soils and soil acidification in the adirondack mountains

146 THE A DI RON DACK JOURNA L OF ENVIRONMENTAL STUDIES

9 : P O S T- VA L L E Y H E A D S D E G L AC AT I O N O F T H E A D I RO N DAC K M O U N TA I N S A N D

A D JAC E N T L OW L A N D S

VOLUME 21 147

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1. Department of Geology, Colgate University, Hamilton, NY 13346, 315.228.7212, [email protected] or [email protected]. Department of Environmental Studies and Geology, Alfred University, Alfred, NY 14802, 607.871.2838, [email protected]

SOILS AND SOIL ACIDIFICATION IN THE ADIRONDACK MOUNTAINS RICHARD APRIL,1 DIANNE KELLER,1 AND MICHELE HLUCHY2

KEYWORDS:

Adirondacks, Soils, Acid Rain, Mineralogy, Weathering, Watershed Liming

ABSTRACT

Soils in the Adirondack Mountains are relatively young, having formed in sediments deposited from the last major glacial episode that ended in the Adirondacks about 12,000 years ago. Most of the region is covered by an acidic, sandy, low-fertility soil called a Spodosol. Over time, Spodosols develop distinct colorful horizons that can be clearly distinguished in soil pits and road cuts. Because Spodosols are naturally acidic and contain marginal concentrations of some elements necessary to sustain a healthy forested ecosystem, they are somewhat fragile and susceptible to chemical changes. A century’s worth of acid deposition (“acid rain”) has depleted these soils of some important nutrients and has mobilized aluminum, an element linked to fish mortality and forest decline. Acid rain has declined over the past several decades, and lakes, streams, and soils are beginning to recover. How long it will take is still unknown.

148 THE A DI RON DACK JOURNA L OF ENVIRONMENTAL STUDIES VOLUME 21 149

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INTRODUCTION

What is Soil?

What is soil and how is it different from sediment or just plain dirt? Sediment is particulate matter composed of inorganic mineral grains derived from both the physical and chemical weathering of rock. Depending on the source, sediment may contain just a few minerals derived from a single rock type or a plethora of minerals, some abundant and some minor, originating from different rock types present in the source area. Dirt is what you sweep from your porch when spring finally arrives or what collects in the crevices of your wood floors or in the corners of your attic. Soil, on the other hand, is an incredibly complex mixture of minerals, liquids, and gases, decomposed organic matter, and microbes. Some have called soil the “skin of the earth,” (Logan 1995) others “the biologically excited layer of earth’s crust” (Richter and Markewitz 1995). Either way it is a precious commodity, too often taken for granted. Soil can take decades to thousands of years to develop and evolve. It is a major component of our planet’s terrestrial ecosystem. Without soil there would be no terrestrial ecosystem.

SOIL IN THE ADIRONDACKS

Adirondack Glacial Sediments – the Precursor of Adirondack Soil

Soils in the Adirondacks are less than 12,000 years old. Any regolith that was present before the last ice age was virtually stripped clean by the erosive action of the Wisconsinan glaciation. Soils in the southern U.S., on the other hand, where no recent glacial event reached, are hundreds of thousands, perhaps millions of years old. The parent material from which soil in the Adirondacks formed is the sediment deposited by the glacier after the great ice sheet melted away (see Franzi, this issue). We can determine what the nature of this parent material is by examining drill cores, sampling exposed sediments from road cuts and sand and gravel quarries, and by excavating soil pits to a depth of a meter or more to where the sediment has remained virtually unaltered by pedogenic processes. The minerals that compose these sediments vary from place to place depending on the composition of the local bedrock. But, in general, the most abundant minerals are quartz (SiO2), potassium feldspar (KAlSi3O8), and plagioclase feldspar (NaAlSi3O8– CaAl2Si2O8) (Table 1). This is not surprising given that granitic bedrock occurs widely in the Adirondacks and is composed mainly of these minerals (see Chiarenzelli and Selleck, this issue). In locations where other types of bedrock dominate, such as metasedimentary rocks, anorthosites, gabbros, and amphibolites, the relative abundance of quartz and the feldspars, as well as amounts of accessory minerals (i.e., those present in the soil in concentrations of only a few percent or less) such as hornblende, garnet, ilmenite, magnetite, and pyroxene, will vary (April and Newton 1983; Newton et al. 1987). Soils initially inherit these same minerals from the sediment in which they form. But, as pedogenesis proceeds, weathering processes modify the mineral suite in soil by creating new ones and eliminating others.


