Monday, 10 July 2006

Landscape Position Affects the Emission of Greenhouse Gases in a Prairie Pot-Hole Soil in Western Canada.

Adedeji S. Dunmola1, David Lobb1, Dan J. Pennock2, Yappa Priyantha1, and Mario Tenuta1. (1) Dept of Soil Science, Univ of Manitoba, 362, Ellis Building, Winnipeg, MB R3T 2N2, Canada, (2) Univ of Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N 5A8, Canada

The first few weeks when frozen soils thaw out in spring has been reported to contribute about 40-70 % of the total annual field emission of nitrous oxide in temperate regions. Agricultural soils in Canada are the major anthropogenic source of nitrous oxide, a greenhouse gas that contributes about 10% of radiative forcing of gases. The large spatial and temporal variability in field emission of nitrous oxide makes modeling difficult. This study was designed to investigate the control landscape position exerts on the growing-season field flux of nitrous oxide with particular emphasis on spring-thaw emission. The field flux of nitrous oxide was monitored from spring of 2005 into the growing season using the static vented-chamber technique. The field site is located 5km North of Brandon, MB and has a hummocky landscape. A transect of 128 chambers at 3m intervals were sampled ten times through 2005. A subset of the chambers were sampled on a more frequent basis for four sections having each one upper, middle, lower and riparian zone landscape positions. The landscape position of all chambers was determined by a landscape segmentation model having upper, middle, lower and depression (riparian) positions. The site was planted to CPS wheat (variety 5701) using zero-tillage. The surface nitrous oxide flux from collars were measured by taking the headspace sample of the vented chambers at 0,8,16 and 24 minutes into evacuated gas vials, analyzing with a gas chromatograph, and calculating the flux. Also, the net nitrous oxide production and denitrification rates from the four sections of the transect were measured later in the growing season by using the acetylene-block technique with laboratory incubation of intact soil cores taken from the field. There was a significant variability in the emission of nitrous oxide with the landscape position, ranging from between 0.078 to 519g N2O-N/ha/day. The highest field flux of nitrous oxide for the period under investigation was observed few days after the frozen soil started thawing early April 2005, with the highest emission from the middle slope position for the most part of the entire spring. Nitrous oxide field flux sharply declined further into the spring, increasing only after the application of fertilizer (74kg/ha of total N) to the wheat crop. The lowest nitrous oxide flux was observed from the riparian zones, from where methane emission was consistently highest, indicating the presence of anaerobic conditions. The emission of carbondioxide was fairly well correlated with nitrous oxide emission. Measuring denitrification rate and net nitrous oxide production from intact soil cores taken in the middle of the growing season showed the middle slope gave the highest denitrification rate and net nitrous oxide production, with the lowest values from the riparian zone. For all the slope positions, net nitrous oxide production was much lower than denitrification rates, implying that dinitrogen oxide was the major gaseous product of denitrification during the latter part of the growing season. The study showed that landscape position contributes to the variability in nitrous oxide flux from the field, both during spring-thaw, as well as into the growing season. Also, spring and early part of the growing season is of more importance in modeling for annual nitrous oxide emission. Current denitrification and/or Nitrous Oxide emission models should take into account this temporal and spatial variability in order to improve the predictive values of such models.

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