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

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, pCO(2) similar to 900 mu atm) 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 degrees C over mean in situ conditions) increased N2O production by similar to 4.2 mu mol-N m(-2) d(-1) and denitrification by similar to 3 mu mol-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 similar to 82 mu mol-N m(-2) d(-1). While D-n was not affected by OA, D-w decreased significantly by 51 mu mol-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.