THE POSSIBLE
RELATIONSHIP BETWEEN LITTER DECOMPOSITION RATE AND METAL ACCUMULATION IN THE
FOREST FLOOR OF MAPLE WOODS ALONG AN URBAN-RURAL CORRIDOR FROM TORONTO,
ONTARIO.
Tom C. Hutchinson
and Eric Sager
Environmental and
Resource Studies Program, Trent University, Peterborough, Ontario K9J 7B8
Email: thutchinson@trentu.ca, Phone/fax: 705/748-1634
Abstract
Urban areas have
been characterized as metal sinks for a wide range of elements, these
originating from traffic, industrial activities, energy production, etc. However, metal deposition rates have
declined substantially in most North American cities over the past 15-20
years. In a study of the effects of
urban and highway factors on the health of mature sugar maple forests, eleven
woodlots were selected along an urban-rural transect northeast from downtown
Toronto. The rates at which “clean”
sugar maple litter decomposed in nylon litter bags set in the forest floor of
the sites was determined. The elemental
chemical content of the original litter and the changing levels over time as
decomposition proceeded was measured.
Despite the Toronto sites not being particularly polluted in this study,
rates of litter breakdown were reduced 15-30% in the urban sites over the first
6 months. During this time, metal
accumulation in the remaining litter residual in the bags increased many fold
so that Pb, Ni and V became high. It is
suggested that litter acts like an exchange resin and accumulates metals from
the surrounding soils to levels inhibitory to microbial breakdown. Such passive accumulation is reminiscent of
pesticide uptake into lipids.
Nutritional elements such as Ca, Mg and K did not show this accumulating
pattern, but rather were released from the litter to be recycled.
Introduction
In North America,
more than 80% of the people now live in urban areas. These towns and cities are connected by roads and highways,
railways and power lines. These cities
swallow up agricultural land as they expand and utilize a very large amount of
resources, including energy, materials, food and water. The impacts on air and water quality and on
soil contamination have been present, however, for at least a century. Fossil fuel combustion contributes
particulates, sulphur dioxide, carbon dioxide and some hydrocarbons. Gasoline driven transportation emits
hydrocarbons, particulates and nitrogen oxides. Air quality is often poor and a health hazard and described as
reducing or photochemical (Los Angeles) smog.
Ozone is now a major consequence of rush hour traffic. Much of the polluted air moves outside of
the city, with impacts downwind on suburban and rural areas. Heavy metals are very much a part of the
mix. Dry and wet deposition add these
elements to gardens, parks, storm run-off, agricultural fields and downwind to
natural ecosystems.
In a major study to
assess the effects of urban factors and highways on the health of forested
ecosystems, we are assessing a number of potentially vulnerable and important
parameters. This paper presents some of
the findings on one key factor, that of the rate of litter breakdown in a
forested system and inquires whether urban influences negatively impact on
nutrient cycling. It especially
considers whether urban and highway generated heavy metals play a role in this.
Methods
Fresh leaf litter
was collected from a clean, old growth forest in November 1994. Litter of white pine needles and of sugar
maple leaves were collected separately, air dried and stored until the spring
of 1995. Nylon mesh bags containing 2 g
of white pine needles or sugar maple leaves were set out at eleven mature sugar
maple sites in May 1995. These were
placed on the forest floor in multiple replicates and retrieved at six-month
intervals. The contents were then
removed from the bags and dried at 600 C before homogenized
sub-samples were prepared for ICP-MS analysis, following nitric acid
digestion. The contents of the bags
were also weighed after drying so that the percentage of litter remaining could
be obtained.
The sites ran in a
general northeast direction from downtown Toronto for 170 km, with the
reference site at 300 km. All sites
were selected for similarities in maturity, tree species, composition soil
type, and all were on level ground.
Sizes ranged from two hectares to 70 hectares. Small woodlots alongside major roads, in Toronto and in rural
areas are compared. Tree ages are
generally in excess of 100 years.
Soil samples were
taken in triplicate close to the litter bags to a depth of 5 cm, in May
1995. These were also oven dried, nitric
acid digested and analyzed by ICP-MS.
