Sustained whole-chain ammonia mitigation in pig manure via acidogenic fermentation by-products

Livestock production generates roughly 65% of global agricultural ammonia (NH₃) emissions. Manure acidification suppresses NH₃ volatilization effectively, but conventional chemical acidifiers are limited by handling risks and site-specific costs. This study evaluated two industrial fermentation by-products - distillers' grains (DG) and vinegar residue (VR) - as alternative acidogenic amendments for whole-chain NH₃ mitigation across pig manure storage, composting, and soil application. A single application of DG and VR at by-product-to-manure ratios of 1:3 and 2:3 reduced storage-stage emissions by 93.7% and 94.5%, statistically indistinguishable from sulfuric acid (94.1%). Unlike the rapid pH rebound observed with the chemical acidifiers, DG and VR sustained low pH throughout storage through continued organic-acid production. The same initial amendments further reduced NH₃ emissions during subsequent composting by 13-27% and extended the thermophilic phase by up to 7 days. At the soil-application stage, the resulting composts suppressed NH₃ volatilization by 49-70% relative to urea alone and enhanced soil enzyme activities. Heavy-metal stabilization was also improved, with greater proportions of Hg, Pb, and Cr shifting to more stable fractions while the final products remained within agricultural safety limits. A screening-level whole-chain assessment indicates that this regional reuse pathway is most favorable within a 10-15 km sourcing radius, where transport-related climate burdens remain below 1.017 kg CO₂e t^-1 manure, with the advantage eroding under longer-distance hauling. These findings identify acidic fermentation by-products such as DG and VR as promising candidates for regional reuse in integrated manure management, pending field-scale, microbial, and techno-economic validation.