Greenhouse gas production and consumption in soils of the Canadian High Arctic

Abstract

Micro-organisms living in the soils of the Canadian High Arctic produce and consume the greenhouse gases (GHGs) CO2, CH4, and N2O, contributing to global nutrient and GHG cycles; however, different vegetation and soil communities differ in their net productions of each gas and the total emissions from the ecosystem. The range of Arctic vegetation communities spans wetlands, tundras, and deserts differing in their soil water contents and other properties such as organic matter content. Previous estimates of total GHG emissions are often imprecise relative to the scale of microbial processes that result in these emissions. Deserts have extremely low levels of both water and organic matter, yet I found that deserts produce nearly as much GHGs as wetter, more fully vegetated tundras. To test the hypothesis that this unexpectedly strong source of GHGs in deserts was a consequence of recently-thawed, organic-rich permafrost, I measured GHG net production throughout the active layer of polar desert soils; both production and consumption of CH4 and N2O, as well as soil respiration were found throughout the profile, indicating no link to thawed permafrost and suggesting these high GHG activities are characteristic features of Arctic polar deserts rather than transient effects of recent warming. I studied the community of microorganisms of the Arctic deserts by examining DNA from soil samples collected from three deserts on Ellesmere Island using DNA microarrays targeted for the functional genes AmoA and pmo. Using Structural Equation Modeling (SEM) I evaluated the hypotheses that the community of ammonia-oxidizers would be causally linked to the observed patterns of N2O net production, and that methane-oxidizers would be causally linked to CH4 net production. The SEM showed the expected link for CH4 production, but not N2O production. Available nitrogen in Arctic desert soils is primarily in the form of ammonia/ammonium, thus I find it surprising that no link could be found to the nitrifying community. Subsequent analysis of the occurence patterns of nitrous oxide reductase, a gene present in denitrifying bacteria and the only known biological sink for N2O, revealed only a weak association. Thus it remains unknown which organisms are responsible for the high levels of N2O emitted from Arctic polar desert soils. Furthermore, I observed several cases of unusual GHG processes, including a positive correlation between net CO2 and net N2O production in only some soils and some soil layers that consumed both CH4 and N2O.