Results and
Discussion
The chemistry of the
soils in the mature sugar maple forests of the three urban-rural categories are
shown in Table 1. Calcium and potassium
concentrations were higher in the urban and highway sites compared with the
rural sites (90-150 km from Toronto) and with the old growth Shaw Woods
site. This corresponds with the
findings of Pouyat et al (1995) who looked at nine oak woodland sites located from
downtown New York out 120 km into Connecticut.
This seems likely to be due to dust deposition, especially from cement
industries and incinerators. Three
heavy metals are shown also in Table 1.
Lead was highest in the urban soils and lowest in the rural sites, while
vanadium and nickel were highest in the woods adjacent to highways. Both vanadium and nickel are present in
heavy engine oils and the high densities of diesel trucks is likely a factor in
this. Vanadium is also discharged from
coal burning and from incinerators while nickel sources in urban areas are,
again, heavy truck traffic and incinerators.
The lead levels in
the soils peaked at 110 ppm.
Interestingly, these are much lower levels than recorded in Toronto in
the early 1970's (Hutchinson, 1972), when levels in excess of 500 ppm were
common, and busy intersections yielded soils with in excess of 2000 ppm. The phasing out of leaded gasoline since
1974 has produced dramatic decreases in lead in air, in rain and in soils in
urban Canada and USA, as well as in remoter rural areas (Miller and Friedland
1994, Friedland et al 1992). It seems
most probable that, for lead at all sites, levels are now much reduced over the
past 20 years. Rainfall-dustfall data
for the sites nevertheless showed elevated lead, nickel and vanadium inputs to
the forests, especially during wet periods and with lower inputs to the rural
and reference site.
Sulphate and nitrate
inputs to the urban sites were up to four times greater in wet months (eg.
October 1996) than to the rural sites.
In contrast, the rural sites received several fold greater phosphate
deposition than the urban sites. Pouyat
et al (1995) also report increase N in the soils of urban New York sites.
Table 2 shows the
percentage decomposition in the first six months of the litter bag experiment. All the initial litter from white pine and
sugar maple was collected from a clean site i.e. Shaw Woods, our reference
site. The rate of white pine litter
decomposition was significantly greater in the rural and reference site than in
the urban Toronto sites, or in the highway sites. Indeed, for white pine, this first six months showed a reduction
of the natural litter decomposition by 45% compared with the rural sites,
while, litter decay in the highway sites was reduced 34%. In sugar maple litter which decomposes
faster than pine needles, 28% was gone after six months in the rural sites
(Table 2) compared with 17% and 13% at the two other categories. The rural sites showed very similar rates of
sugar maple litter decomposition to that of the reference site Shaw Woods,
where the leaves were originally collected.
This time, the reduction in decay rate in the urban compared with the
rural sites was 39%, and 53% at the highway sites. These differences had disappeared by eighteen months for sugar
maple, the faster decaying species, whereas they still are apparent after
eighteen months in white pine. Also, in
both species, the rate of decay of the litter in its site of origin was greater
than at all other sites. This is likely
due to the specific adaptation of the decomposer fauna and flora to the litter
normally encountered.
A decrease in the
rate of litter decomposition has been rather commonly reported in situations of
high heavy metal exposure, especially around mining and smelting sites, and
ascribed to toxic concentrations of the metals interfering with various biotic
components of the normal decompositional microbial succession. (Freedman and Hutchinson 1980, 1981, Strojan
1978, Inman and Parker, 1976. Coughtrey
et al, 1979, McNeilly et al, 1984).
Urban areas have been reported to show reduced rates of litter
decomposition eg. in Naples, Italy, where a study was made of evergreen
Mediterranean oak (Cotrufo et al 1995).
They ascribed it to a loss of fungal decomposer. Similar reports of the fungal components
being especially vulnerable to “urban” factors such as excess heavy metals,
excess sulphur dioxide and sulphate inputs, acidification and ozone comes from
Newshaw et al (1992a and b) and is reported in Cairney and Meharg (1999).
Table 3 shows the
concentrations of selected elements in the six month old litter at the various
sites. The initial content of the
litter used in the experiments is also shown.
Potassium is one of the first elements to be released from fresh litter,
along with magnesium, and this is shown both in white pine and sugar maple
where the rural sites show lower potassium levels than initially. Calcium is released less rapidly and
accumulates a little as decomposition takes place.
