurrent-feed thermophilic semi-continuous systems with captured exhaust, based on batch results.