LOCATION DESCRIPTION
Soil atmosphere fluxes of the trace gases; carbon dioxide (CO2),
nitrous oxide (N2O) and methane (CH4) have been measured at several
locations at the Hubbard Brook Experimental Forest. (HBEF):
Bear Brook Watershed. Trace gas sampling is coordinated with long-term
monitoring of microbial biomass and activity:
(https://portal.edirepository.org/nis/mapbrowse?scope=knb-lter-hbr&identifier=67)
that is keyed to the “west of watershed 6 litter trap transects”
described by Hughes and Fahey (1994). These are four 100 m transects,
50 m apart with five traps per transect located at low, mid, upper and
high elevations - 20 traps per elevation. Trace gas sampling locations
are located within 15 m of five traps at each elevation. Litter
quality, quantity and composition have been monitored on these
transects since 1984 and microbial biomass and activity have been
monitored since 1994. Trace gas sampling on this watershed began in
August 2002 and is ongoing.
Watershed 1. Trace gas sampling locations were co-located with
long-term microbial biomass and activity monitoring sites that are
located near a subset of the lysimeter sites established for the
calcium addition study on this watershed. Our sites include the
"spruce/fir" Lysimeter Site 2 at the top of Watershed 1, the
"high" Lysimeter Site 3, the "mid" Lysimeter Site
4, and the "low" Lysimeter Site 6. These site types and
elevations correspond to those in the Bear Brook Watershed. At each
lysimeter site we chose replicate plots that were all within ~30 m of
the center lysimeter stake. Trace gas sampling on this watershed began
in October 2002 and is ongoing.
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). From 1997 to September 2015 N2O and CO2 were
analyzed on a Shimadzu GC-14 GC with electron capture (N2O) and
thermal conductivity (carbon dioxide, CO2) detectors. From Oct 2015 to
present CO2, N2O, and CH4 were analysed on a Shimadzu GC2014 equipped
with Thermal Conductivity, Electron Capture and Flame Ionization
detectors. 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.
REFERENCES
Bowden R, J. Melillo, P. Steudler and J. Aber. 1991. Effects of
nitrogen additions on annual nitrous oxide fluxes from temperate
forest soils in the northeastern United States. Journal of Geophysical
Research – Atmospheres 96(D5):9321‑9328.
Bowden R, P. Steudler, J. Melillo and J. Aber. 1990. Annual nitrous
oxide fluxes from temperate forest soils in the northeastern United
States. Journal of Geophysical Research – Atmospheres,
95(D9):13,997‑14,005.
Groffman, P.M., J.P. Hardy, M.C. Fisk, T.J. Fahey and C.T. Fahey.
2009. Climate variation and nitrogen and carbon cycle processes in a
northern hardwood forest. Ecosystems 12: 927–943. DOI:
10.1007/s10021-009-9268-y.
Groffman, P.M., M.C. Fisk, C.T. Driscoll, G.E. Likens, T.J. Fahey, C.
Eagar and L.H. Pardo. 2006b. Calcium additions reduce nitrogen cycling
in a northern hardwood forest. Ecosystems 9:1289-1305.
Hughes, J.W. and T.J. Fahey. 1994. Litterfall dynamics and ecosystem
recovery during forest development. Forest Ecology and Management
63:181‑198.
