carbon sequestration in forest soils: effects of soil type, atmospheric co2 enrichment, and n...

Carbon sequestration in forest soils: effects of soil type,atmospheric CO2 enrichment, and N deposition

F . H A G E D O R Na , S . M A U R E R

b , P . E G L Ia , P . B L A S E R

a , J . B . B U C H E Ra & R . S I E G W O L F

b

aSwiss Federal Institute for Forest, Snow and Landscape Research (WSL), ZuÈrcherstrasse 111, 8903 Birmensdorf, and bPaul Scherrer

Institut, 5232 Villigen-PSI, Switzerland

Summary

Soil contains the major part of carbon in terrestrial ecosystems, but the response of this carbon to

enriching the atmosphere in CO2 and to increased N deposition is not completely understood. We studied

the effects of CO2 concentrations at 370 and 570�mol CO2 mol±1 air and increased N deposition (7

against 0.7 g N m±2 year±1) on the dynamics of soil organic C in two types of forest soil in model

ecosystems with spruce and beech established in large open-top chambers containing an acidic loam and

a calcareous sand. The added CO2 was depleted in 13C and thus the net input of new C into soil organic

carbon and the mineralization of native C could be quanti®ed.

Soil type was the greatest determining factor in carbon dynamics. After 4 years, the net input of

new C in the acidic loam (670 6 30 g C m±2) exceeded that in the calcareous sand (340 6 40 g C m±2)

although the soil produced less biomass. The mineralization of native organic C accounted for

700 6 90 g C m±2 in the acidic loam and for 2800 6 170 g C m±2 in the calcareous sand. Unfavourable

conditions for mineralization and a greater physico-chemical protection of C by clay and oxides in

the acidic loam are probably the main reasons for these differences. The organic C content of the

acidic loam was 230 g C m±2 more under the large than under the small N treatment. As suggested

by a negligible impact of N inputs on the fraction of new C in the acidic loam, this increase

resulted mainly from a suppressed mineralization of native C. In the calcareous sand, N deposition

did not in¯uence C concentrations. The impacts of CO2 enrichment on C concentrations were small.

In the uppermost 10 cm of the acidic loam, larger CO2 concentrations increased C contents by 50±

170 g C m±2. Below 10 cm depth in the acidic loam and at all soil depths in the calcareous sand,

CO2 concentrations had no signi®cant impact on soil C concentrations. Up to 40% of the `new'

carbon of the acidic loam was found in the coarse sand fraction, which accounted for only 7% of

the total soil volume. This suggests that a large part of the CO2-derived `new' C was incorporated

into the labile and easily mineralizable pool in the soil.

Introduction

The increase in atmospheric CO2 is indisputable, but sinks for

this carbon are not well understood. Combustion of fossil fuel

and deforestation lead to an emission of 6.6 3 1015 g C year±1,

whereas the atmospheric increase and the oceanic uptake

account for only about 5.2 3 1015 g C year±1. This leaves an

unknown sink of about 1.4 3 1015 g C year±1 (Schimel, 1995).

One hypothesis is that this `missing' C is being stored in

terrestrial ecosystems of the northern hemisphere, probably as

a result of increased plant growth induced by larger

concentrations of CO2 and a greater deposition of N (Ciais

et al., 1995; Townsend et al., 1996; White et al., 2000).

Increases in plant growth in response to CO2 enrichment are

experimentally well documented. Forest inventories show an

accelerated forest growth in the northern hemisphere (Speicker

et al., 1996). Carbon ®xed within plant biomass ultimately

enters the soil, where it may reside for hundreds of years. The

ability of soils to store this additional C, however, is highly

controversial, because there are two contrasting ways in which

the increased input of C may be processed in the soil. First, the

extra-®xed C may become soil organic C. Second, this readily

available source of C may stimulate soil microbial processes

by providing substrates that enhance decomposition of the

organic matter through the so-called `priming-effect' (Paterson

et al., 1997). Strong evidence for a long-term sink for

increased atmospheric CO2 in soils is still lacking

(Schlesinger, 1990; Schimel, 1995; Canadell et al., 1996).

Paper given at Eurosoil 2000.

Correspondence: F. Hagedorn. E-mail: [email protected]

Received 12 October 2000; revised version accepted 3 April 2001

European Journal of Soil Science, December 2001, 52, 619±628

# 2001 Blackwell Science Ltd 619

The impacts of CO2 enrichment on soil C dynamics have

been studied mainly in grasslands and agricultural systems

(e.g. Leavitt et al., 1994; Torbert et al., 1997; van Kessel

et al., 2000). Inputs of `new' C from CO2 into soils of

these ecosystems were estimated by using CO2 that was

depleted in 13C compared with ambient atmospheric CO2.

