LOCATION DESCRIPTION
Watershed 5 is a 22.5-ha watershed extending from an elevation of
503-m at the weir to 755-m at the highest point on the ridge.
SAMPLING DESIGN
Watershed five is divided into 360 25x25m plots. Sixty plots for soil
sampling were identified in stratified random fashion in six elevation
bands. These 60 plots were sampled in the summer of 1983, before tree
harvesting, and after harvesting in 1986, 1991, and 1998. The “pit
number” values in the data set identify the 25x25m plots in the HB
grid system containing the individual pits. The three post-harvest
sampling locations within each plot were identified and marked prior
to the experimental whole-tree harvest in 1983-1984. The harvest of W5
is described in detail , including plot maps, at:
https://portal.edirepository.org/nis/mapbrowse?scope=knb-lter-hbr&identifier=327.
FIELD PROCEDURES
In each sampling year, one soil pit was excavated at one of the
pre-designated sampling locations using the quantitative approach
described by Vadeboncoeur et al. (2012). The “quantitative pit”
excavation procedure employs square soil pits with sides measuring
70.7 cm across, for a surface area of 0.5 square meters. The pits were
excavated to the C horizon or bedrock as described below.
A wooden frame with an opening measuring 0.707 m × 0.707 m was placed
on the ground and secured using rebar. Soil was removed from within
the frame in the following layers: Oi + Oe horizons together as a
single sample; Oa horizon; 0- to 10-cm mineral soil layer; 10- to
20-cm layer; and 20 cm to bedrock or C horizon. A grab sample of the C
horizon was taken if it was present. The distance from the plane of
the frame to the top of each soil surface was measured at 25 set
locations within the frame before excavation and after each layer.
Average layer thicknesses were computed using these values.
Measurements made to protruding rocks or large roots were noted. The
Oi + Oe and Oa samples were bagged in their entirety and returned to
the lab. The mineral soil layers were screened in the field using a
12.7-mm screen. All excavated materials—passing and not passing the
screen—were weighed in the field using a portable dairy scale. A
subsample of the screened material was bagged and returned to the lab.
Many of the pits in each year were sampled by pedogenic soil horizon.
In 1983, half of the pits were selected. In 1986 48 of the pits were
selected for horizon sampling. In 1991 and 1998, all pits were sampled
by horizon. Grab samples of each mineral horizon were taken from all
sides of the pits and composited into one sample per horizon. Note
that U.S. standard soil horizon nomenclature changed between 1983 and
1986. In nearly all cases, the 1983 horizon designations have the
following equivalencies: A2 = E, Bhir = Bs, Bir = Bs1, B23 and B23+ =
Bs2. Pit maps and horizon thicknesses can be found in Johnson and
Johnson, 2021.
LABORATORY PROCEDURES
Soil Processing: All soil subsamples were air dried and sieved with a
stainless steel screen (6 mm for organic horizons and 2 mm for mineral
horizons). For Oie, Oa, 0-10, 10-20, and 20+ cm samples, the mass of
each sample that passed through the sieve was used to calculate the
total air dry weight (ADW) of soil excavated from the pit in that
layer. Subsamples of air-dry soils that passed through the screen were
stored in 4 oz glass jars.
Oven dry weight (ODW) and loss on ignition (LOI) were measured using
the air-dry sieved fraction of the Oie, Oa, 0-10, 10-20, and 20+ cm
samples. A subsample was dried at 105°C to constant weight, weighed,
then ashed overnight at 500°C, and weighed again.
Exchangeable acidity (KCl extraction): A 2.5 g sample of soil was
extracted with approximately 50 mL of 1 M KCl for 12 hours using a
vacuum extractor. The filtrate was collected, and exchangeable acidity
was determined by titrating a subsample to a pH endpoint of 8.2 using
NaOH (typically 0.006 M). An excess of 1 M KF was added to bind Al,
and the sample was titrated back to a pH endpoint of 8.2 to determine
exchangeable acidity due to aluminum. The difference between
exchangeable acidity and exchangeable aluminum in the KCl extract is
reported as exchangeable H.
Exchangeable cations (NH4Cl extractions): A 2.5 g sample of soil was
extracted with 50 mL of 1 M NH4Cl for 12 hours using a vacuum
extractor. The filtrate was collected and analyzed for calcium,
magnesium, sodium, aluminum, and potassium on ICP-OES.
Total carbon and nitrogen: A subsample of air-dried soil was ground in
a mortar and pestle and dried at 80 oC for at least 24 hours. The
dried soil was analyzed for carbon and nitrogen by elemental analysis.
Soil pH: For soil pH, 2 g of organic soil, or 10 g of mineral soil,
was mixed with 9.9 mL DIW. The solution was shaken thoroughly, and pH
was measured. This quantity is noted as pHw. Next, 0.1 mL of 1 M CaCl2
was added to the sample to modify the matrix to be a 0.01 M CaCl2
solution. The sample was shaken periodically through 30 minutes and pH
was measured again. This quantity is noted as pHs.
