The water and nutrients utilized by plants are for the most part obtained from the soil. Therefore, understanding soil physical properties and nutrient status is a critical aspect of ecosystem ecology. We collect soil water with lysimeters, extract samples of the forest floor layers, and profile the mineral soil layers, taking samples of each horizon. All are analyzed for nutrient concentrations to get an idea of how much is available to plants over the long and short terms.
Soil research
1. Lysimeters
2.
Forest floor blocks
3. "Big
dig" soil pits
Many nutrients and toxins are transported
within the soil and through the ecosystem by way of aqueous solution.
Lysimeters allow us to periodically extract soil solutions for chemical
analysis in order to monitor the presence and flow of ions in soils.
In order to keep apprised of the chemical status of soil solutions in the
vicinity of W6, several lysimeters were installed just west of W6 in three
different vegetation zones. Because installation requires quite a
bit of soil disturbance, the lysimeters were not installed within the reference
watershed itself.
Each lysimeter station actually has three lysimeters set up in three different soil horizons--beneath the Oa, beneath the Bh, and within the Bs. They are tension-free lysimeters, meaning water is not extracted from pores in the surrounding soil after creating a vacuum in the lysimeter tube. Rather, free-draining soil solution flows into a buried pan and drains into a collection bottle that is buried deeper underground. The collected soil solution is pumped out of the collection bottles once every month or so. Click here for a diagram of what one looks like underground.
Click here for a further description of methodology and data.
The forest floor layers consist primarily of organic
matter from fallen leaves and needles, branches and logs. Bacteria
and fungi act to decompose the plant parts and release the nutrients originally
locked up in the living cells. The forest floor is abundant in fine plant
roots which are closing the loop and taking these nutrients back up into
living plants. In our studies, we divide the forest floor into two
layers. The Oie is the uppermost layer and consists of whole leaves
and plant material that has been broken down into pieces that are still
large enough to be recognizable as debris from leaves or twigs. The
Oa is a black, somewhat greasy soil layer that lies beneath the Oie and
consists of highly decomposed plant material whose origin is no longer
recognizable.
To get an idea
of the depth of the forest floor and its concentration of various nutrients,
we excavate many small blocks across a watershed according to the procedure
below, and return the samples to the lab for chemical analysis. Although
digging up forest floor samples does cause some disturbance to the forest,
having data specific to W6 is considered important enough that we conduct
these surveys in the watershed itself (rather than just to the west of
the watershed) every five years or so.
a
A 15 x 15 cm wooden template is placed on the surface of the forest floor,
and the soil is excavated from around a pedestal left beneath the template.
b The pedestal is removed and flipped upside down to remove any remaining mineral soil now on the "top" of the block.
c The thickness of the block is measured, indicating the depth of the forest floor in that spot.
d
The Oie and Oa layers are separated and placed in separate plastic bags
for return to the lab for chemical analysis.
Click on the photos below for a
photographic demonstration of the process.
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| Some typical values for the forest
floor:
(taken from the 1997 survey on W6) Thickness LOI Nutrient conc. Nutrient amount (cm) (%) (ppm) (g/m2) Ca Mg K P Ca Mg K P Oie -- 83 3330 375 859 942 10.45 1.21 2.85 3.23 Oa -- 47 715 287 802 880 5.21 2.54 6.81 7.57 Total 7.43 -- -- -- -- -- 15.66 3.75 9.66 10.80 |
The forest
floor is also enriched by particle fallout from the atmosphere in the form
of precipitation or "wet deposition", or in the form of dust or "dry deposition". 
Some
of these particles are important plant nutrients, such as sulfur or calcium,
and some are contaminants, such as lead. Since the jet stream generally
moves from west to east, airborne particles from the West and Midwest are
constantly carried over the Northeast, where some of them fall out by wet
or dry deposition. Before the 1970s, when lead
was phased out of gasoline, a great deal of lead was deposited in New England
from automobile exhaust originating both locally and from more western
states. But as the phase-out progressed, less and less lead fell
out of the sky. We observed this phenomena as the decline in total
lead content of the forest floor over time as more lead was leached out
of the forest floor than was added.
Beneath the forest floor lie the mineral soil layers.
In many cases mineral soil can be sampled quickly and neatly with a soil
corer. This technique leaves a small hole in the ground, but otherwise
causes very little disturbance. However, the relatively young glacial
soils of the Northeast are full of large rocks, making it virtually impossible
to obtain representative samples of all soil horizons with just a corer.
