Alder expansion stimulates nitrogen oxide (NO_x) emissions from southern Eurasian permafrost peatlands

Nitrogen oxides (NO_x) play an important role for atmospheric chemistry and radiative forcing. However, NO_x emissions from the vast northern circumpolar permafrost regions have not been studied in situ due to limitations of measurement techniques. Our goals were to validate the offline analytical technique, and based on this, to widely quantify in situ NO_x emissions from peatlands in the southern Eurasian permafrost region. To this end, we conducted a comparison of online and offline flux measurements in 2018 and 2019 using the synthetic air flushing, steady-state opaque chamber method. With differences in annual average and cumulative fluxes less than 0.1 μg N m^-2 h^-1 and 0.01 kg N ha^-1 year^-1, the online and offline fluxes were in good agreement, demonstrating the feasibility of conducting offline measurements in remote regions without power supply. The flux measurements over 2 years showed obvious NO_x emissions of 0.05-0.14 and 0.13-0.30 kg N ha^-1 year^-1 in the hollow and hummock microtopography of permafrost peatlands, respectively. The rapid expansion of alder (Alnus sibirica) in the peatlands induced by permafrost degradation significantly increased soil mineral N contents and NO_x emissions depending on the age of alder (0.64-1.74 and 1.44-2.20 kg N ha^-1 year^-1 from the alder forests with tree ages of 1-10 years and 11-20 years, respectively). Alder expansion also intensively altered the thermal state of permafrost including the sharp increases of soil temperatures during the non-growing season from October to April and active layer thickness. This study provides the first in situ evidences of NO_x emissions from the northern circumpolar permafrost regions and uncovers the well-documented expansion of alders can substantially stimulate NO_x emissions and thus, significantly affect air quality, radiative forcing, and ecosystem productivity in the pristine regions.