Nitrogen transformation processes in soil along a High Arctic tundra transect

Soil nitrogen (N) transformation processes in the High Arctic tundra are poorly understood even though nitrogen is one of the main limiting nutrients. We analyzed soil samples collected along a High Arctic tundra transect to investigate spatial variability in key nitrogen transformation processes, functional gene abundances, ammonia-oxidizing archaea (AOA) community structures, and key nitrogen transformation regulators. The potential denitrification rates were higher than the nitrification rates in the soil samples, although nitrification may still regulate N₂O emissions from tundra soil. The nutrient (total carbon, total organic carbon, total nitrogen, and NH⁺ 4-N ) contents were important determinants of spatial variability in the potential denitrification rates of soil along the tundra transect. The total sulfur content was the main variable controlling potential nitrification processes, probably in association with sulfate-reducing bacteria. The nitrate content was the main variable affecting potential dissimilatory nitrate reduction to ammonium. AOA and ammonia-oxidizing bacteria amoA, nirS, and anammox 16S rRNA genes were found in all of the soil samples. AOA play more important roles than ammonia-oxidizing bacteria in soil nitrification. Anammox bacteria may utilize NO^- 2 produced through nitrification. Phylogenetic analysis indicated that the AOA amoA sequences could be grouped into eight unique operational taxonomic units (OTUs) with a 97% sequence similarity and were affiliated with three group 1.1b Nitrososphaera clusters. The results indicated that heterogeneous environmental factors (e.g., the carbon and nitrogen contents of soil) along the High Arctic tundra transect strongly affected the nitrogen transformation rate and relevant functional gene abundances in soil.