Constraints to nitrogen fixation by cryptogamic crusts in a polar desert ecosystem, Devon Island, NWT, Canada

Polar desert ecosystems, which dominate the landscape throughout much of the High Arctic, are environmentally stressed and limited in their development. Scattered intermittently over these landscapes are areas of cryptogamic crust development that are associated with increased vascular plant abundance. Since nutrient limitation, especially nitrogen, is significant in these ecosystems. I wished to examine the role of these cryptogamic crusts in the supply of fixed nitrogen and the constraints to that fixation. Nitrogen fixation rates (as measured by acetylene reduction) were highest in sites with a well-developed cryptogamic crust, lowest in sites with only bare mineral soil, and intermediate in sites with a partially developed crust. Highest rates of acetylene reduction (i.e., nitrogen fixation) were seen within a few days of snowmelt (late June to early July) and declined as the season progressed, until near the end of the growing season (1-5 August) when rates were approximately 50% of early season rates. Late season precipitation events restored acetylene reduction rates to near original levels. In manipulative experiments, acetylene reduction rates dropped dramatically as crust moisture content declined and rates increased as soil surface temperature increased to 24 degrees C. A significant finding was that acetylene reduction at 3 degrees C was 40% of that round at 12 to 13 degrees C. Thus, there is a potential for nitrogen accumulation even during the colder periods of the growing season. As calculations show, the quantity of nitrogen fixed by these cryptogamic crusts was adequate to support the nitrogen needs of the mosses and vascular plants of these developing ecosystems.