Table 1: Mineral abundances in the C-horizon determined by point counting of 300 grains (average of 81 samples; total of 100.1 due to rounding; one standard deviation in parentheses) (April et al. 1983).

AVG. VOLUME % STD. DEV

Quartz 39.0 (4.3)

K-feldspar 30.3 (1.9)

Plagioclase 17.8 (2.6)

Hornblende 3.5 (2.2)

Opaques 4.2 (1.1)

Pyroxene 0.7 (0.4)

Epidote 0.8 (0.4)

Garnet 0.6 (0.2)

Others 3.2 (0.6)

Total 100.1

THE BEGINNINGS OF ADIRONDACK SOIL

Life does not take long to establish itself, even in such an inhospitable and barren terrain as the post-glacial Adirondacks. Primitive alpine plants, such as mosses, and composite organisms, such as lichens, took hold on the sand and bare rock left by the retreating glacier. Their presence began the process of turning sediment into soil by assimilating and recycling nutrients, retaining moisture, releasing organic acids, and, upon death, providing organic matter to the substrate. As temperatures warmed and the climate moderated, larger and more complex plants, such as shrubs, grasses, forbs, and trees began to thrive. Macroinvertebrates such as snails, millipedes, and spiders (McCay et al. 2013) joined the soil ecosystem, which was now also filled with microbes, and all of these organisms provided new means to decompose organic matter, aerate the soil, and construct channels for water to percolate. Soon boreal forests of spruce and fir covered the lowlands, spreading north and to higher altitudes as the climate moderated further. Finally, mixed conifer and hardwood forests comprising pine, hemlock, spruce, birch, ash, beech, and maple, to name just a few, covered much of the low- to middle-elevation slopes, with black spruce and tamarack occupying the wetland areas. It is important to note that as vegetation literally took root and organisms moved into new and available habitats in what was once the bare and lifeless glacial sediment, little by little an amazing transformation had begun —the development of soil.

The Soil Profile —Spodosols

As you take a spade and drive it deep into a well-drained Adirondack forest soil, the cutting edge of the shovel will first penetrate a layer of fine roots and decomposed vegetation anywhere from a few centimeters (cm) to 25 cm thick. The vegetation is composed mainly of leaf and




needle litter in various states of decay, with the remains of leaves and needles from the previous autumn senescence still visible on the surface grading to humus below. Some mineral matter will be present in this layer, likely mixed in by frost heave, tree throw, or invertebrate burrowing activity, but not much. This is the “O” or organic horizon of what will likely be a Spodosol (Figure 1), one of the twelve soil orders recognized in the U.S. Department of Agriculture’s soil taxonomy system and the most common soil type in the Adirondacks (Figure 2). A Spodosol is an acid soil, usually sandy and of low fertility, and characterized by a subsurface accumulation of organic matter, clay, and aluminum and iron oxides. It is typically found in regions of the world that experience a cold and wet temperate climate, and it represents little more than 4% of all soil orders mapped on our planet (NRCS 1999). The term Spodosol derives from the Greek “spodos,” which means wood ash, the characteristic color of the next horizon your spade will penetrate. This is the E-horizon, an ash-gray eluvial layer, the color of which is caused when iron and aluminum are leached out of this layer by the constant rain of organic acids produced by the decomposition of organic matter in the O-horizon above. The E-horizon is usually sandy, wavy, sometimes discontinuous, and consists mostly of resistant, uncoated quartz grains and little else. In the Adirondacks, however, clay minerals are also found here as they form from the intense weathering of silicate minerals that were once present in this horizon (April et al. 1986a).

Figure 1: A typical Spodosol profile in the Adirondacks showing distinctive and colorful horizons. Depth from top O-horizon to bottom C-horizon (parent glacial sediment) is approximately 90 cm.


