Sonia Nobrega

Publications (2)6.99 Total impact

• Source

  Available from: post.queensu.ca

  Article: Landscape and Ecosystem-Level Controls on Net Carbon Dioxide Exchange along a Natural Moisture Gradient in Canadian Low Arctic Tundra

  Sonia Nobrega, Paul Grogan
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  ABSTRACT: Our understanding of the controls and magnitudes of regional CO2 exchanges in the Arctic are limited by uncertainties due to spatial heterogeneity in vegetation across the landscape and temporal variation in environmental conditions through the seasons. We measured daytime net ecosystem CO2 exchange and each of its component fluxes in the three major tundra ecosystem-types that typically occur along natural moisture gradients in the Canadian Low Arctic biweekly during the full snow-free season of 2004. In addition, we used a plant-removal treatment to compare the contribution of bulk soil organic matter to total respiratory CO2 loss among these ecosystems. Net CO2 exchange rates varied strongly, but not consistently, among ecosystems in the spring and summer phases as a result of ecosystem-specific and differing responses of gross photosynthesis and respiration to temporal variation in environmental conditions. Overall, net carbon gain was largest in the wet sedge ecosystem and smallest in the dry heath. Our measures of CO2 flux variation within each ecosystem were frequently most closely correlated with air or soil temperatures during each seasonal phase. Nevertheless, a particularly large rainfall event in early August rapidly decreased respiration rates and stimulated gross photosynthetic rates, resulting in peak rates of net carbon gain in all ecosystems. Finally, the bulk soil carbon contribution to total respiration was relatively high in the birch hummock ecosystem. Together, these results demonstrate that the relative influences of moisture and temperature as primary controls on daytime net ecosystem CO2 exchange and its component fluxes differ in fundamental ways between the landscape and ecosystem scales. Furthermore, they strongly suggest that carbon cycling responses to environmental change are likely to be highly ecosystem-specific, and thus to vary substantially across the low arctic landscape.
  Ecosystems 04/2012; 11(3):377-396. · 3.49 Impact Factor
• Source

  Available from: post.queensu.ca

  Article: Deeper Snow Enhances Winter Respiration from Both Plant-associated and Bulk Soil Carbon Pools in Birch Hummock Tundra

  Sonia Nobrega, Paul Grogan
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  ABSTRACT: It has only recently become apparent that biological activity during winter in seasonally snow-covered ecosystems may exert a significant influence on biogeochemical cycling and ecosystem function. One-seventh of the global soil carbon pool is stored in the bulk soil component of arctic ecosystems. Consistent climate change predictions of substantial increases in winter air temperatures and snow depths for the Arctic indicate that this region may become a significant net annual source of CO2 to the atmosphere if its bulk soil carbon is decomposed. We used snow fences to investigate the influence of a moderate increase in snow depth from approximately 0.3 m (ambient) to approximately 1 m on winter carbon dioxide fluxes from mesic birch hummock tundra in northern Canada. We differentiated fluxes derived from the bulk soil and plant-associated carbon pools using an experimental ‘weeding’ manipulation. Increased snow depth enhanced the wintertime carbon flux from both pools, strongly suggesting that respiration from each was sensitive to warmer soil temperatures. Furthermore, deepened snow resulted in cooler and relatively stable soil temperatures during the spring-thaw period, as well as delayed and fewer freeze–thaw cycles. The snow fence treatment increased mean total winter efflux from 27 to 43 g CO2-C m−2. Because total 2004 growing season net ecosystem exchange for this site is estimated at 29–37 g CO2-C m−2, our results strongly suggest that a moderate increase in snow depth can enhance winter respiration sufficiently to switch the ecosystem annual net carbon exchange from a sink to source, resulting in net CO2 release to the atmosphere.
  Ecosystems 04/2012; 10(3):419-431. · 3.49 Impact Factor