The following discussion is centered on lead (Pb),  however, the larger dimensions of the spatial and temporal analyses studied in this project include 9 other elements. While the original design of the work was to determine the patterns of lead movement, data about other elements was obtained concurrently and add other dimensions to the work now and for the future.

Abstract: Comparison between volumetric soil samples of the forest floor and Ap soil horizon of a North Haven pitch pine forest from 1990 and 1998 show vertical redistribution of metals due to podzolization and downward movement of pollutants.  Mineral soil was collected in thickness slices of several centimeters and analyzed for concentration and amount of ten metals: Pb, Zn, Cu, Al, Mn, Mg, Fe, Ca, P, and K.  Of these, most show "bulges" in amounts at a depth of 6-8 cm, where the majority of the increases since 1990 have accumulated.  However, Pb and Cu amounts smoothly decrease with depth, and their inputs appear to be moving downward at different rates. The downward rate of lead movement was estimated at 12 mm/yr.  Most elements increased in concentration in the mineral soil while decreasing in the forest floor.

Air movement patterns across the continental United States funnel high levels of lead, other metals, and other air pollutants to the Northeast (Van Cleef, 1908).  These compounds deposit on New England forest floors, both as dry fallout and in aqueous solution/suspension with rainwater.  In attempts to understand the cycling of nutrients and the effects of anthropogenic emissions on northeastern forest ecosystems, many researchers have documented the deposition of lead and other metals into the forest floor (Heinrichs and Mayer, 1978, 1980; Siccama and Smith 1980; Herrick and Friedland, 1990).

In the cycling of lead, the forest floor is an important sink.  Only a small amount of lead incorporates into biomass, and only a very small percentage leaves in streamflow or groundwater flux (Siccama and Smith 1978; Siccama et. al., 1980; Heinrichs and Mayer 1980).  However, since lead complexes with humus (partially decomposed organic matter) in the soil, decomposer organisms do incorporate it and thus excessive lead may slow decomposition processes (Buchauer 1973).  Initial lead studies from the 1970's documented rapid increases in forest floor lead amounts and concentrations, leading researchers to conclude that lead was accumulating (Siccama and Smith, 1978; Siccama et. al., 1980).  Input-output analyses showed that > 90 % of lead deposited within a given time period accumulated in the forest floor (Benninger et. al. 1975; Siccama et. al., 1980).  Researchers therefore concluded that lead accumulates in the forest floor Oe and Oa layers by complexing with insoluble organic matter, or as inorganic particles, and that it had a residence time ranging from 150-5000 years (Benninger et. al. 1975; Siccama and Smith, 1978; Siccama et. al., 1980; Johnson et. al.; 1982; Friedland and Johnson, 1985).

The single most important source of lead emissions was auto exhaust containing lead additives until 1970 amendments to the Clean Air Act required new cars to use unleaded gasoline.  As a result of this Act, between 1966 and 1990 gasoline-related lead emissions decreased by 96 %, and the amount of lead in precipitation fell by an equivalent amount (Miller and Friedland, 1994).  Subsequent to this reduction, lead concentrations began declining in the environment in general, with reductions documented in soil and major rivers throughout the country (Friedland et. al. 1992).

Because lead was assumed to have a long residence time in the forest floor, the declining rate of deposition was expected to merely slow the rate of accumulation.  However studies across the northeast found significant net decreases in the amount of lead in the forest floor over a timespan of 1-2 decades (Friedland et. al., 1992; Herrick and Friedland, 1990; Craig, 1991).  As lead amounts decreased in upper portions of the forest floor, they increased in lower portions of the forest floor, as well as the mineral soil, suggesting a pulse of lead moving downward through the forest floor and into the mineral soil (Craig, 1991; Marsh and Siccama 1992; Miller and Friedland, 1994).

This downward movement of lead occurs as a front with a measurable speed.  Miller and Friedland quantified downward movement between 1966-1990 to range from ~3 mm/yr in a northern hardwood forest to ~19 mm/yr in a sub-alpine spruce-fir forest.  This rate depends on microbial decomposition and transport of soil organic matter, the acidity of the soil, the texture of the soil and the amount of precipitation (Miller and Friedland, 1994). Since the concentration of lead will eventually reach the water table, this rate has potentially serious human ramifications.  Through a temporal comparison, this study will estimate the rate of movement in a sandy acidic low elevation pitch pine forest in North Haven, CT.