Dig deeper and you encounter the B-horizon, or zone of accumulation. Here, the soil takes on a darker color and then eventually turns a bright orange-red, which then fades with depth. The dark layer just below the E-horizon is a zone where some organic matter (humic material), iron and aluminum sesquioxides, and fine clays washed down from above and accumulated. This top layer of the B-horizon (the Bs- [spodic] horizon, also sometimes referred to as the Bh- [humic] horizon), is not much thicker than 5-10 cm in the Adirondacks, on average. Fine and medium roots are still plentiful at this depth but begin to dwindle away to nothing as the spade penetrates to the next layer of soil below, the B2ir-horizon. This is a zone of accumulation in which iron and aluminum, derived from mineral weathering processes above and carried to depth as soluble metal-ligand complexes, finally precipitate as oxides and hydroxides, some crystalline and others paracrystalline or amorphous. The orange-red color reflects the large amount of iron present in this horizon. Few fine to medium roots are found here, mainly because there are fewer nutrients available for plants to feed on. Cementation, resulting from this influx of iron and aluminum and its subsequent precipitation, sometimes results in the development of a fragipan at depth, a hard, almost impenetrable layer in the profile.


Figure 2: Map of soil orders in and around the Adirondack Mountains. (Modified from the USDA Natural Resources Con-servation Service [NRCS] World Soils Dataset).



Now down about 50 cm into the forest soil, the B-horizon begins to show a transitional gradient for perhaps another 25 cm to a lighter color, perhaps a light orange changing to a tan or olive-gray. This is the BC-horizon, which, depending upon the sediment type from which the soil evolved, is usually gravelly to sandy because it contains less of the fine-grain weathering products illuviated from above. The bottom of this horizon represents the depth in the Spodosol profile at which no visible signs of chemical alteration are apparent. Below this horizon lies the C-horizon, defined as the original parent material—the glacial sediment —from which, and in which, the soil profile evolved.

From the authors’ experience excavating hundreds of soil pits over the past several decades of working in the Adirondacks, a typical Spodosol profile will reach a depth of about a meter, if bedrock does not impede its development. While digging a soil pit, you may encounter boulder-sized rock fragments if the soil developed in till, and a pickax and a crowbar will be necessary for their removal. Soils that developed in outwash sands, on the other hand, are usually devoid of boulders and generally have a sandy, loamy texture and horizons that are more continuous and level.

THE CHEMISTRY AND MINERALOGY OF ADIRONDACK SOIL

Spodosols are characterized as naturally acidic, base cation poor soils, and the Spodosols in the Adirondacks are no exception. Acidity is highest in the upper soil horizons, with pH values averaging about 3.7, and lowest in the C-horizons, with pH values averaging about 4.8 (Table 2). Concentrations of exchangeable base cations (i.e., calcium, magnesium, potassium, and sodium loosely bound to organic matter and clay particles) are quite low in Adirondack soils, but are present in sufficient quantities to support a healthy forested ecosystem (Table 2). The cation exchange capacity (CEC) of the soil, which is a measure of the total number of soil exchange sites available for these cations to bind to, typically shows a close correlation with the percent organic matter. Percent organic matter (determined by loss on ignition, LOI) is highest at the surface, drops sharply in the E–horizon, increases somewhat in the Bs/Bh horizon, and then declines with depth (Table 2). Base saturation, the percent of available exchange sites occupied by base cations, is also highest in the upper horizons where nutrient elements such as calcium are recycled by decomposing plant litter. Recycled elements do not tend to penetrate deeply in the soil so base saturation typically drops to low values at mid-profile depths, then increases again toward the C-horizon where exchangeable cations are not mined much by fine roots.

Bulk soil chemistry, determined by X-ray fluorescence analysis is commonly interpreted using the dimensionless mass transfer coefficient,

τi, j = -1Cj, w Ci, p

Cj, p Ci, w


where τ represents the ratio of the concentration of an element of interest (Cj) normalized to an immobile element (Ci) in the weathered soil (w) and unweathered parent material (p) (Brimhall and Dietrich 1987; Anderson et al. 2002). The τ value is a useful way to compare changes in elemental composition from weathering processes in soils. A τ value of zero indicates an element has not changed from the initial parent composition, whereas a τ value greater than or less than zero indicates addition or depletion of an element relative to the initial parent composition, respectively (Brimhall and Dietrich 1987).

Mineral weathering is most intense in the upper horizons of Adirondack soil profiles, where organic acids are constantly being produced and are reacting with minerals there. Acid is used by these reactions so soil water becomes increasingly more neutralized as it percolates deeper through the soil profile. As can be seen in Figure 3, most elements are depleted within the soil profile. Exceptions to note are calcium, which is highly enriched in the organic horizon from the release of plant-bound calcium in decomposing litter, and iron, which is transported down-profile and accumulates in the B-horizon. All elements show their greatest depletion in the E–horizon, an organic-poor eluviated zone, increasing in concentration to the levels of the unweathered parent material at depth. The lower depletion values above the E-horizon reflect additions of recycled cations from above by decomposing vegetation. It should be noted here that for the past century acid deposition has delivered strong acids (i.e., sulfuric and nitric) to the forest floor, intensifying weathering in the upper soil horizons, and, as shall be discussed later, causing problematic depletion of nutrient cations.