The metals,
especially in the sugar maple litter, accumulate to much higher levels than
were present initially eg. lead goes from 1.2 to 62 ppm in sugar maple in the
urban sites, and vanadium from 0.15 to 9.7 ppm in the same sugar maple
litter. Nickel shows a similar but less
extreme trend (Table 3). Since the
levels of these metals in the soil were not especially high, it seems as if the
decomposing litter accumulates these elements, perhaps acting like a resin or
ion exchange column.
This is clearly
demonstrated in Tables 4 and 5 which show the relative accumulation which has
taken place as a percentage of the initial levels in the litter of the two
species after six and eighteen months, and allowing for the reduction in
overall weight of the litter which has decomposed to different extents at the
different sites (Table 2). Lead and
vanadium accumulate as much as 27 to 36 fold in white pine in the urban sites,
and to an almost equal extent in the highway sites. The accumulation is much less in the rural and reference site
although these metals in clean sites show the same tendency to concentrate in
the litter. Sugar maple litter (Table
5), shows an even greater potential to accumulate these toxic elements to
potentially inhibitory levels and these remain residual in the litter. While potassium is rapidly lost from fresh
litter, and calcium, the structural part of the cell walls, follows more slowly
and contributes to nutrient cycling, the non-essential heavy metals seem to
adsorb to the organic matter on balance and stay with it. It seems at least possible that,
accumulatively, these elements can reach levels in litter at polluted sites
which would inhibit bacterial and fungal microbial decomposition, as well as
interfering with soil microfauna. The
numerous literature reports of retardation of the decomposition processes
around smelters and mine sites, seem to have a counterpart in these urban and
highway inpacted systems. Residual
metal accumulations together with ongoing inputs may result in a significant
reduction in plant growth. Many other
factors could make this worse eg. acidification, sulphur dioxide and ozone
exposures, pesticide deposition, etc.
We acknowledge the
helpful assistant of Sheena Symington in this research.
References
Cairney, J. W. G., A. A. Meharg. 1999.
Influences of anthropogenic pollution on mycorrhizal fungal communities. Environmental Pollution. 106: 169-182.
Cotrufo, M. F., A. V. De Santo, A. Alfani, G.
Bartoli & A. De Cristofaro. 1995.
Effects of urban heavy metal pollution on organic matter decomposition
in Quercus ilix L. Woods.
Environmental Pollution. 89:
81-87.
Freedman, W. & T. C. Hutchinson. 1980.
Smelter Pollution near Sudbury, Ontario, Canada and effects on forest litter
decomposition, in “The Effects of Acid
Precipitation on Terrestrial Ecosystems”, NATO Advanced Research Institute, Ed.
T. C. Hutchinson & M. Havas, pub. Plenum, pp 393-434.
Friedland, A., B. W. Craig, E. K. Miller, G.
T. Herrick, T. G. Siccama and A. H. Johnson.
1992. Decreasing lead levels in
forest floor of the Northeastern USA.
Ambio. 2: 400-403.
Hutchinson, T. C. 1972. The occurrence of lead, cadmium, nickel, vanadium and
chloride in soils and vegetation of Toronto in relation to traffic
density. Symp., Int. Ecol. Soc.
INTECOL, Yugoslavia. Adaptations to
extreme habits.
Kohler, H. R., C. Wein, S. Reiss, V. Storch
and G. Alberti. 1995. Impact of heavy
metals on mass and energy flux within the decomposition process in deciduous
forests. Ecotoxicology. 4: 114-137.
Miller, E. K. and A. J. Friedland. 1994.
Lead migration in forest soils: response to changing atmospheric
inputs. Environ. Sci. Technol., 28:
662-669.
Newsham, K. K., J. C. Frankland, L. Boddy and
P. Ineson. 1992. Affects of dry-deposited sulphur dioxide on
fungal decomposition of angiosperm tree leaf letter. I. Changes in communities
of fungal saprotrophs. New Phytol. 122:
97-110.
Ibid. 1992b.
III. Decomposition rates and fungal
respiration. New Phytol. 122: 127-140.
