Ecological significance of nitrifier and denitrifier spatial patterns in three Arctic ecosystems

Owing to spatial variability of soil properties, microbial communities and their functional role in biogeochemical processes may also vary across multiple spatial scales. Soil and microbial spatial heterogeneity has been studied in various tropical and temperate ecosystems yet no information is available from Arctic permafrost ecosystems. These ecosystems represent a significant proportion of global land mass and contain about one fourth of total soil carbon pool. Soil microbial N transformations such as nitrification and denitrification have significant implications for N availability and N loss in nutrient-limited Arctic ecosystems. This study aims to elucidate 1) the spatial variability of soil attributes and the overall microbial communities 2) the spatial structure of ammonia oxidizer and denitrifier abundance and their activities, and 3) relationships among microbial communities, functional processes, and soil attributes in three Arctic Cryosolic ecosystems. The results show that despite challenging climatic conditions and the regular occurrence of cryopedogenic processes, soil properties and microbial abundance are highly spatially dependent and their spatial autocorrelation is consistent within and between the ecohabitats. Despite similar abundances, the zone of spatial autocorrelation is substantially smaller than other ecosystems. The correlations between moisture content and other soil attributes in Arctic are considerably higher than temperate agricultural and tropical grassland soils, suggesting the critical role of moisture in Arctic soil ecosystems. Ammonia-oxidizing archaeal and bacterial communities and aerobic ammonia oxidation were spatially dependent. Functional groups were spatially structured within 4 m whereas biochemical processes were structured within 40 m. Ammonia oxidation was driven at small scales (<1 m) by moisture and total organic carbon content whereas gene abundance and other edaphic factors drove ammonia oxidation at medium (1-10 m) and large (10-100 m) scales. Denitrifier functional groups and potential denitrification were spatially autocorrelated within a scale of 5 m. Soil moisture, organic carbon and nitrogen content were the predominant driving factors with nirK abundance also correlated to potential denitrification. This is the first study to report high spatial dependence of soil properties, overall microbial, ammonia oxidizing, and denitrifying communities, and functional processes in Canadian Arctic. It disentangles the associations among the aforementioned parameters to identify the key controls on nitrification and denitrification in Cryosolic ecosystems.