Table 2: Average soil chemistry and loss on ignition (LOI) for recent Spodosol profiles sampled at all 21 study sites across the Adirondacks.

Exchangeable Base Cations Average concentration (cmolc/kg)

Horizon Average Soil pH Ca Mg K Na Average LOI

Oi 3.73 7.07 1.18 0.75 0.13 80.61

Oa 3.60 4.00 0.60 0.33 0.11 57.15

E 3.87 0.30 0.05 0.06 0.09 5.13

Bs 3.75 0.58 0.10 0.12 0.13 14.59

B2ir 4.48 0.21 0.03 0.03 0.08 12.19

B2 4.49 0.40 0.04 0.04 0.08 8.83

BC 4.77 0.25 0.02 0.02 0.07 4.88

C 4.76 0.09 0.01 0.01 0.07 2.51



Mineral abundances in Adirondack soils vary depending on location and the nature of the regional bedrock, which in turn influences the composition of the parent glacial sediment from which soil minerals are inherited (Newton et al. 1987). Silicate minerals dominate in Adirondack soils, and as previously mentioned, quartz and the feldspars are ubiquitous and are most abundant. In those locations where carbonate minerals are present in the bedrock, such as in the marble belts near Newcomb in the central Adirondacks or in the calcsilicate-bearing metasedimentary rocks around Bug Lake and Windfall Pond in the southwestern Adirondacks, traces of calcite are occasionally found in the deepest soil samples (Harstad and Newton 1983). More often, the calcite has long ago dissolved in the highly acidic soil, and the only remnant of its former presence may be higher concentrations of soil exchangeable calcium.

The general stability of silicate minerals in acidic soils is fairly well known, with those formed at high temperatures and pressures and containing elements such as magnesium, iron, and calcium more susceptible to weathering than those formed at low temperatures and pressures and containing elements such as potassium and sodium (White 1995; White et al. 1996). For example, calcium-rich plagioclase will weather faster than sodium-rich plagioclase, and both will weather faster than potassium feldspar (Kolka et al. 1996). Hornblende, a silicate mineral rich in calcium and magnesium, can constitute up to 50 wt%

Figure 3: Representative Tau plot depicting enriched and depleted elements in an Adirondack Spodosol profile. Depth of the profile is approximately 100 cm.


or more of the heavy mineral fraction (specific gravity >2.96) of Adirondack Spodosols (April et al. 1986a). April and Newton (1983) showed that concentrations of hornblende can drop from 6 wt% in the C-horizon to zero in the upper few centimeters of Adirondack soil profiles, where chemical weathering is most intense. The congruent weathering and dissolution of hornblende, as well as the weathering of other accessory minerals such as diopside and biotite, both rich in nutrient base cations, was observed in SEM analyses of soil samples collected from the central Adirondacks (April and Newton 1992).

It is important to note here that in Adirondack Spodosols, with relatively low fertility and low availability of nutrient cations for plant growth, the weathering of accessory minerals provides a disproportionately large fraction of nutrient cations to soil exchange sites compared to their low abundance in the soil (April et al. 1986b). Without minerals like hornblende, diopside, and biotite providing nutrient base cations such as Ca2+, Mg2+, and K+, and apatite providing phosphorus, Adirondack soils would be less able to support a healthy forested ecosystem. But, can the weathering of these silicate minerals supply nutrient base cations at a pace that keeps up with the demand of a healthy forest ecosystem? Historically, it appears that this has been the case. But, as we shall see, anthropogenic influences that disturb the delicate balance of natural ecosystems can cause problems.