Results indicated that substantial amounts of atmospheric

CO2 are ®xed in soil: up to 25% of the total organic C in

the soil (700 g C m±2) can be derived from the added

atmospheric CO2 after 4 years (van Kessel et al., 2000).

Forests play a major role in the global carbon cycle.

Between 62 and 78% of the global terrestrial C is

sequestered in forests, and about 70% of this C is stored

in the soil (Dixon et al., 1994; Schimel, 1995). Forest soil

tends to accumulate more C than does soil under

agriculture, because the carbon turns over more slowly

(Guggenberger et al., 1994). Thus, forest soils may store

more C than agricultural soils, and their responses to

increasing atmospheric CO2 concentrations will be signi®-

cant for the future global carbon cycle.

Canadell et al. (1996) have suggested that soil quality may

in¯uence sequestration of C in response to increased atmo-

spheric CO2. Soil fertility may control the C inputs into the

soil, since CO2 enrichment can stimulate plant growth only in

soils with adequate nutrients (Egli et al., 1998). Moreover, soil

properties in¯uence the decomposition of organic C in the soil,

because they determine the living conditions for microbes and

protect C in the soil (Martin & Haider, 1986; Christensen,

1992). Most studies on CO2, however, have been made on only

one soil each, and if fertility was taken into account, fertilizers

were applied in different amounts.

The quantitative contribution of increased deposition of

nitrogen to C sequestration by forest ecosystems of the

northern hemisphere is much debated. Estimates of the

effects of N deposition on C sequestration in forests range

from less than 0.1 up to 1.3 3 1015 g C year±1 (Townsend

et al., 1996; Nadelhoffer et al., 1999; White et al., 2000),

which corresponds to 5±90% of the putative terrestrial C

sink. As increased deposition of N affects both the amount

and the quality of litter, it is not clear how the carbon

dynamics of the soil respond to increased N inputs to

forests.

We have investigated the effect of soil type, N deposition,

and increased atmospheric CO2 on soil organic carbon and

attempted to determine the input of added CO2 into two

different forest soils experimentally. We established spruce±

beech model ecosystems in large open-top chambers on an

acidic loam and a calcareous sand. The ecosystems were

exposed to an atmosphere enriched in CO2 and increased N

inputs for 4 years. The CO2 used for the enrichment was

depleted in 13C relative to the ambient atmospheric CO2,

creating an opportunity to trace C inputs in the plant±soil

system.

Materials and methods

Experimental design

Model ecosystems with spruce (Picea abies Karst) and beech

(Fagus sylvatica L.) were established in 16 open-top chambers

in 1994. Each chamber was 3 m high, had a diameter of 3 m

and contained two soil compartments of 3 m2 surface area.

Each compartment was 1.5 m deep, served as a non-weighable

lysimeter, and had a 0.5-m thick layer of quartz sand with a

drainage outlet at the bottom (for details see Sonnleitner et al.,

2001). The treatments were as follows: ambient CO2

(370�mol mol±1) + low N deposition (0.7 g NH4NO3-N m±2

year±1); enriched CO2 (570�mol mol±1) + low N; ambient

CO2 + high N (7 g NH4NO3-N m±2 year±1); enriched CO2 +

high N. The treatments were arranged as a Latin square with

four replicates for each CO2 3 N treatment. Atmospheric CO2

enrichment started in January 1995. The incoming ®ltered air

was blown through textile tubes at a rate of 3000 m3 per hour

(Landolt et al., 1997). The chambers were irrigated with

electro-osmotically puri®ed tap water with ions added in

concentrations usually found in rain water. Nitrogen deposi-

tion was manipulated by adding NH4NO3 to the irrigation

water. Concentrations of nitrogen in the irrigation water were

0.88 mg N l±1 under low N deposition and 8.8 mg l±1 under high

N deposition. The irrigation water was applied once or twice a

week through nozzles which were kept just above the plant

canopy. To avoid external water and nutrient inputs by rain, a

transparent roof automatically closed the chambers at the onset

of rain.

The experiment was done on two soils transferred from

natural spruce±beech forest sites in Switzerland. One of the

soils was an acidic sandy loam, a Haplic Alisol, referred to

as `acidic loam'. The other soil was a calcareous loamy

sand, a Calcaric Fluvisol, quoted henceforth as `calcareous

sand'. None of the soils showed signs of hydromorphy.

