1

SOIL-CALCIUM DEPLETION LINKED TO

ACID RAIN AND FOREST GROWTH IN

THE EASTERN UNITED STATES

Figure 1. Calcium cycle in forest ecosystems. Inputs to the pool of available calcium in the soil result from weathering

of rocks, atmospheric deposition, and litterfall. Outputs from this pool result from root uptake and leaching out of the soil.

Prepared in Cooperation with The U.S. Department of Agriculture—Forest Service and

New York City Department of Environmental Protection

Root Uptake Forest

Floor

Adsorption to Surfaces

Desorption

from Surfaces

Calcium in Rocks

Weathering

Mineral

Soil

Calcium

in

Soil Water

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca Ca

Exchangeable

Calcium

Ca

Ca

Ca

Ca

Ca

Ca Ca

Rain and Dust

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca

U.S. Department of the Interior WRIR 98-4267

U.S. Geological Survey February 1999

Since the discovery of acid rain in the

1970’s, scientists have been concerned that

deposition of acids could cause depletion of

calcium in forest soils. Research in the 1980’s

showed that the amount of calcium in forest

soils is controlled by several factors that are

difficult to measure. Further research in the

1990’s, including several studies by the U.S.

Geological Survey, has shown that (1) calcium

in forest soils has decreased at locations in the

northeastern and southeastern U.S., and (2)

acid rain and forest growth (uptake of calcium

from the soil by roots) are both factors

contributing to calcium depletion.

CALCIUM CYCLING IN FORESTS

Calcium is an essential nutrient for tree growth

that is used in the formation of wood and in the

maintenance of cell walls, the primary structure

of plant tissue. Trees obtain calcium from the soil

(fig. 1), but to be taken up by roots, the calcium

(a positively charged ion) must be dissolved in soil
2

water. Calcium adsorbed to negatively charged

surfaces of soil particles (termed exchangeable

calcium) is also available to roots because both

adsorption and desorption of calcium occurs readily

through the process of chemical equilibrium. Some

calcium is deposited onto forests in an available

form in dust and precipitation or is returned to the

soil through decomposing leaves and branches that

form the forest floor. Most of the calcium in soil,

however, is bound within the mineral structure of

rocks within the underlying mineral soil, which

prevents it from dissolving in water or becoming

Continued research in the 1990’s has documented

distinct decreases in soil calcium over the past 4 to 5

decades, in both the Northeast (Johnson and others,

1994a) and the Southeast (Richter and others, 1994).

These decreases were attributed primarily to the

uptake of calcium by trees in excess of inputs from

weathering. Follow-up studies have suggested,

however, that both acid deposition (Markewitz and

others, 1998) and a decline in deposition of calcium

from the atmosphere, a trend that has been underway

adsorbed to particle surfaces. Weathering, the

physical and chemical breakdown of rocks,

gradually releases calcium to soil water where it can

(1) be taken up by roots, (2) adsorb to particle

surfaces, or (3) be leached out of the soil by

negatively charged ions in percolating water.

As calcium is released by weathering, acidity in the

form of hydrogen ions is neutralized, which

increases the pH of soil water and the surface waters

into which it flows. Calcium leached out of the soil

into surface waters also serves as an essential

nutrient for aquatic plants and animals.

CALCIUM RESEARCH IN THE 1970’S AND 80’S

increased leaching caused by acid rain (Johnson

and others, 1982).

In the Southeast, the amount of calcium in soils

was found to be generally less than in the

Northeast, but was thought to be somewhat

protected from cation leaching by the high

capacity of these soils to adsorb sulfate, a

negatively charged ion. The fine texture of soils in

the Southeast enhances sulfate adsorption, which

removes the negative charge from soil water that is

necessary to leach the positively charged cations.

Considerable uncertainty as to the status of soil

calcium remained, through the 1980’s, however,

because the processes involved in weathering and

leaching were found to be extremely complex. At

the close of the research phase of the National

Acid Precipitation Assessment Program (NAPAP)

in 1990, decreases in the availability of calcium in

forest soils from either acid rain or

forest growth had not been confirmed, although

the possibility of future depletion from acid

deposition was acknowledged (NAPAP, 1993).

