Effect of excess and restricted inorganic carbon on biokinetics, nitrous oxide emissions, and microbial community in a full-nitrification bioreactor

Abstract

Inorganic carbon (IC) availability was evaluated as a driver of the nitrification process, biokinetics of nitrifying bacteria, nitrous oxide formation, and community structure in a continuous full-nitrification reactor operated under IC sufficiency, limitation, and recovery. Reactor behavior was tracked using dissolved IC and nitrogen profiles (NH4+, NO2−, NO3−), short-term respirometric specific oxygen uptake rates for ammonia oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), on-line N2O monitoring, and 16 S rRNA gene sequencing. IC limitation consistently suppressed AOB respiration and apparent growth capacity, increased effective substrate requirements, and led to NO2− increase together with a decline in biomass. After IC was restored, AOB activity recovered faster than NOB activity, causing nitrite accumulation and the highest N2O emissions. Peak effluent NO2− reached 12.7 mg N/L and gas-phase N2O concentration peaked at about 18 ppmv. The biomass-specific N2O production rate increased by more than an order of magnitude compared to the IC-sufficient baseline. Multivariate and correlational analyses indicated that effluent nitrite and biomass state, rather than absolute IC alone, were the dominant proximate predictors of emission variability. Community composition shifted coherently – a decrease in Nitrosomonas during limitation and rebounding during recovery, a modest increase of Nitrobacter, and transient enrichment of heterotrophs consistent with endogenous carbon cycling under stress conditions. Maintaining adequate IC supply, avoiding abrupt IC changes, and monitoring NO2− and oxygen uptake metrics are suggested as practical methods to sustain complete nitrification while minimizing N2O emission.