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

Rates of denitrification and associated nitrous oxide (N 2 O) 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, p CO 2 ~ 900 µatm) and warming on denitrification rates and N 2 O 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 N 2 O production. Strong warming (+5 °C over mean in situ conditions) increased N 2 O 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 (D w ), but decreased rates of nitrification-coupled denitrification in the sediment (D n ) by ~82 µmol-N m −2 d −1 . While D n was not affected by OA, D w 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 N 2 O production expected from warming alone by up to 64% and reduce a potential positive climate feedback loop by inhibiting denitrification pathways.