CALCIUM RESEARCH IN THE 1990’S

The discovery of acid rain (acids deposited from

the atmosphere in rain, snow, particles and gases,

more accurately referred to as acid deposition) in the

1970’s prompted concern that deposition of acids

could increase leaching of calcium, and other

neutralizing cations, such as magnesium, sodium,

and potassium (referred to as base cations) from

forest soils. Acid deposition provides (1) hydrogen

ions, which displace cations adsorbed to soil

surfaces, and (2) sulfate and nitrate ions, which tend

to keep these cations dissolved in soil water that

eventually drains into streams and lakes. Results of

preliminary research in the 1970’s suggested that

elevated leaching might deplete soil calcium,

decrease forest growth, and acidify surface-waters

(Cowling and Dochinger, 1980). These concerns

were somewhat alleviated in the Northeast by the

discovery that the soil contained large amounts of

calcium. Although most of the soil calcium was

known to be in unavailable forms in rocks and

minerals, the rate at which calcium was released by

weathering was considered to be sufficient to offset
3

since the 1970’s, may also have contributed to the

decrease in the availability of soil calcium in the

East (Johnson and others, 1994b).

Investigations of forest health have identified

relations between low calcium availability and a

reduction in stress tolerance of red spruce (Shortle

and Smith, 1988; DeHayes, 1992; Schlegel and

others, 1992), and sugar maple (Wilmot and others,

Ongoing research by the U.S. Geological Survey, in collaboration with the U.S. Forest Service, the University of

Illinois, the Institute of Ecosystem Studies and Syracuse University, has provided new information on the causes

and magnitude of calcium depletion in soils of the eastern United States. Results of this research have been

detailed in four articles prepared for scientific journals. A general summary of each of these articles and a

description of current USGS calcium research follows.

Aluminum released to water in the mineral soil by acid rain decreases the availability of calcium in the

overlying forest floor, the primary layer for nutrient uptake by roots in forest soils.

Lawrence, G.B., David, M.B., Shortle, W.C., 1995, A new mechanism for calcium loss in forest-floor soils:

Nature, v. 378, p. 162-164.

1995; Long and others, 1997). Related studies have

also suggested that forest harvesting could reduce

calcium availability through the removal of calcium

stored in trees, which could lower the growth rates of

the regenerating stand (Federer and others, 1989;

Hornbeck and others, 1990). Relationships among

acid deposition, calcium availability, and forest

productivity remain uncertain, however.

by capillary movement. Because aluminum has a

much higher affinity for negatively charged surfaces

than calcium, introduction of aluminum into the

forest floor tends to displace adsorbed calcium,

which causes it to be more readily leached. A

continued buildup of aluminum in the Oa horizon

can (1) decrease the availability of calcium for roots

(Lawrence and others, 1995), (2) lower the

efficiency of calcium uptake because aluminum is

more readily taken up than calcium when the ratio of

calcium to aluminum in soil water is less than 1

(Cronan and Grigal, 1995), and (3) be toxic to roots

at high concentrations (Anderson, 1988).