Lastly, it is worth noting that secondary weathering products formed during pedogenesis, such as oxides and hydroxides of iron and aluminum, and clays, such as vermiculite, smectite, and kaolinite, are also common minerals in Adirondack Spodosols (April et al. 1986a). Johnson and McBride (1989) identified microcrystalline goethite, ferrihydrite, and imogolite-like material, too poorly crystalline to be considered true imogolite because it lacked characteristic tubular morphology. As for the clay minerals, vermiculite is ubiquitous in Adirondack Spodosols, forming from the weathering and transformation of biotite and muscovite. Studies of the secondary clay products in soil and glacial deposits from forested sites across the Adirondack Mountains show that the up-profile mineral weathering sequence of mica to mixed-layered mica-vermiculite to vermiculite to low-charge vermiculite (April and Newton 1983; April et al. 1986a) progresses even further in some locations to form smectite in the uppermost soil horizons (April et al. 2004). As these clays weather, they release potassium and magnesium to the soil system. The swelling clays vermiculite and smectite also provide exchange sites for storing nutrient cations and interlayer sites that accommodate water molecules.

ACID RAIN AND SOIL ACIDIFICATION

A century of acid deposition has resulted in changes to soil, soil water, and surface water chemistry in the Adirondack Mountains. Although deposition of both sulfuric and nitric acids has decreased substantially over the past three decades as the Clean Air Act Amendments of 1990 and the Acid Rain Program (1995-2010) prescribed, even with implementation of more recent programs to ameliorate air pollution (e.g., the NOx Budget




Trading Program [2003-2009]; the Clean Air Interstate Rule [2010-2015]; and the Cross-State Air Pollution Rule [2015]), lingering effects of acid rain remain in lake and forest ecosystems (EPA 2013). Acidified lakes and streams, both chronic and episodic, continue to persist. However, there are indications that some lakes and streams are showing signs of recovery, with pH and acid neutralizing capacity (ANC) values drifting slightly higher (Driscoll et al. 2007; Waller et al. 2012; Strock et al. 2014). In Adirondack soils, levels of aluminum and soil acidity have increased over time (Warby et al. 2009). Concomitantly, depletion of exchangeable base cations, especially calcium, which is so critical for plant growth, also has occurred, and base saturation, in general, has declined (Bedison and Johnson 2010). The depletion of calcium from soil may delay the recovery of ANC values in acidified surface waters (e.g., Warby et al. 2005).

Whether the rate of chemical weathering of primary minerals in soil is enough to resupply nutrient base cations to exchange sites has yet to be determined. It may take decades, perhaps many, before mineral weathering reverses the effects of soil acidification that has occurred over most of the past century. To better demonstrate how decades of acid deposition has affected soils in the Adirondacks, briefly presented below are two case studies—one that investigated changes in soil chemistry over time and a second that monitored the effects of liming on soil chemistry.

Figure 4: Location of the 21 watersheds from which samples were collected in 1979, 1982-1983, and 2005-2006 for the “Cation Depletion” case study and the location of the “Town of Webb” sites where the “Soil Liming” case study was conducted.



CASE STUDY 1: CATION DEPLETION IN ADIRONDACK SOILS

Because Spodosols naturally have low base saturation, leaching of base cations by acidic inputs exacerbates an already fragile balance between nutrient supply for vegetation and organisms that live on or within the forest floor, and replenishment of these nutrient cations by litter decay and mineral weathering (atmospheric inputs) are minimal. In 2005, a study to determine changes in Adirondack soil chemistry over a period of approximately two decades was undertaken by comparing the chemical characteristics of newly collected soil samples with those of archived soil samples collected from the same lake-watershed sites (Figure 4) approximately two decades earlier (1979 and 1982-1983) (April and Newton 1983; Newton et al. 1987; Goldstein et al. 1987). Particular attention was paid to determining differences in exchangeable calcium and percent base saturation, both of which were reported in previous studies to show significant declines over time in soils of forested ecosystems in the northeastern U.S. (Lawrence et al. 1997; Sullivan et al. 2006).

Exchangeable Base Cations

A significant decrease of exchangeable Ca in the uppermost horizons of Adirondack soil profiles was observed over the period of study (Table 3 and Figure 5). Rondaxe watershed shows the highest average percent decrease in Ca in Oa-horizons, from a mean of 8.6 cmolc kg -1 to 0.6 cmolc kg -1, a drop of 8.0 cmolc kg -1 or almost 93%. Smaller, yet significant, decreases in exchangeable Ca are seen in the Oa-horizons of all ten other sites as well. Except for Russian and Sagamore watersheds, exchangeable Mg also shows declines in mean values for Oa-horizons over the period of study (Table 3). Exchangeable K and Na show no definitive trends over time because concentrations of these cations are quite low to begin with (K < 1.0 cmolc kg-1 and Na < 0.1 cmolc kg -1) (Table 3).

