Impact of biochar on abiotic nitrous oxide formation from chemodenitrification

• Biochar acts as a catalyst or suppressor of N 2 O production depending on pH. • Acidic pH drives biochar-catalyzed N 2 O production via surface Fe 2+ activation. • At neutral pH, biochar may interact with Fe(II) phases and slow reaction rates. • Biochars and green rust drive NH 4 + formation and suppress N 2 O at alkaline pH Chemodenitrification is a critical but understudied source of nitrous oxide (N 2 O) emissions from soils and sediments. This study investigates biochar’s influence on the most widespread chemodenitrification process, here tested by the reduction of nitrite (NO 2 – ) by iron(II) (Fe(II)) across varying pH. By comparing 13 biochars from 7 different feedstocks produced at two pyrolysis temperatures, we demonstrate that biochar exerts a dual effect on this reaction. At acidic pH (5.5), biochar acts as a catalyst. It accelerates NO 2 – reduction via surface adsorption of Fe(II), leading to a near-stoichiometric conversion of NO 2 – to N 2 O. This catalytic effect was most pronounced for bone meal char. However, at neutral pH (7.0) where part of the Fe(II) is present in Fe(II)-Fe(III) hydroxides (Green Rust, GR), biochar addition delayed the reaction and moderately lowered N 2 O formation likely via surface passivation of reactive solids by biochar, especially Miscanthus straw char. At alkaline pH (8.5), where reactive GR dominates, biochar acted synergistically with the GR to suppress N 2 O. Carboxylic-rich biochars (e.g. oilseed rape, sewage sludge chars) facilitated the GR-driven NO 2 – reduction to ammonium (NH 4 + ), suppressing N 2 O yields to low levels (<11.5%). Our findings highlight that biochar neither adsorbs nor facilitates reduction of N 2 O, and hence its effects on N 2 O emission is through its effects on reactivity of Fe(II). Consequently, biochar is not a universal mitigation tool for N 2 O formed by chemodenitrification, and its efficacy is conditional, requiring alkaline environments and carboxylic-rich biochar amendments to buffer acidic soils or enhance natural suppression mechanisms.