Warming stimulates sediment denitrification at the expense of anaerobic ammonium oxidation

Temperature is one of the fundamental environmental variables governing microbially mediated denitrification and anaerobic ammonium oxidation (anammox) in sediments. The GHG nitrous oxide (N 2 O) is produced during denitrification, but not by anammox, and knowledge of how these pathways respond to global warming remains limited. Here, we show that warming directly stimulates denitrification-derived N 2 O production and that the warming response for N 2 O production is slightly higher than the response for denitrification in subtropical sediments. Moreover, denitrification had a higher optimal temperature than anammox. Integrating our data into a global compilation indicates that denitrifiers are more thermotolerant, whereas anammox bacteria are relatively psychrotolerant. Crucially, recent summer temperatures in low-latitude sediments have exceeded the optimal temperature of anammox, implying that further warming may suppress anammox and direct more of the nitrogen flow towards denitrification and associated N 2 O production, leading to a positive climate feedback at low latitudes. Nitrogen removal occurs primarily through microbially mediated denitrification, which produces the GHG N 2 O, and anaerobic ammonium oxidation (anammox), which does not. Warming stimulates denitrification relative to annamox in subtropical sediments, indicating that warming could lead to greater N 2 O production.