Ocean acidification offsets the effect of warming on sediment denitrification and associated nitrous oxide production

Abstract

Rates of denitrification and associated nitrous oxide (N2O) production are expected to increase with global warming, leading to positive climate feedback. However, previous studies have not considered the combined effect of ocean acidification (OA, pCO2 ~ 900 µatm) and warming on denitrification rates and N2O production. Here we used a series of whole core incubation studies to assess the combined impact of warming and OA on estuarine sediment denitrification rates and N2O production. Strong warming (+5 °C over mean in situ conditions) increased N2O production by ~4.2 µmol-N m−2 d−1 and denitrification by ~43 µmol-N m−2 d−1, fuelled by water column nitrate (Dw), but decreased rates of nitrification-coupled denitrification in the sediment (Dn) by ~82 µmol-N m−2 d−1. While Dn was not affected by OA, Dw decreased significantly by 51 µmol-N m−2 d−1 when OA was coupled with warmer temperatures. We estimate that OA may offset the increase in estuarine sediment denitrification and N2O production expected from warming alone by up to 64% and reduce a potential positive climate feedback loop by inhibiting denitrification pathways.