Each soil was ®lled into one of the two separate

gravitational lysimeters (surface area 3 m2) of each open-

top chamber in spring 1994. The acidic loam was

transferred in two layers, a 0.4-m topsoil layer and a 0.6-

m subsoil layer, into the lysimeters. The calcareous sand,

which had no distinct soil horizons, was transferred in one

layer. Soil properties are given in Table 1. To minimize the

effects of soil disturbance, oats and barley were cropped in

the summer of 1994. In October 1994, model ecosystems

were established in each of the 32 lysimeters (16 per soil

type). Each model ecosystem was composed of eight beech

and eight spruce trees with ®ve understorey species (Carex

sylvatica, Geum urbanum, Ranunculus ®caria, Viola

sylvatica, and Hedera helix). Trees were grown from seeds

collected from selected provenances (beech) and from

clonal cuttings (spruce). The beech trees were 2±3 years

old at the time of planting. Details are given in Egli et al.

(1998).

620 F. Hagedorn et al.

# 2001 Blackwell Science Ltd, European Journal of Soil Science, 52, 619±628

Sampling and analysis

After four growing seasons, in September 1998, the trees

were harvested. Leaves, stems, and roots were sampled and

dried separately. For isotopic analysis, one leaf was taken

from the top, middle, and lower crown section of each of

the eight beech trees. The 24 leaves taken per lysimeter

were pooled for analysis. In each lysimeter, three needles

per age class from the top and lower crown position were

collected from each of the eight spruce trees and pooled.

Fine roots were sampled at three locations per lysimeter

from 0 to 20 cm soil depth with a soil corer (3.5 cm in

diameter), separated into beech and spruce roots and

washed thoroughly. All plant samples were dried at 65°C

for 72 h and ground with a mixer mill (Retsch MM2000,

Haan, Germany). Aliquots of 4.0±4.5 mg were placed in tin

capsules for mass spectrometry analysis.

The soil was sampled prior to tree harvest from 0 to 10 cm

depth with a sharpened metal tube (n = 6 per lysimeter). The

soil from 10±25, 25±50, 50±60, and 90±100 cm depth was

taken with a soil corer (n = 2 per lysimeter). All soil samples

were dried at 60°C for 48 h immediately after sampling and

then stored in a climate chamber. For analysis, visible plant

particles were removed from the soil by hand. After sieving the

soil through a 2-mm sieve, we ground the samples of the bulk

soil ®nely in a ball-mill. The organic C associated with the

sand was separated by dispersing soil aliquots of 30 g

ultrasonically with 3 kJ energy and passing them through a

250-�m sieve as described by Amelung & Zech (1999). The

remaining sand fraction was dried at 60°C for 48 h and ground

with a ball-mill. To remove carbonates from the soil prior to C

analysis, aliquots of 15 mg soil were placed into silver

elemental analysis capsules, moistened, and exposed to vapour

from concentrated HCl in a desiccator for 12 h (David Harris,

personal communication, 1999). This procedure yielded the

same �13C values as the removal of carbonates by titration.

The C content and the �13C values of plant and soil samples

were determined with an automated elemental analyser±

continuous ¯ow isotope ratio mass spectrophotometer (EA-

1110, Carlo Erba, Italy, interfaced with a CONFLO II, Delta-S

Finnigan MAT, Bremen). Results of the C isotope analysis are

expressed in � units (½):

d13C � 1000

Rsample

Rstandard

ÿ 1

� �; �1�

where R = 13C/12C.

The �13C values were referenced to the Pee Dee Belemnite

(PDB) standard.

Calculations and statistical analysis

The fraction of soil C derived from plant input (fnew), the net

new soil C, was calculated with a simple mixing model

(Balesdent et al., 1988):

fnew � dsoil ÿ dsoil; native

dplant ÿ dsoil; native

� �; �2�

where �soil is the �13C value of the organic C in the soil at

harvest, �soil,native that of the organic C before the start of the

experiment (`native' SOC), and �plant is the �13C value of the

trees at harvest. As the biomass of foliage and ®ne roots at

harvest were approximately the same, and the contribution of

both root and ®ne root litter to soil organic C was not known,

we took the mean of the �13C values of leaves and ®ne roots

for �plant. The remaining proportion of native C in the soil is

equal to 1 ± fnew. The quantity of C in native C and the net

input of new C were calculated by multiplying the proportions

with soil C concentrations and dry masses. The mineralization

of native C was assessed by subtracting the amount of native

organic C at harvest from the value prior to the experiment.