To quantify the soil at Hubbard Brook, a technique was developed by Steven
Hamburg to get around the problem of rocks. It entails digging a
large, precise, square hole and very meticulously excavating the soil by
layer. We call the process the "big dig" and the steps are outlined
below. This technique, however, causes a great deal of disturbance
and we don't use it in Watershed 6 for fear of disrupting our "reference".
We have, however, conducted big digs on W1 and W5.
Click on the photos below for a photographic demonstration of the process.
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Soils at Hubbard Brook are predominantly base-poor sandy loams derived from glacial till. A cool climate and abundant precipitation have caused them to develop as spodosols. The cold climate inhibits decomposition, so organic matter builds up on the surface in the form of a thick forest floor (Oie and Oa). As the forest floor becomes moist, very acidic organic compounds are created. When water from a precipitation event percolates down through the soil profile, it carries with it these acidic organic compounds, as well as some soluble organic matter. The acid compounds, in turn, leach out aluminum and iron from the mineral soil layer immediately below the forest floor (A or E). The aluminum and iron are then carried deeper down the soil profile and eventually precipitated as aluminum and iron oxides in what is called a spodic horizon (Bs). The soluble organic matter is also deposited in the spodic horizon, often creating a dark layer just above the reddish horizon containing the aluminum and iron (Bh). When a spodosol has had enough time to develop, sometimes the layer from which aluminum and iron have been eluviated, becomes a light-colored, almost pure, "acid-washed" sand (E). This is because quartz is just about the only mineral not dissolved and carried downward by the acids.
Below the soil, which by definition includes only the region of biological activity, is a layer of relatively unweathered glacial till (C)--the parent material in which these soils developed. The layer is generally greenish brown in color, and its texture is generally a bit finer and siltier than the soil above it. At Hubbard Brook, the top of this layer is occasionally a somewhat impermeable pan, which slows the downward passage of water and may create a perched water table after significant rain events. This is especially true at the time of snowmelt in spring. Where it is present, the hardpan also inhibits root growth and thus restricts the depth of soil development.
In many places,
there is no layer of unweathered till and the soil sits atop bedrock (R).
In some cases there is little or no mineral soil, and the soil profile
consists of Oie and Oa on bedrock.
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At Hubbard
Brook the soil profile is very variable from spot to spot. This is,
in part, the nature of spodosols in that the wetting front within a soil
is uneven, and the characteristic horizons vary greatly in depth depending
on the frequency and extent of water movement at that point. The
E layer, most noticeably, widens and narrows across a profile for this
reason.
However, the variation is also a result of soil disturbance at the base
of uprooted trees, which leads to pit
and mound topography. Every time a tree is uprooted a new pit
and a new mound are created, and the soil-forming process begins anew.
This essentially creates a mosaic of soil at different stages in the development
of a spodosol.
This graph shows probability distributions for soil horizon
depths before (1983) and after (1986) a major disturbance to the forest
(the clearcut of W5). Most
notable is the creation of A horizons at the expense of E, probably in
the process of dragging logs across the soil surface. The original
E horizon was presumably mixed with the forest floor to create an A horizon,
which is a mixed mineral and organic layer. There also seems to be
a significant increase in buried horizons, which are soil layers with characteristics
typical of higher horizons (such as A or E) that are found below a typically
lower horizon (such as B). Buried horizons are a result of new soil
being deposited on top of the original ground surface. Average soil
depth on the Hubbard Brook watersheds is about 60 cm.
Although defining soil horizons is important, it is virtually impossible
to excavate a soil pit by the very variable individual horizons in Hubbard
Brook soils. Instead we excavate our soil pits by depth layers.
Subsamples from each depth layer are returned to the lab to undergo chemical
analysis. Because the soil varies so greatly from point to point,
we need to dig many pits to get a big enough sample size for any kind of
statistical analysis. In the big digs on W5, we dig about 60 pits
every time we sample. Below are some representative values for soils
at Hubbard Brook, averaged from the 60 or so pits dug on W5 in 1983.
| Some typical values for the soil
profile:
(taken from the 1983 W5 study by Chris Johnson) pH Exchangable cations (in water) (g/m2) Ca Mg K Oie 4.29 -- -- -- Oa 3.91 7.40 0.81 1.61 0-10 cm 3.83 7.79 1.21 3.26 10-20 cm 4.20 3.93 0.58 1.82 20+ cm 4.50 7.56 0.90 4.06 C 4.69 -- -- -- Total -- 26.68 3.50 10.75 |