Fig. 4.

Potential pathways of HONO emission from soil (a), and illustration of HONO production during denitrification and anaerobic nitrate reduction (b). a Pathway 1: acid–base equilibrium in soil aqueous solution and the volatilization of nitrous acid according to Henry’s law [17]; Pathway 2: nitrous acid formation and release controlled by soil mineral particle surface acidity [19]; Pathway 3: nitrite accumulation at the outside of the cytoplasmic membrane of nitrate-reducing microbes, resulting in locally increased nitrite concentrations and nitrous acid formation enhanced by proton motive force-dependent acidification. b Nitrite is produced by anaerobic nitrate reductases close to the cytoplasmic membrane, either at the outer or inner side of the membrane. Nitrite produced at the inner side of the membrane is subsequently transported to the outer side via nitrate–nitrite antiporters (AP). The outer side of the membrane is positively charged due to accumulation of protons (proton motive force). Nitrous acid is formed due to the high-proton concentrations from nitrite (acid–base equilibrium) and diffused out of the outer membrane through porin channels to the external environment. HONO (g) and HNO₂ (aq) represent molecular nitrous acid in the gas and aqueous phase, respectively