Impact of biochar on abiotic nitrous oxide formation from chemodenitrification

Chemodenitrification is a critical but understudied source of nitrous oxide (N₂O) emissions from soils and sediments. This study investigates biochar's influence on the most widespread chemodenitrification process, here tested by the reduction of nitrite (NO₂^-) 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 NO₂^- reduction via surface adsorption of Fe(II), leading to a near-stoichiometric conversion of NO₂^- to N₂O. 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 N₂O 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 N₂O. Carboxylic-rich biochars (e.g. oilseed rape, sewage sludge chars) facilitated the GR-driven NO₂^- reduction to ammonium (NH₄⁺), suppressing N₂O yields to low levels (<11.5%). Our findings highlight that biochar neither adsorbs nor facilitates reduction of N₂O, and hence its effects on N₂O emission is through its effects on reactivity of Fe(II). Consequently, biochar is not a universal mitigation tool for N₂O 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.