Soil nitric oxide emissions linking to nitrogen transformations as mediated by carbon availability from maize-cultivated Mollisols

Nitric oxide (NO) is a key precursor to reactive nitrogen (N) species and nitrate aerosols, with agricultural soils being a major emission source. Driven by growing global food demands, increased chemical fertilizer application coupled with accompanied soil organic carbon (SOC) decline can alter soil NO emissions, which remains poorly understood. We conducted a two-year field experiment in two croplands with equivalent SOC quantity but distinct quality (HCA and LCA, i.e., high vs . low C availability) to investigate NO emission intensity relating to substrates supply controlling N transformations. Without N fertilization, annual background NO emissions from HCA were 1.34-fold higher than LCA cropland (0.39 vs . 0.29 kg N ha −1 ), mainly because NO emissions were predominantly driven by nitrification, with soil mineral N supply and the quantity and lability of dissolved organic carbon (DOC) identified as key regulators. Higher SOC availability of HCA than LCA cropland facilitated coupled mineralization (6.87 vs . 2.94 mg N kg −1 d −1 ) and nitrification (13.49 vs . 4.02 mg N kg −1 d −1 ), promoting N supply and thus NO emissions. N application significantly stimulated NO emissions but resulted in lower emissions from HCA than LCA (2.60 vs . 4.55 kg N ha −1 ). Under N fertilization, greater crop biomass and N uptake in HCA cropland accelerated soil mineral N consumption and intensified competition for ammonium between heterotrophic microorganisms and nitrifiers. Consequently, less ammonium supplies restricted nitrification, yielding a lower fertilizer-induced NO emission factor in HCA compared to LCA cropland (1.10% vs . 2.13%). Overall, these findings suggest that SOC quality mediates N transformations and thus NO emissions, highlighting that improving SOC availability through appropriate carbon management can enhance fertilizer N retention and mitigate associated environmental threats.