Effects of iron on greenhouse gases and ammonia emissions and related genes in swine manure composting

Greenhouse gases (GHGs) and ammonia (NH₃) emissions from livestock and poultry manure composting are a crucial environmental problem worldwide. In this study, effects of ten soluble iron (Fe) salts or insoluble Fe and Fe minerals on carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and NH₃ emissions were systematically explored in swine manure composting, respectively. Results illustrated that soluble Fe salts were superior to insoluble Fe minerals on reducing global warming potential (GWP) of GHGs. Furthermore, Fe(Ⅲ) salts were more effective on weakening CH₄, NH₃ and CO₂ emissions compared to Fe(Ⅱ) salts. Specifically, 35.7 % and 67.3 % of CH₄ reduction was attributed that ferrous chloride (FeCl₂) and ferric chloride (FeCl₃) increased initial oxidation reduction potential (ORP) and mitigated mmoX and pmoA downward in initial composting. Abundances of mmoX and pmoA decreased by 45.9 % and 76.3 % in Ctrl treatment, 37.9 % and 60.7 % in FeⅡ treatment, 25.7 % and 51.9 % in FeⅢ treatment, respectively. In addition, main methanogenic archaea of Methanobrevibacter abundances in Ctrl, FeⅡ and FeⅢ obviously decreased by 82.2 %, 89.3 % and 92.2 % in the first 10 days, respectively. Moreover, Fe(Ⅲ) salts enhanced N₂O emission relative to Ctrl and Fe(Ⅱ) salts. NH₄⁺-N content, nirK and norB increment might promote N₂O release in initial period. Notedly, nosZ of Ctrl and FeⅡ separately ascended by 71.2 % and 66.9 %, yet nosZ of FeⅢ descended by 45.8 % in the first 10 days, which might restrict denitrification. Generally, this study proposed a valuable and feasible strategy to alleviate GHGs emissions and nitrogen loss in composting.