ABSTRACT
Soil atmosphere fluxes of the trace gases; carbon dioxide (CO2),
nitrous oxide (N2O) and methane (CH4) have been measured on snow
removal plots at the Hubbard Brook Experimental Forest (HBEF).
In 1997, as part of a study of the relationships between snow depth,
soil freezing and nutrient cycling
(http://www.ecostudies.org/people_sci_groffman_snow_summary.html), we
established eight 10 x 10-m plots located within four stands; two
dominated (80%) by sugar maple and two dominated by yellow birch, with
one snow reduction (freeze) and one reference plot in each stand.
Trace gas fluxes were measured at approximately monthly intervals on
these plots from October 1997 through September 2000. In 2001, we
established eight new 10-m x 10-m plots (4 treatment, 4 reference) in
four new sites; two high elevation, north facing and two low
elevation, south facing maple-beech-birch stands. Trace gas fluxes
were measured at approximately monthly intervals on these plots from
May 2001 through November 2004. To establish plots for the “freeze”
study, we cleared minor amounts of understory vegetation from all
(both freeze and reference) plots (to facilitate shoveling). We then
installed soil solution samplers (zero tension lysimeters),
thermistors for soil temperature monitoring, water content (time
domain) reflectometers (for measuring soil moisture), soil atmosphere
sampling probes, minirhizotron access tubes, and trace gas flux
measurement chambers (described below). All plots were equipped with
dataloggers to allow for continuous monitoring of soil moisture and
temperature. Treatments (keep plots snow free by shoveling through the
end of January) were applied in the winters of 1997/98, 1998/99,
2002/2003 and 2003/2004.
DATA DESCRIPTION
When plots were snow free, trace gas fluxes were measured using the in
situ chamber design identical to that described by Bowden and others
(1990, 1991). Chambers (three per plot) of 287-mm diameter (ID) by
40-mm high polyvinyl chloride (PVC) were placed on permanently
installed PVC base rings immediately prior to measurement. At 0, 10,
20 and 30 min following placement of the chamber on the base, 9-mL gas
samples were collected from gas sampling ports in the center of the
chamber top by syringe. Samples were transferred to evacuated glass
vials and stored at room temperature prior to analysis by gas
chromatography (GC). N2O and CO2 were analyzed on a Shimadzu GC-14 GC
with electron capture (N2O) and thermal conductivity (carbon dioxide,
CO2) detectors. CH4 was analyzed on a Shimadzu GC-8a GC with a flame
ionization detector. These GC’s were capable of detecting ambient
levels of these gases. Fluxes were calculated from the linear rate of
change in gas concentration, the chamber internal volume and soil
surface area. Flux rate calculations were not corrected for actual in
situ temperature and pressure. Single points were removed from
regressions if they were more than six times higher or lower than the
other three values or if they contradicted a clear trend in the other
three points. This procedure prevents inclusion of high flux rates
based on non-significant regressions. Non-significant regressions were
used in flux calculations to avoid biasing the statistical
distribution of rates by setting all non-significant regressions to
zero.
For snow-covered plots, plastic chambers (approximately 5 L) fitted
with septa were inserted 5 cm into the snowpack just prior to
measurement.
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