Nutrient and carbon budgets in forest soils as decision support in sustainable forest management

www.elsevier.com/locate/foreco

Forest Ecology and Management 238 (2007) 167–174

Nutrient and carbon budgets in forest soils as decision support

in sustainable forest management

Cecilia Akselsson a,*, Olle Westling a, Harald Sverdrup b, Per Gundersen c

a IVL Swedish Environmental Research Institute, P.O. Box 5302, SE-400 14 Goteborg, Swedenb Department of Chemical Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden

c Danish Centre for Forest, Landscape, and Planning, KVL, Hørsholm Kongevej 11, DK-2970 Hørsholm, Denmark

Received 22 March 2006; received in revised form 15 September 2006; accepted 2 October 2006

Abstract

Knowledge about the nutrient and carbon budgets in forest soils is essential to maintain sustainable production, but also in several

environmental issues, such as acidification, eutrophication and climate change. The budgets are strongly influenced by atmospheric deposition

as well as forestry. This study demonstrates how budget calculations for nitrogen (N), carbon (C) and base cations (BC) can be used as a basis for

policy decisions on a regional level in Sweden.

The study was based on existing nutrient and C budget calculations on a regional scale in Sweden. The nutrient budgets have been calculated for

each square in a national 5 km � 5 km net by means of mass balances including deposition, harvest losses, leaching, weathering (BC) and fixation

(N). Scenarios with different deposition and forestry intensity have been run and illustrated on maps. A simplified C budget has been estimated by

multiplying the N accumulation with the C/N ratio in the organic layer, based on the assumption that the C/N ratio in the accumulating organic

matter is equal to the ratio in the soil organic matter pool. The budget approaches differ from earlier budget studies since they involve regional high

resolution data, combine deposition and forestry scenarios and integrate different environmental aspects.

The results indicate that whole-tree harvesting will cause net losses of N and base cations in large parts of Sweden, which means that forestry

will not be sustainable unless nutrients are added through compensatory fertilization. To prevent net losses following whole-tree harvesting,

compensatory fertilization of base cations would be required in almost the whole country, whereas N fertilization would be needed mainly in the

northern half of Sweden. The results further suggest that today’s recommendations for N fertilization should be revised in southern Sweden by

applying the southwest–northeast gradient of the N budget calculations. The C and N accumulation calculations show that C sequestration in

Swedish forest soils is not an effective or sustainable way to decrease the net carbon dioxide emissions. A better way is to apply whole-tree

harvesting and use the branches, tops and needles as biofuel replacing fossil fuels. This could reduce the present carbon dioxide emissions from

fossil fuels substantially.

The study shows that high resolution budget calculations that illuminate different aspects of sustainability in forest ecosystems are important

tools for identifying problem areas, investigating different alternatives through scenario analyses and developing new policies. Cooperation with

stakeholders increases the probability that the research will be useful.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Forestry; Sustainability; Deposition; Harvesting; Policy; Decision-support; Sweden

1. Introduction

Sustainability can be seen from an ecological, economical or

social perspective. When these different types of sustainability

do not overlap goal conflicts appear and policy decisions are

needed. In the policy making process decision support is

required. Different scenarios have to be assessed in order to find

* Corresponding author. Tel.: +46 31 7256200; fax: +46 31 7256290.

E-mail address: [email protected] (C. Akselsson).

0378-1127/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2006.10.015

the best possible solution, for which all three types of

sustainability are reached to an acceptable level.

A central part of reaching ecological sustainability is to

counteract the environmental problems acidification, eutrophi-

cation and climate change. The ways to counteract these

environmental problems sometimes leads to negative production

effects and thus negative effects on the economical sustainability.

Moreover, measures to counteract the different environmental

problems are in some cases contradictory. In Sweden, this has led

to a demand of decision support on a national level. Decision

C. Akselsson et al. / Forest Ecology and Management 238 (2007) 167–174168

support can be based on experimental data and/or model

calculations. Regardless of the method national level decision

support has to be scaled up to the national level.

Upscaling procedures have been developed for budget

calculations on a regional scale in Swedish forest soils, based

on the simple mass balance methods for base cations, N and,

with a simplified approach, also for C (Akselsson, 2005).

Scenarios of different N deposition and forestry intensity have

been run. The aims of this paper are to show how the results

from these regional-scaled budget calculations and scenarios

can be used as decision support on a national level in Sweden,

and to briefly exemplify the process of integrating science and

policy. Three issues, related to the environmental problems

acidification, eutrophication and climate change, are discussed:

whole-tree harvesting, N fertilization and the C issue.