To estimate this rate, I sampled five plots of soil at a pitch pine site in North Haven, CT.  This location has been sampled twice in the recent past.  In 1980 Mark Groff sampled the forest floor with 20 sampling sites and analyzed for dry mass, organic matter, lead, zinc, copper, calcium and magnesium (Groff, unpublished data). Groff did not study the mineral soil.  In 1990, Anne Marsh and Thomas Siccama sampled soil plots in several sites throughout the northeast.  In this study, we revisited one site, the North Haven pitch pine forest to measure the change in the vertical distribution of metals between 1990 and 1998.  We reanalyzed the 1990 soil concurrently with the new samples to prevent laboratory conditions, technical changes, or processing differences from affecting the results.  By temporal comparison, we hope to observe the patterns of change in vertical metal distribution and infer soil organization processes and rates and patterns of downward movement of the several elements.

Besides lead, the 1990 and 1998 samples were analyzed for nine other minerals: copper, zinc, manganese, magnesium, calcium, aluminum, iron, phosphorous, and potassium.  For more information on the biogeochemical properties of these minerals, please go to the soil minerals page.

The geology, vegetation and land use history of the Pine stand creates an ideal area for studies of vertical mineral stratification in soil.  Pitch pines and black oaks are the dominant trees above an understory of mixed hardwoods.  The parent material at this site is a layer of the reddish arkosic sand of the Central Lowlands, overlaying whitish schist/gneissic granitic sand from the Western Highlands.  The sand here results in excessively drained soil that slows decomposition of organic matter.  Due to the minerology (quartz), coarse texture (sand) and slow decomposition (dry, nutrient poor), the soil is highly acidic.

Early settlers cleared the area and plowed the soil, destroying native soil horizons and creating a homogenized layer of soil called an Ap or plow horizon.  Plow horizons characteristically are 20 cm (8 inches) thick, the depth of standard plows. This Ap horizon provides a homogenous environment for measuring the movement of pollutants without the high spatial variation inherent in native (undisturbed by plowing) soil horizons.  Further, since the primary deposition of lead and most pollutants occurred since plowing ended, the Ap layer serves as a "blank slate"  in which anthropogenic inputs can be discerned from the background levels in 1860.

Now, 125 years since plowing was ended, the previously homogenized soil (Ap layer) is beginning to chemically restratify, showing evidence of two somewhat visually distinct levels in the Ap horizon.  The end point of this natural process will create layers of leached soil and layers of high concentration of nutrients and organic matter. In the figure above the extreme endpoint which might be expeced in this soil is illustrated. In the other figure above, a long ago plowed soil, there is just the beginnings of a visible reestablishment of the distince horizonation. The "grey" layer is called an E horizon- which is a leached layer. Material (organic matter and various elements) leached from this layer accumulate in a slightly lower layer (the Bhs horizon).  Marsh and Siccama (1997) found 'bulges' in the Cu and Zn curves, which they ascribe to this natural soil ‘reorganization’ process.   Unseen processes, biologic and chemical, are working to restratify this soil.  In addition to the measures of the pollutant lead, this study will discuss the extent to which these natural processes have  occurred.

Besides the plow layer, another factor making the site ideal to measure downward movement of lead is that in the interposing years since the end of plowing, very little bioperturbation has occurred.  Bioperturbation is the mixing which occurs by various soil animals especially earthworms, however in this dry acidic soil the organisms are absent.  The boundary between the bottom of the plowed layer and the underlying soil (yellow b horizon) is very sharp.  Small notches made by the tip of the plow bottom on its last pass over the field 125 years ago can still be distinguished on the face of soil pits.  This means that the stratification of recent inputs will migrate due to natural soil/water relations without inordinate biological mixing in the soil.

The site location is near many potential sources of pollution, including one of the largest interstate highways in the northeast.  Lead distribution patterns show a corridor of high lead amounts, which the site is within, along I-91 and I-95 (Johnson et. al., 1982).  Both copper and lead concentrations at this site are exceptionally high compared to other New England locations (Marsh and Siccama, 1997).  The amount of lead could result from the high degree of urbanity in the surrounding area, but copper levels are high enough to suggest a local anthropogenic point source.

In the acidic/ cool temperate forest ecosystem, the annual leaf fall is not rapidly incorporated into the mineral soil and builds up as a partially decomposed layer over the mineral soil.  This layer varies in the upland setting from from a trace (1-2 cm) to 10-20 cm, or more.  The layer is quite distinct and separate from the underlying mineral soil.  While walking in the woods, it is the soft "carpet of leaves" underfoot in New England. Interestingly the thick forest floor in the North Haven Pitch Pine stand is itself an artifact of anthropogenic activities -- or lack thereof. If this were a totally natural setting this sytem would burn frequently and thus obliterate the thick organic layer. However, fire protection in the urban setting has precluded its burning off.
 
 

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