The Oa horizon has generally been considered

immune from acidification by acid deposition

because the typical pH of soil water in this horizon is

less than 3.8 whereas the pH of rain in the East

typically ranges between 4.0 and 4.5 (lower pH

indicates greater acidity). The low pH of soil water

in the Oa horizon is caused by organic acids that

form naturally through the decomposition of plant

matter. Organic acids that move downward in

percolating water from the Oa horizon into mineral

Decreases in concentrations of exchangeable

calcium are generally attributed to displacement by

hydrogen ions, which can originate from either acid

deposition or uptake of cations by roots (Johnson

and others, 1994a, Richter and others, 1994). A

regional survey of soils in northeastern red spruce

forests in 1992-93 (fig. 2) has revealed that

decreases in exchangeable calcium concentrations

in the Oa horizon (a layer within the forest floor,

where uptake of nutrients is greatest) can also result

from increased concentrations of exchangeable

aluminum, which originated in the underlying

mineral soil (Lawrence and others, 1995). By

lowering the pH of the aluminum-rich mineral soil,

acid deposition can increase aluminum

concentrations in soil water through dissolution and

ion-exchange processes. Once in solution, the

aluminum (although not a nutrient) is taken up by

roots and transported through the trees to be

eventually deposited on the forest floor in leaves

and branches. Dissolved aluminum in the mineral

soil can also be transported into the forest floor by a

rising water table, or during unsaturated conditions,

1. DISCOVERY OF A NEW MECHANISM OF CALCIUM DEPLETION IN FOREST SOILS
4

Figure 2. Locations at which soil-calcium assessments

have been made in the papers by Lawrence and others,

(1995,1997) and Huntington and others (USGS approved).

horizons, which contain little organic

matter, tend to be removed from solution

through further decomposition or by

adsorption to mineral surfaces. Once

removed from solution, they are incapable

of acidifying the soil, which results in a

higher pH in the mineral soil than in the

forest floor. Unlike these natural organic

acids, sulfuric and nitric acids from acid

deposition neither decompose (because

they are inorganic) nor adsorb to soil

particles readily; rather, they tend to remain

in solution within the mineral soil where

they lower the pH, thereby releasing

aluminum to soil water and causing the

leaching of calcium.

The effects of these processes can be

seen through a comparison of soils that

have been acidified to varying degrees

(table 1). The ratio of exchangeable

aluminum to exchangeable calcium in the

Oa horizon was lowest at Groton, Vt, where

the pH of water in the mineral soil was

highest, and the pH of soil water in the Oa

horizon was lowest. At Big Moose Lake,

N.Y. and Mt. Abraham, Vt. the pH of soil

water in the B horizon was considerably

lower than at the Groton site and the ratios

of exchangeable aluminum to exchangeable

calcium in the Oa horizons were greater

than 1.0, the threshold above which

aluminum can interfere with uptake of

calcium by roots.

Table 1. pH and ratio of exchangeable aluminum to 

exchangeable calcium (Al:Ca) in soil water from forest floor 

(Oa horizon) and mineral soil (upper 10 centimeters of B 

horizon) in three red spruce stands of the northeastern 

United States. 

* ratio expressed as centimoles of charge per kilogram Aluminum 

per centimole of charge per kilogram Calcium

Forest floor Mineral soil

Site location pH Al:Ca* pH Al:Ca*

Groton, Vt. 3.3 0.3 4.8 5

Big Moose Lake, N.Y. 3.5 1.2 4.1 27

Mt. Abraham, Vt. 3.7 5.6 4.1 16

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1

2

3

4

5

7 6

8

9

10

11

12

13

17. Shenandoah National Park

18. Duke Forest, N.C.

19. Panola Mountain, Ga.

20. Coweeta Hydrologic Lab, N.C.

21. Stewart Co., Ga.

22. Clemson, S.C.

23. Gainesville, Fl.

18

15 14

20 22 16

19

21

23

1. Kossuth, Me.

2. Bear Brook, Me.

3. Howland, Me.

4. Crawford Notch, N.H.

5. Bartlett, N.H.

6. Cone Pond, N.H.

7. Hubbard Brook, N.H.

8. Sleepers River, Vt.

9. Groton, Vt.

10. Mt. Abraham, Vt.

11. Whiteface Mountain, N.Y.

12. Big Moose Lake, N.Y.

13. Neversink River, N.Y.

14. Great Smoky Mts. Nat. Park

15. Oak Ridge, Tn.

16. Calhoun Forest, S.C.

17
5

Decreases in available calcium in forest soils of

the Northeast have been indicated in several studies

in the 1990’s (Shortle and Bondietti, 1992; Bailey

and others, 1996; Johnson and others, 1994a;b), but

the results of these studies could not be directly

related to each other because the measurements of

soil-calcium concentrations were made with

different methods of unknown comparability. To

address this problem, soil samples collected by

Lawrence and others (1995) in the survey of red

spruce forests (fig. 2) were analyzed by the four

most common methods currently used for measuring

exchangeable calcium (Lawrence and others, 1997).

