Exploring Sulfate as an Alternative Electron Acceptor: A Potential Strategy to Mitigate N₂O Emissions in Upland...

Agricultural activities are a significant source of nitrous oxide (N₂O), accounting for approximately 60% of global emissions, highlighting the urgent need for innovative strategies to mitigate N₂O emissions. Microbes conserve nearly as much energy with nitrate (NO₃ ^-) as oxygen (O₂) respiration under limited O₂ availability. Thus, microorganisms prioritize NO₃ ^-, limiting exploration of alternative electron acceptors (EAs) to inhibit N₂O emissions through NO₃ ^- respiration in upland arable soils. Current approaches remain insufficient, and the interactions between alternative EA reduction and pathways for N₂O emissions remain poorly understood. This study evaluated oxidized iron, manganese, and sulfate as alternative EAs to reduce N₂O emissions, along with the effects of zero-valent metals (ZVMs). Metal sulfates (MSs) significantly minimized N₂O emissions by inhibiting denitrification rather than altering nitrification in microcosms, as supported by isotope mapping and inorganic nitrogen concentrations. Among others, putative complete denitrifiers, N₂O reducers, and sulfate reducers were stimulated, whereas ZVMs stimulated N₂O emissions and 16S rRNA gene abundance. Moreover, the abundance of denitrifier-related genes (nirK, nirS, norB, and nosZ) consistently decreased under MS treatments, while dsrA mRNA abundance significantly increased. Sulfate (SO₄ ^2-) addition reshaped the soil microbial community by enriching sulfur-cycling taxa-including sulfate-reducing and sulfur-oxidizing bacteria-while suppressing nitrifiers such as Nitrospira, potentially disrupting nitrification-denitrification coupling. Ureibacillus thermosphaerius, harboring genes for denitrification and SO₄ ^2- reduction, increased under MS treatment. These shifts likely redirected electron flow toward SO₄ ^2- respiration, reducing NO₃ ^- utilization and contributing to N₂O mitigation. Field-based manipulation experiments over 2 years demonstrated the feasibility of MSs in upland arable soils, reducing yield-scaled N₂O emissions by 21.5% without compromising crop yields. A systematic literature review and meta-analysis revealed that SO₄ ^2- application mitigated N₂O emissions by an average of 9%, with over 70% of observations showing a decreasing trend, underscoring its potential as an effective soil amendment for sustainable agriculture.