PhD Thesis, 2010 - University College Cork

Nitrous oxide flux dynamics of grassland undergoing afforestation
concentration of the gas sample using tuneable diode laser absorption spectroscopy
which allows for fast and sensitive measurements (Edwards et al. 2003). The sonic
anemometer and air intake for gas measurements was installed at 6 m height on the EC
tower.
While setup costs of the EC technique are higher than those of more widely used
closed chamber method, the maintenance costs and requirements are typically much
lower. Ground instrumentation included soil temperature probes at the depth of 1.5 cm
(Campbell Sci, USA), time-domain reflectometry (TDR) probes for soil moisture at 5
cm (Campbell Sci., USA) and rain gauges (Campbell Sci., USA) installed within 5
metres of the EC tower in the grassland sector. Due to the special location of the
converted forest sector to the west of the EC tower, all flux coming from the direction
corresponding to the forest sector was considered to be the forest flux (Figure 5.1).
The basic idea behind the eddy covariance method is that of measuring the high
frequency (in our case 10 Hz) vertical wind speed and gas species concentration. In the
case of stationary atmospheric conditions, the time series of these two parameters are
Reynolds-averaged and the covariance represents the ecosystem flux from the footprint
area. The analytical model of Hsieh et al. (2000) was used to calculate the footprint of
the EC system. Such experimental setup allows for continuous measurements while the
more traditional closed-chambers technique typically is at a frequency of about one
measurement per day or per week.
The flux of the trace gas can be expressed as:
F χ = wρ χ (5-1)
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