A comparison was also done between the most

common current method for determining acidextractable calcium and the method used by

Heimburger (1934), and also by Johnson and others

(1994a) in resampling the plots first sampled by

Heimburger 50 years earlier (Lawrence and others,

1997). The soil survey of Lawrence and others

(1995, 1997) in red spruce forests entailed the

selection of 12 sites that together encompassed the

range of environmental conditions represented by

red spruce stands in the Northeast.

Considerable variability in exchangeable-calcium

concentrations was observed among the 12 sites;

concentrations ranged from 2.1 to 22.0 centimoles of

charge per kilogram in the Oa horizon, and from

0.11 to 0.68 centimoles of charge per kilogram in the

upper mineral soil. The primary cause of this

variability was mineralogical differences in parent

material (rock that weathers to form soil), but results

also suggested that acidic deposition had decreased

concentrations of exchangeable calcium in the

mineral soil at these sites (Lawrence and others, 1997).

Results of the methods-comparison tests were

used to relate the long-term decrease documented in

the study by Johnson and others (1994a) to the range

of calcium variability measured in the study by

Lawrence and others (1997). The average value

determined by Johnson and others (1994a) for 38 red

A regional assessment of forest-soil chemistry supports evidence of soil-calcium depletion in the Northeast;

acid rain identified as a probable factor.

Lawrence, G.,B., David, M.B., Bailey, S.W., Shortle, W.C.,1997, Assessment of calcium status in soils of red

spruce forests in the northeastern United States: Biogeochemistry, v. 38, p.19-39.

spruce stands in the Adirondack Mountains of New

York, was near the middle of the range obtained by

Lawrence and others (1997), and in the upper range

of values obtained in a study by McNulty and others

(1991). The average value obtained for Adirondack

sites measured by Heimburger (1934) 50 years

earlier was in the upper part of the range measured

by Lawrence and others (1997). Although

considerable variability was caused by local

differences in environmental factors such as

mineralogy of parent material, and acid deposition

rates, the results support the probability indicated by

the combined studies of Johnson and others

(1994a;b), Shortle and Bondietti (1992), Lawrence

and others (1995), and Bailey and others (1996), that

concentrations of available calcium in northeastern

red spruce forests have decreased in the second half

of this century.

ppp pppppppp

p

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£


£


£


£


Acid-Extractable Calcium, in millimoles per kilogram 

0 10 20 30 40 50 60 70 80 90 100 110

1992–93

1987–88

1984 1930–32

EXPLANATION

Mean concentrations of calcium in soils of individual

 sites in New York, Vermont, New Hampshire and Maine

 sampled by Lawrence and others (1997) in 1992–93.

Mean concentrations of calcium in soils of individual

 sites in New York, Vermont, New Hampshire and Maine

 sampled by McNulty and others (1991) in 1987–88.

Mean concentrations of calcium in soil of 38 samples

 collected by Heimburger (1934) throughout the

 Adirondack Mountains of New York in 1930–32.

Mean concentrations of calcium in soil of 59 samples

 collected by Johnson and others (1994a) throughout

 the Adirondack Mountains of New York in 1984.

Figure 3. Acid-extractable calcium in Oa horizons of red

spruce stands in the studies of Lawrence and others

(1997), McNulty and others (1991), Heimburger (1934),

and Johnson and others (1994a).

2. STATUS OF SOIL CALCIUM IN THE OA HORIZON OF RED SPRUCE STANDS OF

THE NORTHEAST
6

Relations among acid deposition rates, base-cation concentrations in soils, and stream-water acidification

indicate that changes in soil-calcium status can affect long-term trends in stream chemistry.

Lawrence, G.B., David, M.B., Lovett, G.M., and others, In Press, Soil-calcium status and the response of stream

chemistry to changing acidic deposition rates in the Catskill Mountains of New York: Ecological Applications.

3. DEPLETION OF SOIL CALCIUM AFFECTS THE RESPONSE OF STREAM CHEMISTRY

TO DECLINING RATES OF ACID DEPOSITION IN THE NORTHEAST

Rates of acid deposition have been declining in

the Northeast since the 1970’s, but many acidified

surface waters in the region have unexpectedly

shown little or no corresponding increase in acidneutralizing capacity, an integrated measure of the

degree to which a water body is acidified. Longterm records of stream chemistry at the Hubbard

Brook Experimental Forest in New Hampshire (fig.

