In November of 1998 four rural, forested plots were established at Oregon Ridge Park in Baltimore County northeast of the Gwynns Falls Watershed. Oregon Ridge Park contains Pond Branch, the forested reference watershed for BES. Two of these four plots are located on the top of a slope; the other two are located midway up the slope.
In June of 2010 measurements at the mid-slope sites on Pond Branch were discontinued. Monuments and equipment remain at the two plots. These plots were replaced with two lowland riparian plots; Oregon upper riparian and Oregon lower riparian. Each riparian sites has four 5 cm by 1-2.5 meter depth slotted wells laid perpendicular to the stream, four tension lysimeters at 10 cm depth, five time domain reflectometry probes, and four trace gas flux chambers in the two dominant microtopographic features of the riparian zones - high spots (hummocks) and low spots (hollows).
Four urban, forested plots were established in November 1998, two at Leakin Park and two adjacent to Hillsdale Park in west Baltimore City in the Gwynns Falls. One of the plots in Hillsdale Park was abandoned in 2004 due to continued vandalism.
In May 1999 two grass, lawn plots were established at McDonogh School in Baltimore County west of the city in the Gwynns Falls. One of these plots is an extremely low intensity management area (mowed once or twice a year) and one is in a low intensity management area (frequent mowing, no fertilizer or herbicide use). In 2009, the McDonogh plots were abandoned due to management changes at the school.
Two grass lawn plots were established on the campus of the University of Maryland, Baltimore County (UMBC) in fall 2000. One of these plots is in a medium intensity management area (frequent mowing, moderate applications of fertilizer and herbicides) and one is in a high intensity management area (frequent mowing, high applications of fertilizer and herbicides).
In 2012 trace gas rings were installed in three more sites. The first (Cahill) is an urban site, located along a tributary to the Gwynn’s Falls in Leakin Park in the urban core of the watershed. Two sites (Glyndon, Gwynnbrook) are in suburban areas of the watershed; one just upstream from the Glyndon BES long-term stream monitoring site in the headwaters of the Gwynns Falls, and one along a tributary that enters the Gwynns Falls just above the Gwynnbrook BES long-term stream monitoring site farther downstream.
In 2012 and 2013 three storm water detention basins were instrumented with trace gas rings as part of a study to determine the denitrification potential these structures provide. These detention basins are all located within the Dead Run sub-watershed. Each of the three sites (Woodlawn, Winterset and Woodlawn BIO) had 3 plots, with plot 1 in the wettest part of the basin (though not water-filled), plot 2 in an intermediate zone and plot 3 in the driest zone.
Trace gas (N2O, CO2, and CH4) fluxes from soil to the atmosphere were measured every four to six weeks at each plot using an in situ chamber design. The lawn plots had chambers identical to those used by Bowden et al. (1990) installed flush with the soil surface to allow mowing. These (three per plot, at least 5 m apart and at least 5 m from the plot boundary) consisted of 28.7-cm diameter (ID) by 4.0-cm high polyvinyl chloride (PVC) cylinders which were placed on permanently installed PVC base rings immediately prior to measurement.
From 1998 through September 2007, the forest plots had the chambers described by Goldman et al. (1995), constructed from 16.5 cm wide by 20 cm long pieces of PVC pipe fitted with a septum and an air-tight well cap. These chambers were placed 4 cm into the soil and had a total volume of 2 L. Beginning in September 2007, the Bowden et al. (1990) chambers, which are used at many research sites across the world, were installed at the forest sites. A brief period of side by side measurements suggested that the Bowden chambers produce lower gas fluxes than the Goldman chambers. The replacement dates for chambers at different sites are:
Leakin 1 September 2007
Leakin 2 September 2007
Hillsdale 1 Sept 2007
Oregon Ridge Upper 1 October 2007
Oregon Ridge Upper 2 October 2007
Oregon Ridge Middle 1 September 2007
Oregon Ridge Middle 2 September 2007
At 0, 10, 20 and 30 min following placement of the chamber on the base or installation of the well cap, 9-mL gas samples were collected from gas sampling ports in the center of the chamber top by syringe. Samples were injected into 9 mL evacuated glass vials sealed with septa and aluminum rings. Samples were sent to the Cary Institute of Ecosystem Studies (CIES) for gas chromatography (GC) analysis. Concentrations of CO2 and N2O are measured on a GC equipped with thermal conductivity and electron capture detectors aligned in sequence. Concentrations of CH4 are measured on a GC equipped with a flame ionization detector. Samples and standards are introduced into this GC via syringe and sample loop. Fluxes were calculated from the linear rate of change in gas concentration, the chamber internal volume and soil surface area.
In May 2016 sample collection and analysis change. Samples are now collected 5.9 mL exetainers (Labco Ltd, UK). 8 ml of samples were injected into each vial. Analysis is now performed on a GC equipped with thermal conductivity, electron capture, and flame ionization detectors (GC2014, Shimadzu Inc.)
Spike standards are processed along with each batch of flux samples. These standards are generated by filling sample vials from a standard tank in the field laboratory in Baltimore and then processing these samples along with samples taken from flux chambers in the field. Percent recoveries of the trip standards are used to correct flux values for leakage or gas absorption that may occur between sampling and analysis. Exceptionally low recoveries for N2O (traced to a change in septa manufacture) led to loss of flux data for this gas for several dates from 2007 - 2009.
Literature Cited
Bowden R, Steudler P, Melillo J and Aber J. 1990. Annual nitrous oxide fluxes from temperate forest soils in the northeastern United States. J. Geophys. Res.Atmos. 95, 13997 14005.
Goldman, M. B., P. M. Groffman, R. V. Pouyat, M. J. McDonnell, and S. T. A. Pickett. 1995. CH4 uptake and N availability in forest soils along an urban to rural gradient. Soil Biology and Biochemistry 27:281-286.
