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

Stewart K.J., D. Coxson and S.D. Siciliano. 2011. Soil Biology and Biochemistry

Abstract

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 %u03B4 15N 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.

Key Words

Nitrogen fixation, Soil nitrogen, Phosphorus limitation, Nutrient availability, Low arctic, N2-fixing cyanobacteria, Biological soil crusts