2) showed that the ratio of base cations (calcium,

magnesium, sodium, and potassium) to acid anions

(sulfate, nitrate and chloride) decreased steadily

through the 1970’s, then remained fairly stable

through the 1980’s and early 1990’s (Likens and

others, 1996). This response was interpreted as an

indication that past declines in soil concentrations of

calcium and other base cations were impeding

recovery of acidified stream-water at this site.

Additional information on the relations between

soil-calcium depletion and the status of acidified

surface waters was obtained in a recent USGS study

by Lawrence and others (in press) in the Neversink

River Basin in the Catskill Mountains of New York

(fig. 2). Results showed that rates of atmospheric

deposition of sulfuric acid increased from the lowest

elevations to the highest elevations of a subbasin,

whereas the concentrations of exchangeable-base

cations in natural soil decreased from lowest to

highest elevations (fig. 4 a, b).

Relations between the elevational trends in

atmospheric deposition rates and soil chemistry were

further investigated through a procedure in which a

large quantity of soil was collected at a low

elevation in the basin, where exchangeable basecation concentrations were somewhat higher than in

the subbasin being studied. This soil was thoroughly

mixed, placed in mesh bags, then buried in the soil

in stands of mixed northern hardwoods at varying

elevations within the subbasin. Analysis of the

samples after 1 year, confirmed a distinct upslope

decrease in exchangeable base cation concentrations

750 850 950 1050 1150 1250

ELEVATION ABOVE SEA LEVEL, IN METERS

0

0

0.1

0.2

0.3

0.4

0.5

0.6 Initial concentration

0.7

0.5

1

2

1.5

150

200

250

300

350

400

500

450

A. Atmospheric deposition

B. Mineral soil

C. Soil bags collected after 1 year

D. Stream water

EXCHANGEABLEBASE-CATION

CONCENTRATION,

IN CENTIMOLES

OF CHARGE PER

KILOGRAM OF SOIL

EXCHANGEABLEBASE-CATION

CONCENTRATION,

IN CENTIMOLES

OF CHARGE PER

KILOGRAM OF SOIL

RATIO OF BASECATION (Ca+Mg+

Na+K) EQUIVALENTS TO ACIDANION (SO4+NO3

+Cl) EQUIVALENTS

ANNUAL MEAN

ATMOSPHERIC

SULFATE

DEPOSITION,

IN MOLES

PER HECTARE

0.25

0.3

0.4

0.35

P < 0.01

P = 0.08

P < 0.01

P < 0.01

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Figure 4. Elevational gradients for selected chemical

constituents within a subbasin of the Neversink River, N.Y.

A. Annual mean atmospheric sulfate deposition, June

1993 through May 1995. B. Exchangeable-base cationconcentrations in mineral soil samples collected in 1994.

C. Exchangeable-base-cation concentrations in initially

uniform mineral soil samples that were placed in mesh

bags and leached within the mineral-soil profile from

June 1994 through May 1995. D. Ratio of base-cation

concentrations to acid-anion concentrations from the

samples collected on dates when the highest 20% of

stream flows occurred.
7

(fig. 4c), and in base saturation (the base-cation

concentration as a percentage of total cation

exchange capacity). Detailed analysis of other

factors that could cause an elevational gradient in

soil acidity and leaching of base cations indicated

that the effects of these factors were either minimal

or undetectable. These results led to the conclusion

that the upslope increase in the rate of atmospheric

deposition was the most probable cause of the

upslope decrease in base-cation concentrations (and

base saturation) in the soil.

The elevational trends in atmospheric deposition

and soil chemistry were also reflected in an

elevational trend in stream chemistry; the ratio of

base cations to acid anions in stream water (an index

of acidity) decreased with

increasing elevation, which

indicates an upslope increase in

stream-water acidity (fig. 4d).

