Acidification Offset Warming-Induced Increase in N₂O Production in Estuarine and Coastal Sediments

Global warming and acidification, induced by a substantial increase in anthropogenic CO₂ emissions, are expected to have profound impacts on biogeochemical cycles. However, underlying mechanisms of nitrous oxide (N₂O) production in estuarine and coastal sediments remain rarely constrained under warming and acidification. Here, the responses of sediment N₂O production pathways to warming and acidification were examined using a series of anoxic incubation experiments. Denitrification and N₂O production were largely stimulated by the warming, while N₂O production decreased under the acidification as well as the denitrification rate and electron transfer efficiency. Compared to warming alone, the combination of warming and acidification decreased N₂O production by 26 ± 4%, which was mainly attributed to the decline of the N₂O yield by fungal denitrification. Fungal denitrification was mainly responsible for N₂O production under the warming condition, while bacterial denitrification predominated N₂O production under the acidification condition. The reduced site preference of N₂O under acidification reflects that the dominant pathways of N₂O production were likely shifted from fungal to bacterial denitrification. In addition, acidification decreased the diversity and abundance of nirS-type denitrifiers, which were the keystone taxa mediating the low N₂O production. Collectively, acidification can decrease sediment N₂O yield through shifting the responsible production pathways, partly counteracting the warming-induced increase in N₂O emissions, further reducing the positive climate warming feedback loop.