Treatment effects were estimated with analyses of variance

(ANOVA). Following the experimental design, the main effects

of CO2 and N and their interaction were tested against the

`chamber mean squares', while the main effects of soil type

and its interactions with CO2, N, and CO2 3 N were tested

against the `lysimeter mean squares'. Effects of CO2 and N

were additionally tested in separate analyses for the acidic and

the calcareous soil.

Table 1 Properties of the soil before the experiment

pH Altota Fetot

a CECeffb N (KCl)c P (Olsen)c Sandd Siltd Clayd

(CaCl2) ___________ /g kg±1 __________ /mmolc kg±1 _________ /mg kg±1 _________ _________________ /% ________________

Acidic loam topsoil 4.11 8.14 9.26 42 3.0 2.3 55 29 16

Acidic loam subsoil 3.80 8.56 13.18 60 2.9 BDL 55 27 18

Calcareous sand 7.16 2.88 5.78 127 11.0 6.0 84 10 6

aExtracted with concentrated HNO3.bEffective cation exchange capacity (CECeff) was determined by extraction with 1 N NH4Cl.cDetermined at harvest.dFrom Sonnleitner et al. (2001).

BDL, below detection limit.

Carbon sequestration in forest soils 621


Results

Soil carbon

After the 4 years, the concentrations of organic C in the

calcareous sand decreased throughout all depths (see dotted

lines in Figure 1). This decrease reached 3 g C kg±1, which

corresponds to a loss of 23% of the initial carbon content. The

total loss of C in the whole soil compartment (100 cm depth) of

the calcareous sand ranged between 1300 and 2500 g C m±2. In

contrast, the changes of C concentrations in the acidic loam

were small (Figure 1). Contents of C in the acidic loam

decreased only below 25 cm depth. The total loss of C was

between 0 and 600 g C m±2.

Under large deposition of N, concentrations of C in the

uppermost 10 cm of the acidic loam were 9% greater than

when little N was added (P < 0.05, main effect pooled across

the CO2 treatments). At depths below 10 cm, N inputs had no

signi®cant effect on C concentrations (Figure 1). At the end of

the experiment, the C contents in the whole soil compartment

of the acidic loam were 230 g C m±2 greater in the model

ecosystem receiving 7 g N m±2 year±1 than in those receiving

0.7 g N m±2 year±1. In the calcareous sand, N deposition had no

effect on the C concentration of the topsoil, but concentrations

at 50 and 100 cm soil depth were about 14% (P < 0.05) greater

under the larger N inputs.

With increased atmospheric CO2, C concentrations in the

uppermost 10 cm of the acidic loam and of the calcareous sand

were 8% and 5% greater than at ambient CO2 (Figure 1), but

signi®cant only in the acidic soil (P < 0.05). Below 10 cm

depth, elevated CO2 had no signi®cant impact on the C

concentration of either of the soils. The increase in C content

of the uppermost 10 cm of the acidic loam induced by the CO2

accounted for 180 g C m±2 under low and for 70 g C m±2 under

high N deposition (Figure 4).

New and old soil carbon

The 4-year exposure of the trees to increased atmospheric CO2

with a �13C value of about ±16½ decreased the �13C values of

®ne roots and leaves by 10½ as compared with the biomass

produced under ambient CO2 (Table 2). The ratios of C to N of

®ne roots were increased by CO2 enrichment, but were

decreased by the greater N deposition (Table 2; P < 0.01 for

beech and spruce across both soils). Hagedorn et al. (2000)

noticed similar effects for C/N ratios of foliage.

The decline in 13C through the fumigation with CO2 was

also re¯ected in soil organic C. The �13C values of the C were

signi®cantly less under enriched CO2 than under ambient

concentration (Figure 2), indicating that new C was incorpo-

rated into the soils. The shift in �13C was highly signi®cant

down to a depth of 50 cm. In the acidic loam, this decline in

�13C was more pronounced than in the calcareous sand

(P < 0.01). As the C concentrations of the two soils were

similar before the experiment, the greater decrease in 13C

indicates that more new C was incorporated in the course of

the experiment. In contrast to soil type, N inputs did not

signi®cantly in¯uence the shift in �13C (Figure 2).

Under CO2 enrichment, the calculated fraction of new C in

total organic C in the uppermost 10 cm was 22 6 2% (6 SE) in

the acidic loam and 17 6 2% in the calcareous sand (Figure 4).

