Small-scale spatial patterns in N2-fixation and nutrient availability in an arctic hummock-hollow ecosystem

Atmospheric nitrogen that is fixed by associative cyanobacteria can be released into the surrounding soil environment providing a key source of N for arctic ecosystems. Yet, little is known about nitrogen fixation by Biological Soil Crusts (BSCs) within hummock-hollow complexes that are typical of many arctic environments. In this study, we examined spatial and temporal patterns in N₂-fixation, dinitrogenase reductase (nifH) gene abundance and release of N in a low arctic hummock-hollow ecosystem. The impacts of cyanobacteria on N status in soil were evaluated by assessing soil nitrogen in relation to the cyanobacterial associations found on Hummock and Hollow BSCs. In addition, potential P limitation of N₂-fixation by cyanobacteria was assessed for Hummock and Hollow BSCs. The tops of hummocks and the bottoms of hollows were areas of high N₂-fixation, whereas minimal N₂-fixation occurred on the sides of hummock-hollow complexes. Compared with Hummock BSCs, Hollow BSCs had a higher mean growing season N₂-fixation rate, a higher mean growing season nifH abundance, a higher mean total %N and δ¹⁵N values closer to that of atmospheric N₂. Soil N status was linked to rates of N₂-fixation by BSCs indicating that these N₂-fixing associations act as important point sources of soil N in this low arctic ecosystem. Over the course of a growing season temporal variation in N₂-fixation and nifH abundance were weakly linked suggesting that N₂-fixation was carried out by complex communities of diazotrophic microorganisms and that factors such as nutrient availability may limit N₂-fixation to a greater extent than nifH abundance.