Total (acid-extractable) element concentrations: In 1983 and 1986,
organic horizon soils were digested for total element concentrations.
Ground, oven-dry soil (0.5 g) was placed in a pre-weighed crucible and
ashed overnight at 500 oC. The ash was eluted with 10 ml of 6N HNO3
and heated for a few minutes (to a simmer) on a hot plate. The hot
extract solution was poured through filter paper (Whatman # 41
ashless) into a 50 ml volumetric flask. The crucible and the material
on the filter paper were rinsed several times with distilled deionized
water. The volume in the flask was brought to 50 ml with distilled
deionized water. The 50 ml of extract solution was stored at room
temperature until chemical analysis with ICP-OES. Analysis of plant
tissue standard reference materials (e.g. apple leaves, pine needles)
has consistently yielded near-100% recoveries, indicating that this
digestion procedure is effective for digesting organic matter.
However, digestion of Oa horizon soil samples is incomplete, as
evidenced by residual mineral matter on the filter paper. It is
therefore safest to refer to the results as
"acid-extractable" rather than "total" element
content.
CALCULATIONS
Thickness (reported): The average depth (cm) of all measured points in
a layer that did not land on a root or a rock
Thickness (net, not reported): The average depth (cm) of all measured
points in the layer, including those that landed on a root or a rock
Soil volume (cm^3; not reported) = 5000 cm^2* reported thickness (cm)
Air dry mass (ADM) of soil removed from the pit (not reported; kg) =
Mass of field-moist soil removed from the pit (kg) * (air dry mass of
subsample returned to lab)/(mass of field-moist subsample returned to
lab)
Oven dry mass (ODM) of soil removed from the pit (not reported; kg) =
ADM of soil layer (kg) * ((100-% moisture)/100)
ADW of soil layer (kg/m^2) = ADM of soil removed from the pit/pit
surface area (0.5 m^2)
ODW of soil layer (kg/m^2) = ODM of soil removed from the pit /pit
surface area (0.5 m^2)
% Moisture (not reported) = 100* (mass air dry subsample – mass oven
dry subsample)/mass air dry subsample
LOI (%) = 100 * (mass oven dry subsample-mass ashed subsample) / mass
oven dry subsample
Organic matter mass (kg/m^2) = ODW of soil layer (kg/m^2) * (LOI/100)
Coarse fragment volume (CFV, %) = 100*(a+b+c)/soil volume
Where:
a is the volume of rock in the layer that was not removed from the
pit:
a (cm^3) = (reported thickness-net thickness (cm)) * 5000 cm^2;
b is the volume of large stones (>12.7 mm) removed from the pit:
b (cm^3) = mass of large (>12.7 mm) rocks not passing the field
sieve (g)/ (2.6 g/cm^3);
c is the volume of rocks between 2 and 12.7 mm (i.e., stones that
passed through the field sieve but were retained by the sieve in the
lab:
c = (ADM of soil removed from the pit (g) * (mass of rocks not passing
2 mm sieve (g) /air dry mass of subsample returned from the field (g))
/ (2.6 g/cm^3)
Note: 2.6 g/cm^3 is the average density of rock.
Bulk Density (g/cm^3)= ODM of soil from the pit (g)/ (Soil volume
(cm^3) * (1-(CFV/100))
Exchangeable cation concentration (cmolc/kg) = [cation]
(mg/L)*(1g/1000 mg)*Molecular Weight cation (g/mol)*mol cation/mol
charge associated with cation)* (100 cmol charge/mol charge)*( Extract
solution volume (L)/soil extracted (kg))
Exchangeable cation stock (g/m^2) = [cation] mg/L * (g/1000 mg)*
(Extract solution volume (L)/soil extracted (kg))*ADW of soil layer
(kg/m^2)
Exchangeable acidity concentration (cmolc/kg) = Soil extracted
(kg-1)*volume titrant used (L)* concentration of NaOH titrant (
mol/L)* 100 cmolc/mol NaOH * volume extracted/volume titrated
Total element concentration (mg/kg) = [element] (mg/L) * (digest
volume (L)/ soil digested (kg))
Total element (pooled; g/m^2) = total element (mg/kg) * ADW soil
(kg/m^2) * (1 g/1000 mg)
REFERENCES
Johnson, A.H. and C.E. Johnson. 2021. Hubbard Brook Experimental
Forest: Soil Profile Maps and Horizon Thicknesses on Watershed 5,
1983-1998 ver 1. Environmental Data Initiative.
https://doi.org/10.6073/pasta/64db0cd6d007ca9c111b9e0eb1b71738
(Accessed 2021-07-20).
Vadeboncoeur, M.A., S.P. Hamburg, J.D. Blum, et al. 2012. The
quantitative pit method for measuring belowground carbon and nitrogen
stocks. Soil Sci. Soc. Am. J. 76: 2241–2255.
