SITE DESCRIPTION
Samples were collected in Watershed 1 of the Hubbard Brook
Experimental Forest. Watershed 1 is forested by typical northern
hardwood species (sugar maple, beech and yellow birch) on the lower
90% of its area and by a montane boreal transition forest of red
spruce, balsam fir and white birch on the highest 10%. Hydrology and
stream water chemistry have been monitored since 1963. Standard
surveying techniques were used in 1996 to install a grid system of 200
grid cells, each 25 x 25 m square (slope corrected). These grid units
serve as the coordinate system for sample plot selection. More
detailed information on the elevation zones and plot layout can be
found under the 'watershed/research site' tab on
https://hubbardbrook.org.
SAMPLING DESIGN
Plots were chosen to be sampled prior to each sampling trip by using a
random number generator to identify 100 plots over the entire
watershed. The location of the sampling site within each plot was
determined by selecting two random integers between 0 and 24
("x" and "y") and navigating to the selected
coordinates from the northwest corner of the 25 x 25 m plot. The
number of plots sampled varies from 58 to 101 per sampling year.
FIELD COLLECTION PROCEDURES
Once a sampling point was located by taping (occasionally by pacing),
a 15 x 15 cm template was placed on the forest floor and 4 nails were
inserted through pre-drilled holes, one in each corner. The nails hold
the template in place on the forest floor and retain the forest floor
material within the sample area beneath the template. A keyhole saw,
pruning shears and a trowel were used to make a clean cut around the
template, thus isolating a block of the forest floor. The surrounding
forest floor material was scraped back from the block leaving it as a
pedestal. This block was then undercut at the organic-mineral soil
boundary with a trowel, and pruning shears used to sever any roots.
Using a trowel inserted beneath the block and hands on top, the block
was carefully picked up and turned over such as to rest on the
template. Any remaining mineral soil was carefully scraped away
"up" to the bottom of the Oa layer of the forest floor.
After all mineral soil was removed, the total thickness of the block
and the thickness of the combined Oi and Oe horizons (referred to in
this data set as "Oie") were measured at 5 cm intervals
along each of the four sides for a total of 8 measurements. The
average thickness of the Oa horizon could then be determined by
subtraction of the average Oie thickness from the average total
thickness for each block. The block was separated into Oie and Oa
horizons in the field. Each horizon was placed in a separate plastic
bag for return to the laboratory.
After removal of the organic horizons, the upper 10 cm of the mineral
soil was sampled by coring. A stainless steel corer (diameter 1.362
inches 1996-2010, diameter 1.374 inches 2014 onward) was inserted in
the soil below the forest floor. If the core length exceeded 10 cm,
the core was cut to 10 cm and bagged. If the core length was less than
10 cm, usually due to refusal by stones, the entire core was bagged.
Two full 10-cm cores, or up to four partial cores, were collected at
each site, unless mineral soil was absent.
LABORATORY PROCEDURES
Upon return to the laboratory, all samples were air dried to a
constant weight. For 1996-2006, the Oie horizon sample was oven dried
at 80oC. From 2010 onward, the Oie was air dried. The Oie samples were
ground in a Wiley mill to pass a 2-mm screen. Large solid sticks and
roots larger than the size of a pencil and obviously alive or fresh
material were removed and weighed. These large pieces were not ground
and were not included in the mass calculations. The Oa horizon was
sieved through a 5-mm stainless steel screen. Sticks, roots and debris
(coarse fraction) not passing through the screen with mild pressure
were weighed. Stones, which were rarely present in the Oa horizon,
were also included in the coarse fraction. Material passing through
the screen was weighed and rebagged. The mineral soils were passed
through a 2-mm stainless steel screen. Material passing through the
screen was weighed and rebagged. Material not passing through the
screen, including sticks, roots, and stones, was weighed. A subsample
of each processed soil was weighed, oven dried at 80oC, and weighed
again. An air dry to oven dry conversion factor was determined in
order to express the total soil mass for each horizon on an oven dry
weight basis. Samples of the ground/screened Oie, Oa, and mineral
soils were put in 4 oz. glass jars for long-term preservation in the
Hubbard Brook archives.