A followup laboratory soil

experiment demonstrated that

this ratio in soil water was

directly related to soil-base

saturation. The upslope increase

in atmospheric deposition can

therefore be linked to the

upslope increase in streamwater acidification through

the elevational trend in soilbase saturation.

Data from detailed site studies and regional surveys suggest that soil-calcium depletion is common in forests

of the Southeast and could diminish long-term forest productivity.

Huntington, T.G., Hooper, R.P., Johnson, C.E., Aulenbach, B.T., and Cappellato, A., in review, Calcium depletion

in forest ecosystems of the southeastern United States: Submitted for publication as an article in a technical

journal (USGS Approved).

The effect of the upslope increase in atmospheric

deposition on soil chemistry and stream chemistry

may be analogous to the effect of past increases in

acid deposition rates on temporal trends of soil and

stream chemistry, and supports the explanation put

forth by Likens and others (1996) that the observed

minimal response of stream chemistry to declining

acid deposition rates in New Hampshire was due to

depletion of base cations in soil. Long-term trends

in concentrations of calcium and acid-neutralizing

capacity have both shown significant decreases

(indicating increases in acidity) for 1984-97 in the

Neversink Basin (fig. 5) despite a consistent

decreasing trend in rates of atmospheric deposition

during this period.

Accumulating evidence indicates possible

depletion of soil calcium in forests of the

Southeast, as well as the Northeast. Long-term

studies at several southeastern forest sites have

documented declines in the exchangeable-calcium

concentration of soils. In these studies, calcium

inputs from weathering and atmospheric deposition

were found to be insufficient to replenish the losses

that occur through uptake by vegetation and

leaching (Johnson and others, 1988; Johnson and

others, 1991; Knoepp and Swank, 1994; Richter

and others, 1994).

4. CALCIUM DEPLETION IN FOREST ECOSYSTEMS OF THE SOUTHEAST

-50

-40

-30

-20

-10

0

10

20

30

40

50

83 85 87 89 91 93 95 97 84 86 88 90 92 94 96

(P< 0.05)

Ca2+

ANC

(P < 0.01)

YEAR

CONCENTRATION, IN MICROEQUIVALENTS PER LITER

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.

Figure 5. Calcium concentrations and acid-neutralizing capacity (ANC) of

stream water in a subbasin of the Neversink Basin, 1984-1997.
8

Several factors suggest that the available calcium

in southeastern soils is prone to depletion. These

soils have weathered in place for a few hundred

thousand to 2 million years and many of them have

little weatherable calcium left in soil particles and

rocks. Also, poor farming practices caused

extensive loss of soil and nutrients on lands that are

now used for commercial forestry (Nelson, 1957;

Brender, 1974; Trimble, 1974). Forest growth

places a continued demand on the remaining

calcium in these soils, and when the stands are

harvested, the calcium in the trees is removed from

the ecosystem. Acidic deposition during the 20th

century has placed further strain on soil calcium

availability by enhancing calcium leaching rates at a

time when the rates of atmospheric deposition of

calcium have declined (Lynch and others, 1996).

The potential for calcium depletion in southern

forest soils, was evaluated in detail through the

development of a calcium budget for Panola Mountain

watershed, in the Piedmont region near Atlanta, Ga.

(Huntington and others, USGS approved); this budget

was then compared with data from other watershed

studies and soil surveys to assess the extent of calcium

depletion within the Southeast. Calcium-budget

calculations for Panola Mountain watershed (averaged

over the roughly 90 year life of the current forest

stand), indicate that the rate of calcium leaching from

the soil approximates the rate of calcium input from

atmospheric deposition. Also, chemical and

mineralogical analyses of soil and saprolite (the highly

weathered layer of residual rock beneath the soil)

indicated that only trace amounts of weatherable

mineral calcium remain, which suggests that inputs

from weathering are negligible. The annual rate of

calcium depletion in this watershed is, therefore,

similar to the annual rate of uptake by trees of 10.7

kilograms per hectare. Continuous depletion at this

rate for 150 years would reduce the total reserves of

calcium in the soil to the approximate amount required

by a hardwood stand to reach a marketable size.