In the acidic loam, the net input of new C under elevated CO2

was 670 6 30 g C m±2 in the whole soil compartment with no

Figure 1 Concentrations of soil organic carbon in the acidic loam and in the calcareous sand of the spruce±beech model ecosystem after

4 experimental years. The dotted lines represent the carbon concentrations before the experiment. Low N represents a deposition of 0.7 g N m±2

year±1 and high N a deposition of 7 g N m±2 year±1. Means and standard errors of four replicates per treatment.



effects of N additions (Figure 3). In the calcareous sand, the

net input of new C was 450 6 20 g C m±2 and 320 6 50 g C m±2

at low and at high N inputs (P < 0.05). The mineralization of

native C was signi®cantly greater in the calcareous sand than

in the acidic loam (P < 0.001). Increasing the N deposition did

not lead to an increase in net input of new C in the acidic loam,

but decreased the mineralization of native C. In the whole soil

compartment of the acidic loam, the decrease in mineralization

of native C accounted for 240 g C m±2 (Figure 3), but it was

signi®cant only at 0±10 cm depth (P < 0.05; Figure 4). In the

calcareous sand, N deposition did not signi®cantly in¯uence

the mineralization of native C.

Carbon associated with sand

After the 4 years, the concentrations of C in the coarse sand

fraction, representing the particulate organic matter, were

substantially larger than those of the native acidic loam (Table

3). In contrast, C concentrations in the particulate fraction of

the calcareous sand remained constant at enriched CO2 and

Table 2 The �13C values of leaves and ®ne roots and C/N mass ratios of ®ne roots. Means and standard errors of four replicates per treatment

Spruce Beech

Needles Fine roots Fine roots Leaves Fine roots Fine roots

Treatment N CO2__________________ /½ _________________ C/N mass ratio __________________ /½ _________________ C/N mass ratio

Acidic loam Low Ambient ±30.7 6 0.1 ±29.1 6 0.1 62.7 6 1.5 ±30.2 6 0.1 ±29.1 6 0.2 80.8 6 2.7

Low Enriched ±41.2 6 1.0 ±38.8 6 0.6 66.9 6 1.9 ±40.9 6 1.2 ±40.3 6 1.0 80.6 6 1.0

High Ambient ±30.7 6 0.1 ±28.7 6 0.1 53.3 6 1.6 ±30.6 6 0.2 ±29.0 6 0.1 67.6 6 1.8

High Enriched ±40.7 6 0.9 ±40.5 6 1.4 56.4 6 2.4 ±40.2 6 0.9 ±39.8 6 0.9 73.8 6 1.6

Signi®cance CO2 0.001 0.001 NS 0.001 0.001 NS

N NS NS 0.001 NS NS 0.001

Calcareous sand Low Ambient ±30.2 6 0.2 ±28.3 6 0.2 42.2 6 4.5 ±30.8 6 0.5 ±30.4 6 0.9 53.8 6 2.8

Low Enriched ±40.5 6 0.2 ±37.1 6 0.4 48.5 6 3.4 ±39.7 6 0.6 ±38.5 6 0.4 62.9 6 1.8

High Ambient ±30.3 6 0.1 ±28.4 6 0.3 35.4 6 1.5 ±30.7 6 0.3 ±29.4 6 0.6 46.4 6 0.3

High Enriched ±41.0 6 1.0 ±39.7 6 0.4 40.3 6 1.4 ±41.2 6 0.9 ±39.1 6 1.1 52.5 6 1.8

Signi®cance CO2 0.001 0.001 0.01 0.001 0.001 0.002

N NS NS 0.001 NS NS 0.001

Figure 2 The �13C values of soil organic carbon in the acidic loam and in the calcareous sand of the spruce±beech model ecosystem after

4 experimental years. The dotted lines represent the �13C values before the experiment. Low N represents a deposition of 0.7 g N m±2 year±1

and high N a deposition of 7 g N m±2 year±1. Means and standard errors of four replicates per treatment.



decreased by about 25% at ambient CO2. The estimated

fraction of new C in the C associated with sand of the acidic

loam ranged from 50 to 75%. In the calcareous sand, these

proportions were 24 to 31%. Fertilization with N increased

concentrations of C in the acidic loam, but had no signi®cant

effects in the calcareous sand. Enrichment with CO2, in

contrast, did not signi®cantly in¯uence this C fraction in the

acidic loam, but increased it in the calcareous sand (Table 3).