Pouyat, R. V., M. J. McDonnell, and S. T. A.
Pickett. 1995. Soil characteristics of oak stands along an
urban-rural land-use gradient. J.
Environ. Qual. 24: 516-526.
Pouyat, R. V., R. W. Parmelee and M. M.
Carreiro. 1994. Environmental effects of forest
soil-invertebrate and fungal densities in oak stands along an urban-rural land
use gradient. Pedobiologia. 38: 385-399.
Table 1. Chemistry of surface soils
(0-5 cm) collected 1995 at three urban Toronto, 3 major highway sites and 3
rural sites. Each value is the mean of
3 sites, each with 3 replicates. Values
in ppm.
|
Elements |
|
Ca |
K |
Pb |
V |
Ni |
|
Urban |
x |
8652 |
1318 |
71.3 |
31.0 |
26.3 |
|
|
s.d. |
3894 |
770 |
35.2 |
7.8 |
14.6 |
|
Highway |
x |
12024 |
3370 |
37.7 |
44.0 |
36.7 |
|
|
s.d. |
4898 |
764 |
31.8 |
7.8 |
5.5 |
|
Rural |
x |
4608 |
929 |
21.0 |
26.7 |
10.3 |
|
|
s.d. |
2303 |
13 |
13.2 |
1.5 |
3.8 |
|
Reference site |
x |
6526 |
955 |
19.0 |
32.0 |
35.0 |
|
|
s.d. |
2499 |
180 |
13.0 |
4.0 |
32.0 |
Table 2. Percentage decomposition
of white pine and sugar maple litter, placed in field in May 1995, after 6 or
18 months. Sites are grouped into urban,
rural and highway i.e. immediately adjacent to major highway. Each grouping consists of 3 sites with 3
replicated litter bags per site. Litter
collected from the Shaw Woods reference site, an old growth forest, in October
1994. Mean and standard deviation
shown.
|
Litter Decomposition Percent |
|||||
|
|
|
White Pine |
Sugar Maple |
||
|
Sites |
|
6 months |
18 months |
6 months |
18 months |
|
Urban Toronto |
x |
10.2 |
31.1 |
17.2 |
61.1 |
|
|
s.d. |
1.8 |
4.5 |
9.8 |
24.7 |
|
Major Highway |
x |
12.4 |
24.0 |
13.2 |
62.8 |
|
|
s.d. |
0.8 |
1.2 |
8.6 |
6.8 |
|
Rural |
x |
18.6 |
38.7 |
28.1 |
52.5 |
|
|
s.d. |
3.6 |
9.6 |
10.4 |
2.5 |
|
Reference site |
x |
17.8 |
57.2 |
28.3 |
68.0 |
Table 3. Elemental composition of
leaf litter after 6 months of decomposition at the urban, highway and rural
field sites and at an old growth forest reference site. The data for the litter used in the
experiments at time zero is also given.
Data are in ppm. Three sites per
category were used.
|
White Pine |
|
Ca |
K |
Pb |
V |
Ni |
|
Urban |
x |
22.3 |
1.7 |
25.3 |
3.8 |
12.1 |
|
|
s.d. |
2.5 |
0.5 |
10.7 |
1.1 |
0.7 |
|
Highway |
x |
19.7 |
1.5 |
15.3 |
3.9 |
15.8 |
|
|
s.d. |
0.6 |
0.2 |
4.2 |
1.8 |
8.4 |
|
Rural |
x |
20.7 |
1.3 |
11.7 |
1.4 |
18.4 |
|
|
s.d. |
3.2 |
0.4 |
1.5 |
2.5 |
3.3 |
|
Reference site |
x |
20.0 |
1.2 |
10.0 |
0.9 |
8.2 |
|
|
s.d. |
4.0 |
0.3 |
8.0 |
0.8 |
2.6 |
|
Original litter at time zero |
x |
17.0 |
1.52 |
1.8 |
0.3 |
5.2 |
|
|
s.d. |
1.6 |
0.43 |
1.0 |
0.7 |
1.8 |
|
Sugar Maple |
|
Ca |
K |
Pb |
V |
Ni |
|
Urban |
x |
30.7 |
2.0 |
62.0 |
9.7 |
10.5 |
|
|
s.d. |
3.5 |
0.5 |
37.6 |
3.7 |
1.6 |
|
Highway |
x |
34.7 |
2.8 |
30.7 |
11.2 |
12.4 |
|
|
s.d. |
4.7 |
0.3 |
16.8 |
3.0 |
1.7 |
|
Rural |
x |
25.7 |
1.9 |
7.3 |
3.7 |
7.1 |
|
|
s.d. |
2.1 |
0.4 |
5.9 |
3.2 |
5.1 |
|
Reference site |
x |
28.0 |
2.5 |
7.0 |
2.3 |
7.2 |
|
|
s.d. |
6.0 |
0.6 |
3.3 |
2.9 |
2.1 |
|
Original litter at time zero |
x |
32.8 |
9.2 |
1.2 |
0.15 |
6.4 |
|
|
s.d. |
1.4 |
1.2 |
0.4 |
0.03 |
3.5 |
Table 4. Amount of elements, in
percentage, remaining in leaf litter following 6 months or 18 months
decomposition in the field at urban, highway or rural locations as percentage
of that present at time zero. Data are
also given for an old growth reference site.