Budget calculations for other forested study sites

in the Southeast showed a wide variation in

sensitivity to soil-calcium depletion (table 2). The

most sensitive forests are dominated by hardwood

species, which have a higher calcium demand than

softwoods; at some sites more calcium is contained

by the trees than by the soil. Stands that are the

least sensitive tend to be dominated by softwood

and are in areas that receive the highest amounts of

calcium from atmospheric deposition. The rate of

calcium depletion estimated for Panola Mountain

0 1000

0

20

40

60

80

100

20

40

60

80

100

2000 3000 4000 5000 6000

Southern Blue

Ridge Province 

(n = 99)

Northeast

(n = 280)

A. SOIL

EXCHANGEABLE SOIL CALCIUM,

IN KILOGRAMS PER HECTARE

CUMULATIVE PERCENTAGE OF SITES

Southern Blue Ridge Province

Southern Appalachians

Piedmont

0 250 500 750

0

1000 1250 1500

CALCIUM CONCENTRATION,

IN MICROEQUIVALENTS PER LITER

CUMULATIVE PERCENTAGE OF SITES

B. STREAM WATER

watershed was in the middle of the range of values

reported for other forested sites in the Southeast.

Lower availability of calcium in soils of the

Southeast than the Northeast is suggested by surveys

of exchangeable-calcium concentrations in soils.

About 70 percent of the sites sampled in the

Southern Blue Ridge region through the U.S. EPA

Direct-Delayed Response Program (Church and

others, 1989) had concentrations of exchangeable

calcium less than 1000 kilograms per hectare,

whereas in the Northeast, about 50 percent of the

Figure 6. Cumulative percentage of sites as a function of

(A) exchangeable calcium concentrations in soil and (B)

calcium concentrations in stream water in selected

regions of the eastern United States. Figure A adapted

from Turner and others (1993); Figure B adapted from

Sale and others (1988).
9

sites sampled had calcium concentrations less than

1000 kilograms per hectare (fig. 6a). Calcium

concentrations in southeastern streams also tended to

be distributed toward low values, as indicated by the

steeply ascending lines in fig. 6b.

The assessments of the calcium budgets at Panola

Mountain watershed and other forested sites in the

Southeast, in conjunction with the surveys of soil

and stream chemistry, suggest that (1) depletion of

calcium in soils is common, (2) the growth of some

forests may be limited by insufficient amounts of

calcium in soils, and (3) calcium availability will be

lowered by forest harvesting of sites currently low

in calcium.

Table 2. Calcium (Ca) contents of soil, calcium fluxes, net calcium depletion, and sensitivity to calcium depletion at 

selected forest sites in the eastern United States.

[kg/ha, kilograms per hectare; (kg/ha)/yr, kilograms per hectare per year; ND, no data)

a Value indicates number of years until current rates of depletion reduce exchangeable stores to twice the expected requirement

for a marketable forest stand

b References: 

1. Johnson and Lindberg, 1992; 2. Johnson and Todd, 1990; 3. Richter and others, 1994. 

4. Written commun. from J.R. Webb (University of Virginia), June 1995.

5. Huntington and others (USGS approved0; 6. Huntington, 1996b; 7. Johnson and others, 1988.

c

 Soils already depleted to less than twice the calcium requirement for marketable timber

d Estimated from typical forest growth rates in this area, on basis of USDA Forest Service—Forest Inventory Analysis data and calcium 

concentration for oak (Quercus spp.)

e Either soils are accumulating calcium, or output equals input

Type of 

forest

Calcium content 

of soil (kg/ha)

Calcium flux

[(kg/ha)/yr]

Net 

Calcium 

depletion

Ecosystem 

sensitivity

to Calcium 

depletiona

Source 

of 

datab Site location

Exchangeable Total 

Net 

uptake by 

trees

Total 

deposition

Leaching from

soil

A. Sites that are highly sensitive to calcium depletion

Great Smoky 

Mountains Nat. Park

Beech 57 2,500 15.9 7.5 6.3 14.7 highc 1

Oak Ridge, Tenn. Oak 140 1,800 8 5.4 1 3.6 highc 2

Oak Ridge, Tenn. Oak 110 1,180 17 5.4 12 23.6 highc 2

Calhoun Forest, S.C. Pine 245 ND 7.5 2.8 10.2 14.9 highc 3

Shenandoah Nat. 

