CO Deficit in Temperate Forest Soils Receiving High Atmospheric N-Deposition 2

CO Deficit in Temperate Forest Soils Receiving High Atmospheric N-Deposition 2
Siegfried Fleischer
CO2 Deficit in Temperate Forest Soils Receiving
High Atmospheric N-Deposition
Evidence is provided for an internal CO2 sink in forest soils,
that may have a potential impact on the global CO2-budget.
Lowered CO2 fraction in the soil atmosphere, and thus
lowered CO2 release to the aboveground atmosphere, is
indicated in high N-deposition areas. Also at forest edges,
especially of spruce forest, where additional N-deposition
has occurred, the soil CO2 is lowered, and the gradient
increases into the closed forest. Over the last three
decades the capacity of the forest soil to maintain the
internal sink process has been limited to a cumulative
supply of approximately 1000 and 1500 kg N ha–1. Beyond
this limit the internal soil CO2 sink becomes an additional
CO2 source, together with nitrogen leaching. This stage of
“nitrogen saturation” is still uncommon in closed forests in
southern Scandinavia, however, it occurs in exposed forest
edges which receive high atmospheric N-deposition. The
soil CO2 gradient, which originally increases from the edge
towards the closed forest, becomes reversed.
Root and microbial respiration implies that soils are a carbon
dioxide (CO2) source, constituting half of the CO2 from respiration on the continents around the globe. An almost equivalent
amount is released directly from terrestrial plants to the atmosphere (1, 2). CO2 is transported to the atmosphere directly from
the soil surface, or with drainage water to streams. The lowest
CO2 concentration in the soil atmosphere, even after total inhibition of CO2 production in the soil, is considered to be that in
the aboveground atmosphere, e.g. under frozen or dry conditions
when no CO2 is produced (3). However, low soil CO2 concentrations during warm and wet conditions, even concentrations
slightly below the aboveground atmospheric concentration, were
recorded at some sites on the Swedish west coast where atmospheric N-deposition is high. These findings initiated the extensive 10-yr study reported here. In order to determine whether
or not there was a soil process strong enough to decrease CO2
emissions to the atmosphere, I collected 4594 soil atmosphere
samples from 1992 through 2000, including one sampling occasion in June 2001 in the NW Pacific rainforest, Washington,
(5–12) and by I. Stjernquist (pers. comm. relating to the Konga
experimental beech forest site).
Atmospheric N-deposition can be taken up by leaves/needles,
leaving an unknown fraction that may reach the ground directly.
This fraction (throughfall) is, among other conditions, dependent on whether the site has reached nitrogen saturation or not.
Although easy to measure, it is not used as a measure of N-deposition in this study.
N-deposition refers to the EMEP (13, G. Lövblad, pers.
comm.). The cumulative N-input over the last 30 yrs was estimated based on the same annual atmospheric N-deposition during 1970–2000, and a 10% lower deposition during the late
1960s (L. Granat, pers. comm, 14).
At fertilized sites N added is included. Thirty-year N-accumulation was chosen, to include in the study the well-defined
Figure 1. The consequence of thinning for soil
CO2 concentration. A) CO2 in the soil
atmosphere at 20 cm in a spruce stand on
peatland at Amböke, SW Sweden. Thinning
resulted in a rise in summer groundwater
levels from about 90 cm to 50 cm.
Concurrently with the drastic increase in
surficial soil CO2, CH4 concentrations up to
more than 12% (v/v) were recorded at 40 cm.
B) CO2 at 0–40 cm depth at the Mahult spruce
site one year before (1996), and the first (1997)
and second year (1998) after thinning. Annual
averages from 6, 7, and 10 complete profiles,
respectively. Annual precipitation is indicated.
The increase in CO2 the first year after
thinning can not be related to increased
precipitation, and in the second year the effect
of thinning is decreasing despite increased
precipitation that year.