Table 3: Changes in exchangeable base cations Ca, Mg, K, and Na over two decades in Oa-horizons from eleven representative sites across the Adirondacks.



Figure 6: Average base saturation by horizon for archived and recent samples collected from the 21 study sites. Base saturation decreased over time due to cation depletion.

Figure 5: Average change in exchangeable calcium over two decades at eleven representative sites across the Adirondacks. The greatest depletion of calcium occurs in the Oa-horizon at all sites.


Base Saturation and Exchangeable Acidity

Percent base saturation decreased in all horizons of Adirondack soils over the period of study, with the greatest change occurring in the uppermost soil horizons (Figure 6). Decreases in base saturation are caused by base cations being leached from exchange sites and replaced with H+ from strong acids (acid rain) and with monomeric aluminum (Al3+) derived from mineral weathering. One effect of soil acidification has been to increase the mobility of monomeric aluminum, a potential toxin linked to fish mortality and forest decline (Godbold 1988; Cronan and Schofield 1990; Cronan and Grigal 1995).

Overall, the study showed calcium depletion, and, to a lesser extent, magnesium depletion in the uppermost soil horizons of the Adirondack Mountains over a period of approximately two and a half decades. These changes to soil chemistry are most noticeable in Oa-horizons but can reach depths of up to 25 cm. Results of this study are consistent with other studies of soil chemical changes in the northeastern U.S. Warby et al. (2009) found that organic soils (Oa-horizons) across the northeastern U.S. continued to acidify even though significant decreases in sulfate (SO4

2-) and hydrogen ion (H+) deposition has occurred across the region (Driscoll et al. 2003; Kahl et al. 2004). Sullivan et al. (2006, 2007) found that acidic deposition depleted soil-base cation pools and that, despite CAAA mandated cuts in air pollution, continued leaching of cations may restrict the extent to which acidified lakes and streams recover in the future. Loss of soil calcium could hinder the recovery of the acid neutralizing capacity (ANC) of surface waters, especially for those lakes and streams that have remained chronically acidic and are located in watersheds underlain by thin till and granitic bedrock, and where surface water ANC values are less than 25 µequiv L-1 (Warby et al. 2005). Finally, calcium depletion may negatively impact forest health and productivity for years to come (Lawrence et al. 1997; Shortle et al. 1997; Bailey et al. 2004; Schaberg et al. 2006; Sullivan et al. 2013). There is still a lot to be learned about the long-term effects of low base saturation and increasing exchangeable acidity on this fragile Adirondack forest soil and the biotic community it supports.

CASE STUDY 2: SOIL LIMING

As another aspect of the 2005 study, a total of 1.6 tonnes of limestone powder (98.5% CaCO3) was applied, in two equal applications (half in 2005 and half in 2006), to each of four forested sites in the southwestern Adirondacks near Old Forge, NY (Town of Webb). Each site was divided into two circular plots (22.5 m radius), a control plot that was not limed and an experimental plot treated with the calcium carbonate. One of the objectives of this liming study was to monitor whether the calcium released by the dissolution of the lime over time replaced exchangeable soil calcium that had been depleted by decades of acid rain. Prior to liming, soil samples from pits excavated in both the control and experimental plots at the four sites were collected and analyzed for pH, base cations, and base saturation.




pH and Soil Acidity: Ten years later

The soil pH of both the limed and control sites ten years later are shown in Figure 7. Below 20 cm, soil pH values for both the limed and unlimed control plots are about the same, averaging approximately 4.2. Above 20 cm, soil pH values steadily decline to an average of 3.5 in the unlimed control plot, a value typical for the acidic O- and E-horizons of soil in the Adirondacks, but soil pH values rise to an average of 6.3 in the limed plot, responding to the neutralizing capacity of the dissolved calcium carbonate.