Organic content and acid-extractable metals determination
For determination of organic content and acid-extractable metals
concentration, 0.5 g of the oven-dry ground material was placed in a
pre-weighed crucible and ashed overnight at 500oC. The ashed material
was cooled and weighed. The loss-on-ignition was computed and used as
a measure of organic matter content. 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 a filter paper (Whatman # 41
ashless) in a funnel and 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 up to
50 ml with distilled deionized water. The 50 ml of extract solution
was stored at room temperature until chemical analysis. The extract
solution was analyzed for metals on a combination of ICP-OES (Ca, Mg,
K, P, Mn, Al, Fe) and ICP-MS (Cu, Pb, Zn). 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 and mineral soil samples in
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. However, Johnson et al. (2014) showed that this extraction
procedure can account for the Ca added to this watershed as
wollastonite in 1999.
Exchangeable acidity and exchangeable aluminum (KCl extraction)
A 2.5 g sample of soil was extracted with ~50 mL of 1 M KCl for 12
hours using a mechanical vacuum extractor. The filtrate was collected,
and exchangeable acidity was determined by titrating a subsample to
the phenolphthalein endpoint using NaOH. In 2006 and 2010 a subsample
of the filtrate was also analyzed for aluminum on ICP-OES.
Exchangeable base cations and exchangeable aluminum (NH4Cl
extractions)
A 2.5 g sample of soil was extracted with ~50 mL of 1 M NH4Cl for 12
hours using a mechanical vacuum extractor. The filtrate was analyzed
for calcium, magnesium, sodium, and potassium on ICP-OES. In all years
but 2010, aluminum was included in the ICP-OES analysis. From 2002
onward, silica was also included in the ICP-OES analysis.
Total carbon and nitrogen
A subsample of air-dried soil was ground in a mortar and pestle and
dried at 80oC for at least 24 hours. The dried soil was analyzed for
carbon and nitrogen by elemental analysis.
Soil pH
Two grams of organic soil, or 10 g of mineral soil, was mixed with 9.9
mL DIW. The solution was shaken thoroughly and pHw was measured. Next,
0.1 mL of 1 M CaCl2 was added to create a 0.01 M CaCl2 solution. The
sample was shaken periodically through 30 minutes and pHs was
measured.
CALCULATIONS
Concentrations of carbon, nitrogen and exchangeable cations in all
samples were calculated on an oven-dry-weight basis. Exchangeable
cation concentrations are corrected for blank samples. One procedural
blank sample was included for each ten soil samples. For organic
horizons, total mass is the sieved mass of soil for the horizon
multiplied by the ratio of oven dry soil to air dry soil masses and
divided by the area of the template used for sampling (15 x 15 cm).
For the mineral horizon, total mass is the average sieved mass of soil
in the cores multiplied by the ratio of oven dry soil to air dry soil
masses and divided by the diameter of the soil corer. Organic mass is
the total mass of soil in the horizon multiplied by the loss on
ignition for the horizon. Concentrations of elements in the forest
floor were calculated on an oven dry weight basis. Total amounts of
the elements were estimated on a per square meter basis by
extrapolating up from the 15 x 15 cm sample mass. Forest floor
thickness, species contributing litter to the sample site, and E
horizon intensity for each plot are given in dataTable 1. Total mass,
organic matter mass, LOI, acid extractable metals, and CN are
presented in Table data2.
NOTES
Pools are the product of the chemical concentration and the total mass
for each horizon. Exchangeable hydrogen can be estimated by
subtracting exchangeable Al from exchangeable acidity. Effective
cation exchange capacity can be estimated by summing the exchangeable
base cations (Ca, Mg, K) and exchangeable acidity. Effective base
saturation can be calculated as the percent of effective CEC accounted
for by base cations.
Between 2006 and 2010, sampling responsibilities were transferred from
field crew at Yale University School of Forestry to Syracuse
University Civil and Environmental Engineering. Notes on tree species
in Table ww1_ffe.txt are much less detailed beginning in 2010.