Sampling was carried out in (number of samples within parentheses): Sweden (4200), Denmark (232), The Netherlands (55),
USA (31), Norway (17). In addition, samples were also taken
in Argentina, Brazil, Canada, The Czech Republic, Germany,
Greece, Italy, and Polen (altogether 59). Samples were taken in
spruce forest (3243), pine forest (253), deciduous forest (588),
clearcut forest (227), grassland (57), agricultural land (196), and
different types of vegetation, e.g. heathland (30). Sampling depth
was generally 0–20 cm, but at some sites deeper layers were
sampled, but less frequently. A total of 785 analyses of CO2 were
carried out in soil or the above soil atmosphere at the laboratory. Air samples were also analyzed (4). Site descriptions (fertilizer supply, N-leaching) are given in several research reports
© Royal Swedish Academy of Sciences 2003
Ambio Vol. 32 No. 1, Feb. 2003
southern Swedish research sites that were started during the late
1960s, and since then have been administered by the Swedish
University of Agricultural Sciences.
A soil depth of 0–20 cm was generally selected to make the
unsaturated layers at the sites comparable. Samples from deeper
layers were not collected from all sites. A stainless steel tube,
outside diameter 6 mm, with a perpendicular hole on the side
close to the closed bottom end, was used for sampling. With a
hand-operated vacuum pump the soil atmosphere was transferred
into a connecting polybutene tube. The dead volume was first
pumped out, and the polybutene tube was instantly closed at both
ends. Samples were analyzed within 2 days, but usually on the
same day (overseas samples within 1 week). The sampled soil
atmosphere was taken from the tube with a syringe and immediately analyzed by gas-solid chromatography, separation on
Haye Sep Q with a Varian 3300 instrument, and a Varian 4400
Intergrator. Reference and carrier gases were from Air Liquide
Gas Company.
To characterize the soil atmosphere that relates solely to N-deposition, long-term stable conditions were needed. Forest sites with
anthropogenic disturbances other than high N-load, such as
clearcutting, or even thinning—leading to higher groundwater
Figure 2. A) Annual average CO2 in the soil
atmosphere as a function of the cumulative
atmospheric N-deposition (30 yrs) at 45
nonfertilized temperate forest sites. The
sites are sampled in frostfree conditions all
over the year, depth 0–20 cm (unsaturated
zone). The curve shown is a 2nd polynomial
regression, adjusted R2 = 0.653; p < 0.001;
B). Logarithmic plot of the same relationship as in A). CO2 at 4 nonfertilized
Scandinavian arctic forest sites and at
fertilized or otherwise anthropogenically
(draining, clearcutting, agriculture)
influenced sites is also shown. Breakdown
of the CO2 sequestration capacity was
observed during the study period at the
Skogaby experimental site. The slightly
increased nitrate in soil solution (1992–1995
at the N-fertilized plots increased more than
30 times above the control plots, with
subsequent drastic N-leaching (9). This
change in N-cycling occurred simultaneously with the almost threefold increase in
soil CO2 indicated (1997–1998). At this site
the available CO2 data 1992–1995 were from
July and the late autumn, and were
compared with the same seasons 1997–
1998. At the experimental site at Mellby,
N is supplied to the reference plots solely
as commercial fertilizer, while both commercial fertilizer and manure are added to
plots receiving recommended N-supply and
a double dose manure is added at the high
N-treatment plots. Half the plots have/have
not catch crops. The annual average from
the NW Pacific mixed rainforest is indicated
by a blue circle and is based on 3 sampling
occasions in the Hoh River area (April, June
and November) and one rainforest sampling
at Victoria Island, BC (July).
Ambio Vol. 32 No. 1, Feb. 2003
level as a result of decreased evapotranspiration, and manifesting methane production in the summer (15) (Fig. 1), as well as
drained and limed sites—were treated separately.