Exchangeable Calcium and Base Saturation: Ten years later

The amount of exchangeable calcium is higher by more than ten-fold near the soil surface in the limed sites compared to the unlimed control sites and remains consistently higher down to a depth of 35 cm (Figure 8). A simple calculation reveals that over the 10-year period following liming, calcium translocated down profile at a rate of ~3.5 cm/yr, to a depth well beyond the rooting zone of most vegetation. In addition, exchangeable magnesium values also are higher in the upper 10 cm of the limed sites, which is not surprising as chemical analysis of the powdered limestone revealed that it contains about 1.0 wt% MgCO3 (Figure 8). Exchangeable potassium and sodium show similar values for both the limed and unlimed plots, indicating that the lime had no effect on the concentration of these base cations. Base saturation is 20% to 40% higher in the limed sites throughout the soil profile and reaches 100% in the upper 10 cm of the amended soils. In addition, base saturation in the limed soils remains above 50% down to a depth of ~30 cm compared to a depth of only ~15 cm in the unlimed plots (Figure 8). In a companion study of the biota, McCay et al. (2013) found that invertebrate populations differ markedly in limed versus unlimed plots, with snails increasing in abundance and millipedes and spiders decreasing in abundance in the limed plots. The decrease in millipedes, especially, may have contributed to the reduction in the rate of litter decay in limed plots.

Results of this study suggest that amending soil with lime can modify soil chemistry rather quickly and dramatically by raising soil pH to near neutral values, by replacing exchangeable calcium and magnesium depleted by soil acidification, and by increasing base saturation. Watershed and lake liming are expensive undertakings, however, but aside from providing benefits that counteract the effects of acid deposition, liming has been linked to some detrimental effects on biota that inhabit these ecosystems and normally thrive in calcium-poor and less alkaline environments. (Smallidge 1993; Moore 2014).



Figure 8: Average exchangeable calcium, magnesium and base saturation in limed and unlimed plots after 10 years.

Figure 7: Average soil pHw in limed and unlimed plots after 10 years.



SUMMARY

How long it will take for the acidified systems of the Adirondacks to recover from a century of acid deposition? This is a question no one can answer with certainty—not yet, anyway. Once such a fragile and complex ecosystem as the Adirondacks is altered by anthropogenic activities, it is difficult to project what will become of it, for nothing like this has ever happened before. When will acidified lakes return to their historical pH and ANC values? Will fish species decimated by aluminum toxicity reestablish old habitats, and if so, when? How long will it take for mineral weathering in soils to replenish depleted nutrient base cations necessary for the maintenance of a healthy forest ecosystem? When will the populations of terrestrial biota affected by changes in soil chemistry and changes in the proportions of species at different points in the food chain recover to historical norms? Unfortunately, we do not even know precisely what the “historical norm” was, because the natural state of the Adirondack ecosystem prior to the early part of the 20th century, before the onset of acid deposition, is largely unknown.

Fortunately, scientists are monitoring the health of the Adirondack forests and lakes and there are some promising signs. Lawrence et al. (2015) found that the acidification of forest soils is finally beginning to show a reversal, and Strock et al. (2014) measured a recent acceleration in the rate of recovery of lakes as acidic deposition in the northeastern U.S. declines. Josephson et al., (2014) measured a slight, but definite, increase in phytoplankton abundance in Honnedaga Lake and also noted the persistence of the brook trout population, but they state that further recovery will likely be slow. Trout have begun a resurgence in other lakes in the Adirondacks (Mitchell 2014) as well, as have chrysophyte and diatom species in Big Moose Lake (Arseneau et al. 2011). While this is all good news, a recent study by Bishop et al. (2015) alerts us to the worry that sugar maple (Acer saccharum) is showing symptoms of stress and elevated mortality rates across the Adirondacks – all the more reason for scientists to remain vigilant and to continue monitoring the health of the Adirondack ecosystem over time. Let us hope that the recovery continues and that the deleterious effects of almost one hundred years of acid deposition over the Adirondack Mountains vanish over our lifetime.

ACKNOWLEDGEMENTS

Research was funded by the National Science Foundation (DBI-0442222 and EAR 0725019). Thanks to the Town of Webb for permission to conduct studies on town land and to students Ashlynne Rando, Lauren Frisch, and others who assisted with sample collection and laboratory analyses over the years. A.R. and L.F. conducted some of the case study work presented as part of their senior thesis. Thanks to J. Chiarenzelli, who reviewed our manuscript.


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Sullivan, T.J., B.J. Cosby, A.T. Herlihy, C.T. Driscoll, I.J. Fernandez, T.C. McDonnell, C.W. Boylen, S.A. Nierzwicki-Bauer, and K.U. Snyder. 2007. “Assessment of the extent to which intensively-studied lakes are representative of the Adirondack region and response to future changes in acidic deposition,” Water, Air, and Soil Pollution, 185: 279-291.

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soils and soil acidification in the adirondack mountains

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