Discussion

Effects of soil type

Our results show that the soil itself was the most important

factor in¯uencing the carbon dynamics of the soils and the

response of soil carbon to CO2 enrichment. In the 4 years of

the experiment, the contents of organic C decreased in the

calcareous sand, probably as a result of disturbance. This loss

in C indicates that changes in land use, such as deforestation or

clear-cutting, strongly affect storage of C in soils (Balesdent

et al., 1988; Davidson & Ackerman, 1993). Our results show

that loss of C due to disturbance depends upon soil type, since,

in contrast to the calcareous sand, the concentrations of C in

the acidic loam remained almost constant during the 4 years

(Figure 1). The net input of new C was about 75% greater in

the acidic loam than in the calcareous sand (Figure 3), despite

a 40% greater biomass of trees on the calcareous sand (Dieter

Spinnler, personal communication, 2000; for year 2, see Egli

et al., 1998). These ®ndings strongly suggest that C turned

over more rapidly in the calcareous sand than in the acidic

loam. This is supported by a greater mineralization of native C

in the calcareous sand (Figure 3) and by a 50% increase in

respiration measured by the nightly CO2 ef¯ux (Maurer et al.,

1999). Further, losses of nitrate and dissolved organic carbon

through drainage were more than 10 times greater in the

calcareous sand than in the acidic loam (Hagedorn et al.,

2000).

There are several reasons for the organic C to mineralize

more rapidly in the calcareous sand than in the acidic loam.

First, many microbes are more active at a neutral pH than in an

acidic milieu. For instance, there was a net nitri®cation in the

calcareous soil, but not in the acidic one (Hagedorn et al.,

2000). Second, the calcareous sand was less dense than the

acidic loam, indicating better aeration and more favourable

conditions for C mineralization. Third, the acidic loam

contained more clay than the calcareous sand (Table 1).

Clay interacts with organic compounds and protects organic C

from microbial decay. Fourth, the acidic loam contained more

Fe and Al than the calcareous sand (Table 1), suggesting that it

contained more oxides on which organic C could build stable

organo-mineral complexes (Martin & Haider, 1986).

Figure 3 The net input of new carbon and the mineralization of

native soil organic carbon of the whole soil compartment down to

100 cm depth in the acidic loam and in the calcareous sand of the

spruce±beech model ecosystem after 4 experimental years. Low N

represents a deposition of 0.7 g N m±2 year±1 and high N a

deposition of 7 g N m±2 year±1. Means and standard errors of four

replicates per treatment.

Figure 4 Total soil organic carbon as well as new and old carbon

under enriched atmospheric CO2 at 0±10 cm depth. The black lines

represent the organic carbon contents of the soils before the

experiment. New and old carbon could not be estimated for ambient

CO2, since the CO2 was not labelled with 13C. Means and standard

errors of four replicates per treatment.



The differences between the two soil types were even more

pronounced in the C associated with the coarse sand. This C

consists mainly of particles and represents the most labile pool

of C in the soil (Balesdent et al., 1988; Christensen, 1992). The

C in the coarse sand fraction of the calcareous sand declined

by as much as 30% during the 4 years, which is considerably

more than the loss of C from the bulk soil. In contrast, in the

acidic loam, the amount of C associated with coarse sand

doubled during the experiment. Since the physico-chemical

stabilization of organic C in the coarse sand fraction is likely to

be equally small in both soil types, the contrasting response of

the soils strongly suggests that microorganisms found living

conditions unfavourable in the acidic loam, and so the organic

C was preserved more than in the calcareous sand.

Net input of new C into the forest soils

For the net input of new C, the soil type was more important

than the productivity of trees, as indicated by a larger net input

of new C in the less productive acidic loam than in the

calcareous sand (Figure 3). The net input of new C into the

acidic loam was 670 g C m±2, corresponding to 45% of the C

stored in the trees (above- + below-ground biomass). In the

calcareous sand, the net input of new C was only about

380 g C m±2, which was equal to 18% of the total C in the trees.

The net input of new C per unit of plant biomass in the acidic

loam was greater than that under agriculture. In cropping

systems with sorghum and soybean, the net input of new C

corresponded to 15 and 22% of the biomass produced

(calculated from Torbert et al., 1997). In a pasture study, this

proportion was about 15% (calculated from Hebeisen et al.,

1997 and van Kessel et al., 2000). One explanation for the

larger C input per unit biomass in our acidic model forest is a

slower decomposition of the new C due either to the quality of

the litter or to unfavourable conditions for decomposers as

discussed above. Moreover, the litter input was probably larger

in our forest than in agriculture, where a part of the biomass

produced is harvested and removed from the land each year.

Another reason could be a greater investment of trees into

below-ground biomass. In our acidic forest, root biomass was

about ®ve times greater than recorded by Hebeisen et al.

(1997) in the pasture experiment. These ®ndings suggest that

acidic forest soils can sequester more C than soils under

agriculture.