Forest edges receive considerably higher N-depositions compared to closed forests (16–18). The latter study (18) was on the
same site as that studied by me, i.e. a soil CO2-gradient from
the forest edge (Stubbaröd in Fig. 4). Of the 45 sampling sites
in undisturbed forests (Fig. 2) 5 were within 50 m from forest
edges and N-depositions were adjusted to fit a gradient from
100% deposition at 50 m (closed forest) to 156% deposition 5
m from the forest edge (18).
The accumulated N-deposition on undisturbed sites showed
a clear influence on CO2 concentrations in the soil atmosphere
(Fig. 2). However, high N-deposition does not only deplete soil
CO2 incidentally, but more importantly, soil CO2 concentrations
above the atmospheric concentration (the predominating situation) are lower than those resulting from respiration processes
alone. In areas where cumulative atmospheric N-depositions—
over the last 30 years—have reached 800–1000 kg ha–1 yr–1, soil
CO2 concentrations are less than half of those in low deposition
sites. These conditions are now widespread and the indicated sink
may have large-scale implications.
I assumed that the indicated CO2-gradient in areas with low
to high cumulative N-deposition should also occur locally where
there is a similar N-deposition gradient. This deposition gradi45 unfertilized forest sites (n = 1451)
Scandinavian arctic sites (!n = 74)
Northwest Pacific Rainforest (n = 20)
N-fertilized forest research sites (!n = 545)
Skogaby (N-fertilized), initial N-leaching, 92–95, (n = 17)
Skogaby, drastic N-leaching, 1997–1998 (n = 32)
Harplinge, 2 sites before clear-cutting (!n = 28)
Harplinge, 2 sites clear-cut (!n = 72)
Site 920 H, N-fertilized (n = 47)
kg N ha–1 30 yrs–1
Site 920 H, section with spruce killed by the pest lps
typógraphus (n = 25)
Dutch mixed forest dominated by pine (n = 32)
Average of 4 clear-cut sites, except Harplinge (n = 82)
Grassland, two sites (!n = 57)
Birch, (n = 67)
Drained mire
Experimental agricultural fields, reference plots, not
fertilized since 1984 (n = 24)
Experimental agricultural fields, recommended N-supply
(n = 63)
Experimental agricultural fields, high N-supply (n = 54)
Farmland SW Sweden (n = 51)
kg N ha–1 30 yrs–1
Figure 3. Soil CO2 at
0–20 cm in transects
from spruce forest
edges into the closed
forests, studied from
mid-1999 to late-2000.
The central Swedish
sites Smedjebacken
(average of 6 complete
profiles) and Storfors
(average of 7 complete
profiles) have an annual
atmospheric Ndeposition to the closed
forest of about 8 and 9
kg ha–1, respectively.
© Royal Swedish Academy of Sciences 2003
ent occurs from the closed forest to the forest edge (18). Results
from transects in central Sweden, where N-deposition to the
closed forest is lower than in the south, support this hypothesis
(Fig. 3).
A cumulative N-input of about 1000–1500 kg N deposited
over a 30-yr period becomes critical. Beyond this N-accumulation level, soil CO2 concentrations increase, concurrently with
increasing nitrate leaching. During the course of the study this
breakdown of the soil N- and CO2-sequestration capacity was
indicated at the Skogaby research site. Increased N-supply (from
approx. 1500 to 1700 kg N deposited over 30 years) initiated a
shift from low N-leaching into extreme leaching, simultaneously
with an almost 3-fold increase in soil CO2 (Fig. 2B). The same
situation was also obvious for forest edges in a high N-deposition area in SW Sweden and in Denmark. Instead of the originally enforced soil CO2 sink, the forest edge had become an increased CO2 source (Fig. 4).
Low soil CO2 concentrations were found even in the summer
when intense soil respiration is expected. This could not be explained solely by decreased CO2 production in the soil, as a result of, e.g. drying, or by day/night fluctuation of the aboveground CO2, especially as the soil CO2-depletion sometimes was
more pronounced at deeper levels (19). Photosynthetic CO2 consumption does not occur in the soil. To my knowledge, nitrification, a chemoautotrophic CO2 consuming process, has not been
considered as a potential CO2 sink in forest soils. However, from
wastewater-treatment processes it is well-known that nitrification alters alkalinity (CO2 is removed and H+ is added) (20).