Park, Va.

Mixed pine, 

hardwood

380 ND 10d 3.9 2.7 8.8 highc 4

Oak Ridge, Tenn. Pine 820 2,500 16 5.4 15 25.6 highc 2

Coweeta Hydrologic 

Laboratory, N.C.

Mixed 

hardwood

1,370 11,500 12.2 7.1 1.5 6.6 high 97 1

B. Sites that are moderately sensitive to calcium depletion

Stewart Co.; Ga. Pine 840 840 6 3.2 1.1 3.9 med. 123 6

Duke Forest, N.C. Pine 2,130 4,900 13.6 8.1 8.1 13.6 med. 130 1

Panola Mountain, Ga. Mixed pine, 

hard wood

2,200 2,200 10 2.25 2.9 10.7 med. 150 5

C. Sites that are insensitive to calcium depletion

Clemson, S.C. Pine 1,450 ND 3.4 2.9 3.9 4.4 low 310 7

Oak Ridge, Tenn.,

plot 1

Pine 6,900 8,600 5.5 5.4 19.2 19.3 low 350 1

Gainesville, Fla. Pine 240 758 5.9 8 2.1 0 lowe 1

Coweeta Hydrologic 

Laboratory, N.C.

Pine 667 3,770 3.9 7.1 2.2 -1 lowe 1

Great Smoky

Mountains Nat. Park

Spruce 230 1,800 1.2 19 10 -7.8 lowe 1
10

UNANSWERED QUESTIONS

2. How do rates of soil-calcium weathering and

leaching vary throughout the East?

3. How is forest growth affected by low levels of

available calcium?

4. How does forest harvesting affect the

availability of soil calcium and long-term trends in

stream chemistry?

ONGOING USGS RESEARCH

Unanswered questions regarding the current and

future status of calcium in the East are being

addressed through the USGS Hydrologic Benchmark

Network (HBN) and the National Trends Network

(NTN). The HBN sponsors monitoring of stream

flow and chemistry, in undeveloped, medium-sized

basins (100 to 200 km2

 drainage areas). The NTN,

which is closely affiliated with the National

Atmospheric Deposition Program (NADP), supports

the monitoring of chemical deposition in rain and

snow (wet-only deposition) at about 100 locations

nationwide. The HBN and NTN programs are

currently supporting investigations in three HBN

watersheds in the Northeast (the Wild River, in New

Hampshire and Maine, the Neversink River, in N.Y.,

and Young Womens Creek, in Pennsylvania.) and

two in the Southeast (Little River, in Tennessee, and

Cataloochee Creek, in North Carolina). These

investigations will (1) measure atmospheric

deposition rates, stream chemistry, and soil

chemistry, (2) determine soil-weathering rates,

forest-growth history and past harvesting in these

watersheds, and (3) assess the effects of future tree

harvesting. A primary objective of this work is to

gather detailed information on calcium cycling that

can be used to assess current conditions for each of

the five basins. This information will be integrated

with the long-term monitoring data to better

understand past and future changes in soil-calcium

status in the eastern United States.

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increasing, decreasing, or remaining stable?
11

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1994a, Acid rain and soils of the Adirondacks. I.

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For more information contact:

1 Gregory B. Lawrence

U.S. Geological Survey

425 Jordan Rd.

Troy, NY 12180

E-mail address:

glawrenc@usgs.gov

2 T.G. Huntington

U.S. Geological Survey

3039 Amwiller Rd.

Atlanta, Ga 30360

E-mail address:

thunting@usgs.gov

Copies of this report can be purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286, Federal Center

Denver, Co 80225-0286

This report and related information can

be found on the World Wide Web at:

http://bqs.usgs.gov/acidrain

Additional earth science information can

be obtained by accessing the USGS “Home

Page” on the World Wide Web at:

http://water.usgs.gov

 By 1Gregory B. Lawrence and 2T. G. Huntington