A large proportion of the net input of new C was found in

the organic matter associated with the coarse sand fraction

(Table 3). Although the coarse sand fraction accounted for

only 7% of the total soil by volume in the acidic loam, up to

40% of the total new C input occurred in it. Because the

turnover of the C associated with the coarse sand is usually

rather rapid, with half-lifes less than 10 years (Martin et al.,

1990), the large proportion of new C in this fraction strongly

suggests that only a small portion of it becomes stable organic

matter. However, only long-term experiments can clarify how

much of new C inputs enters passive pools of organic C in the

soil.

Effects of CO2 enrichment

The impact of an atmosphere enriched in CO2 on the dynamics

of C in the soil was small in comparison with the large

differences between the two soil types and the large losses of C

Table 3 Coarse sand fraction: content in the total soil volume, soil organic carbon concentrations, �13C values, and the fraction of new C in

soil organic carbon (SOC). Means and standard errors of four replicates per treatment

Coarse sand SOC �13C New Ca

Treatment N CO2____________________ /% ____________________ /½ /%

Acidic loam Before experiment 6.9 1.53 ±27.7

Low Ambient 7.0 6 0.5 3.03 6 0.32 ±29.0 6 0.2 ND

Low Enriched 7.0 6 0.2 3.06 6 0.19 ±35.2 6 0.6 59 6 4

High Ambient 6.9 6 0.1 3.49 6 0.49 ±28.9 6 0.1 ND

(After 4 years) High Enriched 7.0 6 0.1 3.57 6 0.07 ±35.8 6 0.5 64 6 4

Signi®cance CO2 NS NS 0.001 ±

N NS 0.03 NS NS

Calcareous sand Before experiment 23.3 1.07 ±27.3

Low Ambient 22.7 6 0.3 0.76 6 0.08 ±27.1 6 0.2 ND

Low Enriched 23.2 6 0.9 1.03 6 0.11 ±30.1 6 0.5 24 6 4

High Ambient 22.5 6 0.8 0.84 6 0.07 ±27.6 6 0.1 ND

(After 4 years) High Enriched 21.9 6 0.4 1.05 6 0.09 ±31.5 6 0.9 31 6 6

Signi®cance CO2 NS 0.03 NS ±

N NS NS NS NS

aCalculated with the mean �13C value of ®ne roots and leaves.

ND, not determined.

NS, not signi®cant.



from the calcareous sand during the experiment (Figure 4).

The small increase in C concentrations of the bulk soil in

response to CO2 enrichment accorded with the marginal and

insigni®cant increases of C contents under enriched CO2 as

reported by Leavitt et al. (1994), Hungate et al. (1996) and van

Kessel et al. (2000). The increase in organic C induced by CO2

in the uppermost 10 cm of the soil ranged between 50 g C m±2

in the calcareous sand and 180 g C m±2 in the acidic loam

(Figure 4). This is similar to the observed increases of up to

400 g C m±2 after 4 years in the pasture study of van Kessel

et al. (2000) and in a tallgrass prairie exposed to increased CO2

for 8 years as found by Williams et al. (2000). In contrast to

most other studies, the CO2-induced increase in soil C was

signi®cant in the acidic loam of our study. This was probably

because there was little variation in the organic carbon content

of the soil in this study (CV < 15%, Figure 4) as a result of the

homogenization prior to the experiment and the large number

of replicates ± eight per CO2 level and soil type.

The small increase in total organic C through CO2

enrichment can be attributed either to an increased input of

new C or to a decreased mineralization of native C under

enriched CO2. Because CO2 of ambient air was not labelled

with 13C, we could not distinguish between these two

mechanisms. However, ®ne roots were stimulated through

CO2 enrichment by about 20% (Dieter Spinnler, personal

communication, 2000), suggesting that a part of the CO2-

induced increase in soil C was caused by a greater net input of

new C into the soil. This is consistent with experiments in

grasslands that were previously cropped with C4 plants where

increased atmospheric CO2 led to an increase in new C by 37

and 50%, respectively, in the soil (Nitschelm et al., 1997;

Loiseau & Soussana, 1999).

Assuming that soils of temperate forests respond similarly to

increased CO2 as the two soils in this experiment, and given

that the world's temperate forests cover about 6 3 1012 m2

(Ajtay et al., 1979), we can estimate that the gain in soil

organic C by the experimental increase of 200�mol CO2

mol±1 amounts to 0.1±0.27 3 1015 g C year±1. This

corresponds to 7±20% of the yearly missing C sink (1.4 3

1015 g C year±1; Schimel, 1995). However, as the annual rise of

CO2 concentrations of the atmosphere is about 1.4�mol CO2

mol±1 year±1 (Houghton et al., 1990), the actual contribution of

a CO2-induced increase in organic C in the soil is probably at

least one order of magnitude less.