Soil N-fertilization studies in the laboratory supported the results, but did not result in CO2 concentrations below the aboveground air (21). Reduced soil CO2 release as a result of applied
inorganic N has been observed earlier. Interpretations for this
phenomenon have involved i) decreased microbial activity, explained by decreased C input or availability; ii) restriction of Cmineralization by high N-concentrations in the organic substrate;
iii) disturbed balance between decomposers; iv) hampered production of enzymes; and v) formation of decomposing products
being toxic or inhibitory (22–24). The impact of fertilization on
soil respiration has also been reported as being low (25).
Nitrification has been demonstrated to occur even in acid forest soils (26–28). Nitrification in Swedish and Danish spruce forests was a chemoautotrophic process, with a high potential in
the 10–50 cm soil layer (29). Liming stimulated nitrification, and
reduced the CO2 evolution rate, except when a nitrification inhibitor was used (23).
The present strong terrestrial, biospheric CO2 sink, indicated by
the 13C/12C ratio in atmospheric CO2 (30) is located in temperate latitudes where it became significant during the late 1980s
(31). At that time considerable atmospheric N-deposition with
subsequent soil N-accumulation had occurred in extensive temperate areas. The “missing sink” was related to N or CO2 fertilization in Russian forests and North American forest regrowth.
Biospheric CO2 sinks as a result of terrestrial N-fertilization, have
so far been related to increased forest growth (32, 33), or to forest
regrowth on abandoned agricultural land (34), but have recently
been questioned as the putative CO2 sink in temperate forests
This inbalance of the global carbon budget is estimated as 0.4–
3.4 . 109 tonnes C yr–1 (36–40). Estimates of the amount of CO2
that could be assimilated, based on the energy yield from net
nitrification (41), indicated that only a minor part of the missing CO2 sink could be explained by this process. Recent findings from a study in western America (28) that gross nitrification in forest soils largely exceeds net nitrification, totally
changed this view.
Figure 4. Same soil CO2 transect measurements in spruce forests as in Figure 3,
sampling from late 1998 to late 1999. One
Danish site (Draved in south Jutland, average
of 7 complete profiles) and 2 southern Swedish
sites (Åstorp, average of 6 complete profiles
and Stubbaröd, average of 8 complete profiles).
These forests receive about 3 times higher Ndeposition than the forests in Figure 3.
From areas with low N-deposition on undisturbed forests in
New Mexico and Oregon, USA, Stark and Hart (28) reported
gross nitrification up to 304 mg N m–2 day–1 during spring. This
is in the magnitude of twice the total annual atmospheric N-deposition cycled in one day. A discrete estimate based on one third
of that value (100 mg N m–2 day–1), lasting for 250 days per year,
implies that 250 kg N ha–1 would be nitrified annually. However, high atmospheric N-deposition areas in Europe receive, and
retain, 10–20 times more N every year than do the American
sites studied by Stark and Hart. A 10-fold increase in the N-cycling rate in these high N-deposition areas would result in 159
kg C ha–1 yr–1 (41). The efficiency of the nitrification reaction
was set to 10%. Providing that 100 kg of the C reassimilated
within the soil is preserved as organic material, and this process occurs in areas corresponding to half of the world’s moist
temperate and boreal forest soils (only 8.4% of Haldridge’s Life
Zones), gross nitrification would account for an annual sink 0.11.
109 tonnes C (0.40 . 109 tonnes CO2).
The American study (28) showed that NO3 produced by nitrification is effectively assimilated by microorganisms. In this
way, NO3 leaching is prevented, which explains the effective Nretention in most forest soils. In the mature forest ecosystem,
nitrogen is rapidly remobilized from the organic material (dead
microbial biomass) and the NH4 released makes repeated nitrification possible. According to my results and those of the
American study (28), a soil “CO2 -pump” is operating, run by
N-cycling in the forest soil. Every turn of the cycle implies CO2sequestration, resulting in the high potential soil CO2-sink indicated where N-deposition has increased (Figs 2, 3).