Each value is the mean of 3 sites.
Sugar maple and white pine litter decomposition are shown.
|
White Pine after 6 months |
||||||
|
|
|
Ca |
K |
Pb |
V |
Ni |
|
Urban |
x |
111 |
116 |
1655 |
1062 |
249 |
|
|
s.d. |
19 |
38 |
775 |
336 |
37 |
|
Highway |
x |
105 |
102 |
1031 |
1126 |
343 |
|
|
s.d. |
1.5 |
15 |
286 |
507 |
186 |
|
Rural |
x |
101 |
84 |
711 |
382 |
397 |
|
|
s.d. |
14 |
20 |
138 |
139 |
24 |
|
Reference site |
x |
97 |
78 |
616 |
243 |
167 |
|
White Pine after 18 months |
||||||
|
|
|
Ca |
K |
Pb |
V |
Ni |
|
Urban |
x |
168 |
224 |
2758 |
3658 |
329 |
|
|
s.d. |
2.3 |
42 |
574 |
1161 |
134 |
|
Highway |
x |
174 |
185 |
1591 |
3360 |
225 |
|
|
s.d. |
27 |
15 |
1342 |
728 |
98 |
|
Rural |
x |
193 |
139 |
422 |
1189 |
127 |
|
|
s.d. |
58 |
6 |
92 |
555 |
22 |
|
Reference site |
x |
207 |
204 |
311 |
621 |
82 |
Table 5. Amount of elements, in
percentage, remaining in leaf litter after 6 and 18 months in the field in
relation to how much was present at time zero, and taking account of how much
decomposition has taken place per site.
Data used for 3 sites in each category.
|
Sugar Maple after 6 months |
||||||
|
Sites |
|
Ca |
K |
Pb |
V |
Ni |
|
Urban |
x |
78 |
18 |
4427 |
5436 |
137 |
|
|
s.d. |
17 |
4.5 |
3123 |
2364 |
35 |
|
Highway |
x |
93 |
26 |
2132 |
6555 |
166 |
|
|
s.d. |
23 |
5 |
1039 |
2414 |
14 |
|
Rural |
x |
57 |
15 |
481 |
1925 |
85 |
|
|
s.d. |
13 |
4.4 |
434 |
1856 |
71 |
|
Reference site |
x |
61 |
19 |
402 |
116 |
80 |
|
Sugar Maple after 18 months |
||||||
|
|
|
Ca |
K |
Pb |
V |
Ni |
|
Urban |
x |
125 |
41 |
7777 |
17241 |
431 |
|
|
s.d. |
18 |
12 |
2534 |
11474 |
283 |
|
Highway |
x |
126 |
33 |
4625 |
13125 |
178 |
|
|
s.d. |
42 |
.06 |
2317 |
4773 |
17 |
|
Rural |
x |
114 |
24 |
1007 |
4911 |
133 |
|
|
s.d. |
21 |
2 |
266 |
2013 |
37 |
|
Reference site |
x |
130 |
34 |
756 |
2833 |
73 |