Total nitrogen (NTot) was calculated as the sum of nitrate, nitrite, and ammonium concentrations. Non-detectable nitrogenous species such as hydroxylamine and gas-phase nitric oxide, nitrous oxide, and dinitrogen (ΔNTot) were quantified as the difference between NTot before and after performance of CA to specific e -/NO3 -. FE of species 𝑖 (𝐹𝐸 𝑖 ) was calculated as,