Warming reduces the increase in NO emission under nitrogen fertilization in a boreal peatland.

Peatlands are known as NO sinks or low NO sources due to nitrogen (N) limitation. However, climate warming and N deposition can modulate this limitation, and little is known about the combinative effects of them on NO emission from boreal peatlands. In this study, experimental warming and N fertilization treatments were conducted at a boreal peatland in western Newfoundland, Canada. Contrary to previous studies on permafrost peatland and alpine meadows, the effect of warming treatment on NO flux was not detectable during the growing seasons of 2015 and 2016. The N fertilization treatment significantly increased the NO flux by 1.61 nmol m s due to increased N availability. Noticeably, warming reduced the effect of N fertilization treatment on NO flux with high significance in the middle growing season of 2015. This can be attributed to low N availability caused by stimulated vegetation growth in the warming treatment. In addition, the results showed that total nitrogen was the main control on NO emission under N fertilization, while dissolved organic carbon was the main driver under the combined treatment of warming and N fertilization. Due to elevated NO emissions under N deposition/fertilization, the contribution of NO to global warming and ozone depletion should not be ignored.