Effects of N deposition

The response of C dynamics to inputs of N depended upon the

type of soil. In the calcareous sand with much available N

(Hagedorn et al., 2000), increased deposition of N did not

affect the concentrations of organic C, but it caused a decrease

of new C (Figures 3 and 4). This is consistent with a smaller

amount of root biomass at high than at low N deposition under

CO2 enrichment in the calcareous sand. In the acidic loam with

little available N, concentrations of organic C in the uppermost

10 cm were increased by increased deposition of N (Figure 4).

Since the increase of N inputs also stimulated tree growth on

this soil (root biomass: +25%; Dieter Spinnler, personal

communication, 2000), one would expect that the increase in

C resulted from enhanced C input into the soil. However, the

net input of new C was only slightly stimulated by increased N

deposition in the acidic loam (Figures 3 and 4). The N-induced

increase in C was caused mainly by a suppressed mineraliza-

tion of native C. Our results accord with the results from other

experiments; they show that additions of mineral N stimulate

the decomposition of plant residues (i.e. `new' C), but reduce

decomposition of recalcitrant organic matter (Fog, 1988;

Green et al., 1995). Magill & Aber (1998) measured an

accumulation of lignin under large deposition of N during a

long-term experiment, and they explained their ®nding as the

suppression of the production of ligninolytic enzyme by soil

microbes, particularly fungi. Global carbon models including

increased N deposition neglect potentially retarded decay of

soil C (Townsend et al., 1996; White et al., 2000). Our results

give this as about 240 g C m±2 during 4 years in the acidic

loam, which is consistent with the values estimated by Magill

& Aber (1998). This corresponds to 50% of the N fertilization

effect found for tree growth, and it is in the same order of

magnitude as the annual leaf fall in mature forests (Vogt et al.,

1986). Since soil organic C represents the pool of carbon with

the longest turnover in ecosystems, this effect of added N

could be quantitatively even more important with respect to

the global carbon cycle.

Conclusions

Our results show that soils play a key role in the sequestration

of carbon. The net input of new C was larger in the acidic loam

than in the calcareous sand, despite a considerably larger

biomass production in the fertile calcareous sand. In

comparison with agricultural systems, the net input of new C

per unit of biomass produced was large in the acidic loam,

suggesting that acidic forest soils have a large potential to store

C. The pronounced differences between the two soil types in C

sequestration indicate that properties such as soil pH, texture,

and mineralogy control the stabilization of C in the soil. The

mechanisms by which organic matter is stabilized in the soil

need further investigation, because they determine whether the

carbon will be respired to the atmosphere in a fairly short time

or remain in the soil and become `long-term' organic C. Our

®ndings that a large proportion of new C was in the coarse

sand fraction of the soil, the most labile pool of organic C,

suggest that only a small portion of the new C input becomes

stable. The different dynamics of C in the two soil types have

implications for estimates and models of global carbon

budgets, which currently neglect soil properties and concen-

trate mainly on the effects of temperature and litter input.



The large losses of C from the calcareous sand during the

4 years indicate that changes of land use can severely decrease

the carbon storage in the soil. In comparison with the large

differences between the two soil types, the effects of

increasing the CO2 and the increased N deposition on the soil

carbon were small. Enrichment in CO2 increased the contents

of C signi®cantly only in the acidic loam at 0±10 cm depth.

The small increase in C through increased CO2 compared with

the missing carbon sink suggests that an accelerated storage of

C in soils makes a minor contribution to C sequestration.

Increased deposition of N increased concentrations of organic

C in the uppermost acidic loam signi®cantly. The major reason

was a suppressed mineralization of native C as indicated by a

negligible in¯uence of N inputs on the fraction of new C.

According to our results, the signi®cant reduction in miner-

alization of native C due to increased deposition of N was

quantitatively important and may lead to underestimates of the

effect of nitrogen fertilization on C sequestration.

Acknowledgements

This experiment was designed by C. KoÈrner, J.B. Bucher, and

W. Landolt as part of the Swiss contribution to COST 614

under the coordination of C. Brunold. The CO2 enrichment

was maintained by U. Bleuler and W. Landolt. We are grateful

to M. Bundt for helpful discussions and carefully reading the

script. Funding was provided by the Board of the Swiss

Federal Institute of Technology and by the Swiss National

Science Foundation.

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