Author
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Skinner, Robert |
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Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/24/2007 Publication Date: 5/14/2007 Citation: Skinner, R.H. 2007. Winter carbon dioxide fluxes in humid-temperate pastures. Agricultural and Forest Meteorology. 144(1-2):32-43. Interpretive Summary: The burning of fossil fuels and deforestation are considered to be primary causes of human induced increases in atmospheric carbon dioxide which contributes to rising global temperatures. However, storage of carbon by terrestrial ecosystems has reduced the rate of carbon dioxide accumulation in the atmosphere. Because of their vast size, grazing lands have the potential to store significant quantities of carbon, slowing the increase in atmospheric carbon dioxide and reducing the risk of global warming. Although carbon dioxide uptake during the growing season can be substantial, losses following defoliation, during periods of drought, and during winter months can significantly reduce annual storage and frequently turn grazing lands from sinks to sources of carbon dioxide to the atmosphere. This research addressed the question of how much carbon dioxide is lost from managed pastures during the winter. The dormant season was defined as the period beginning when pastures started losing carbon dioxide to the atmosphere on a daily basis in the fall and ending when they began accumulating carbon dioxide again in the spring. The dormant season for an alfalfa based pasture was 20 d longer than that of a grass based pasture in 2003-04. However, total carbon dioxide loss from the alfalfa pasture was less than from the grass pasture because the daily rate of loss was less. Pastures required 25 to 39 days of active uptake in the spring to replenish the carbon dioxide lost during the winter. Warmer wintertime temperatures increased the length of the dormant season and increased the amount of carbon dioxide lost from these pasture systems. Warmer winters, resulting from increasing atmospheric carbon dioxide accumulation, could potentially reduce the ability of pastures to store carbon dioxide, hindering their ability to help reduce the negative effects of fossil fuel emissions. Technical Abstract: Because of their vast size, grazing lands have the potential to sequester significant quantities of carbon, slowing the increase in atmospheric CO2 and reducing the risk of global warming. Although C uptake during the growing season can be substantial, losses during winter months can significantly reduce annual sequestration, potentially turning grazing lands into net carbon sources. The goal of this research was to quantify dormant-season fluxes and examine their impact on annual carbon storage. The study was conducted on two pastures near State College, Pennsylvania. The first, a grass-based pasture, was dominated by a mixture of cool-season grasses, while the second was dominated by alfalfa (Medicago sativa L.), intermixed with cool-season grasses. Pasture-scale CO2 fluxes were quantified using eddy covariance techniques. The dormant season was defined as the period beginning when daily net ecosystem exchange (NEE) became positive (CO2 loss to the atmosphere) in the fall and ending when daily NEE became negative (CO2 uptake by the pasture) again in the spring. The dormant season for the alfalfa pasture was 20 d longer than that of the grass pasture in 2003-04. However, total CO2 loss from the alfalfa pasture was reduced by 17% compared with the grass pasture due to a 28% decrease in the daily rate of CO2 efflux. In the absence of snow cover and on sunny days, photosynthetic CO2 uptake occurred at daytime air temperatures well below freezing, beginning at -3 degrees C in the alfalfa and -8 degrees C in the grass pasture. However, nighttime efflux from the system was greater than daytime uptake so that the pastures remained CO2 sources throughout the winter. Because of lower respiration rates, total dormant-season CO2 loss from the alfalfa pasture was less than that from the grass pasture, despite the fact that the alfalfa pasture had lower photosynthetic rates and a dormant season that was 20 d longer. |