High evapotranspiration is known to increase soil CO2 production (3). However, the sites with highest evapotranspiration
(Pacific rainforest) and lowest (two arctic sites in Scandinavia)
all show high soil CO2 concentrations (Fig. 2B). Atmospheric
N-deposition is very low at these sites.
The results support the explanation that nitrogen cycling (nitrification, followed by microbial NO3 uptake), causes the soil
CO2 sink. If production of CO2 were disturbed or hampered by
increased nitrogen supply, which was the alternative interpretation (22–24) this would probably enforce the sink, and not turn
it into an increased CO2 source when still more nitrogen is deposited. Whether the explanation is nitrification alone, or in combination with an inhibitory effect, N-deposition causes extensive
reductions of forest soil CO2 concentrations.
In summary, N-saturation causes increased N-leaching (42)
and this coincides with loss of the internal soil CO2 sequestra-
© Royal Swedish Academy of Sciences 2003
Ambio Vol. 32 No. 1, Feb. 2003
tion capacity, which implies increased CO2 release from the soil.
Effective N-cycling is still the most widespread process in forests (43). Therefore, the internal soil CO2 sink still dominates.
My results indicate that this sink can even be enforced extensively by additional N-deposition except in some very high deposition areas, e.g. in Europe, where N-saturation and subsequent
increased CO2 emissions from the soils are present. Support for
this CO2 sink in order to counteract the increase of CO2 in the
atmosphere, will need better knowledge of its areal distribution,
and probably monitoring programs coupled with effective feedback measures. When other aspects such as biodiversity are considered, increased N-deposition may be undesirable.
Using my findings some results, published quite recently, may
be re-interpreted. Soil warming experiments in Massachusetts
(44) and in northern Sweden (45) did not result in the expected
increase in CO2 release from the soil. Acclimatization of soil res-
piration was suggested to occur under temperature increase. According to my findings, the alternative interpretation may be that
increased temperature, as expected, enhances soil respiration but
also the internal soil CO2 consuming process discussed here.
Within the comprehensive Skogaby project in southern Sweden, run from 1988 to 2001, the C and N soil pools were calculated. Instead of expected C/N ratios of 25 at the N-fertilized
plots, and 30 at the control plots which received solely atmospheric deposition, the ratios calculated were unexpectedly high,
37 and 145, respectively. Overestimation of the soil carbon content, or underestimation of organic material mineralization was
suggested (46). My hypothesis, implying repeated soil CO2 uptake as a result of N-cycling, supports considerable soil carbon
sequestration. It also appears that soil respiration is not equivalent to CO2 released from the soil, especially where the internal
soil CO2 uptake is high as a result of high N-deposition.
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46. Persson, T. and Nilsson, L.-O. (eds) 2001. The Skogaby Experiment. Swedish Environmental Protection Agency, Report 5173, 220 pp. (In Swedish).
47. This study was supported by the Brita and Sven Rahmn foundation and by the WWF.
I thank Gunnar Jacks and Jan Pokorny for their critical review of the manuscript, and
Jonas Svensson and Lars Stibe for re-counting.
48. Further information, including site descriptions, is available from the author.
49. First submitted 19 April 2002, accepted for publication 12 Sept. 2002.
Siegfried Fleischer is professor of limnology at the Wetland
Research Center, Halmstad University, Sweden. Nitrogen
cycling and its coupling to greenhouse gas production in
the aquatic and terrestrial environment have been
emphasized in his research, as well as interdisciplinary
approaches to environmental problems. His address:
County Administration Board, SE-301 86 Halmstad, Sweden.
E-mail: [email protected]
© Royal Swedish Academy of Sciences 2003
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