2. Budget calculations on a regional scale

Nutrient budget calculations for forest soils are useful in

predicting the nutrient sustainability of a forest ecosystem. The

N and base cation budget calculations in the present study

were performed by means of a simple mass balance model

where the inputs of N and base cations to the soil were

compared with the outputs. The C budget was based on the N

budget and the close relation between C and N in organic

matter. Only the net fluxes into and out of the soil were

considered in the mass balance calculations (deposition, base

cation weathering, N fixation, harvesting and leaching), not the

internal circulation of organic matter (uptake, litterfall and

decomposition). The degree of net accumulation or net loss (D)

of N and base cations in the soil was thus estimated as:

D ¼ net inflow� net outflow (1)

A positive value of D indicates net accumulation and a negative

value indicates net loss. Net changes in either direction occur in

natural ecosystems, but normally at low rates. Accumulation

and net losses at high rates may indicate a risk of adverse

environmental effects.

The budget calculations in the present study were based on

static mass balances, i.e. the mass balance terms are assumed to

be constant over time. The results give indications of the nutrient

sustainability of a system. If the budget is negative it can be

concluded that one or more of the mass balance terms will

eventually change since the output cannot be larger than the input

in the long term. When this happens depends on the nutrient store

in the soil and thus measures of soil stocks are required in

combination with the mass balance estimates. The same

reasoning applies to positive mass balances. The results from

budget calculations can be used for discussions on future risks in

different regions of N leaching and lack of nutrients, and for

estimates of the C sequestration potential, but the potential of

using static budget calculations for future projections is limited

since the dynamics of nature are not included. To include the time

aspects and make long-term predictions dynamic models are

required as a complement to budget calculations, as stated by

Forsius et al. (1998), e.g. ForSAFE (Wallman et al., 2005) and

MAGIC (Cosby et al., 2001).

The regional calculations were performed on a Geographical

Information System (GIS) platform in a grid of 5 km � 5 km

grid cells. For each input parameter the best available regional-

scaled data were incorporated into the input database and mass

balance calculations were then made in each of the ca. 17,000

grid cells in Sweden (Akselsson, 2005). The methodology and

input data for budget calculations of base cations, N and C are

described briefly below and more thoroughly in Akselsson

(2005).

2.1. Base cation budgets

The base cation budget was calculated for the root zone,

which was assumed to be about 50 cm, since the aim was to

interpret the results in a nutrient limitation perspecitve for trees.

Atmospheric deposition and weathering of soil minerals in the

root zone constitute the input of base cations to the forest

ecosystem. The outflow of base cations from the forest

ecosystem consists of harvested biomass and leaching. If

reprecipitation of base cations into new minerals is neglected

and if only vertical percolation is considered the nutrient

budgets for base cations can be calculated as:

D ¼ DepositionþWeathering� Harvesting� Leaching (2)

where D = accumulation (+) or loss (�).

The accumulation/loss is the change in the pool of

exchangeable cations in soil and the change in the pool of

base cations bound to soil organic matter. Whereas current

rates, or approximations of current rates, can be used for the

deposition, weathering and leaching terms, the harvesting term

must be regarded in the perspective of a whole forest rotation.

Thus, the results of the calculations give the yearly net change

as an average for a forest rotation, provided that the other terms

are constant over time.

The deposition was modelled using the MATCH model, a

Swedish dispersion model dealing with transport of emitted

substances, wet deposition and dry deposition for different land

use types (Langner et al., 1996). Deposition from 1998 was

used. Weathering rates were modelled with the PROFILE

model (Sverdrup and Warfvinge, 1995), a steady state model

including process-oriented descriptions of solution equilibrium

reactions, chemical weathering of minerals and leaching and

accumulation of dissolved chemical components. The model

has been extensively tested and validated on field data under a

wide range of conditions, see Sverdrup and Warfvinge (1995)

for a thorough discussion about validation and uncertainties.

Harvest losses were estimated based on data from the Swedish

National Inventory of Forests and leaching was derived from

Swedish measurements of concentrations in soil water and

runoff. The methodology and input data of the base cation

budget calculations are described more thoroughly in

Akselsson et al. (2004a,b, in press).

2.2. Nitrogen budgets

Deposition and fixation constitute the inflows of N to the

soil, while harvested biomass and leaching account for the

C. Akselsson et al. / Forest Ecology and Management 238 (2007) 167–174 169

losses. If only vertical percolation is considered the nutrient

budgets for N can be calculated as:

D ¼ Depositionþ Fixation� Harvesting� Leaching

� Denitrification (3)

where D = accumulation (+) or loss (�).

As in the case of the base cation budget the static N balance

calculations give the yearly net losses as an average for a forest

rotation, provided that the budget terms are constant over time.

The budget calculations for nitrogen account for the whole soil

profile.

The N deposition data was modelled with the MATCH

model (Langner et al., 1996) and deposition from 1998 was

applied. Harvest losses were estimated based on data from the

Swedish National Inventory of Forests and leaching was

derived from Swedish measurements of concentrations in soil

water and runoff (Akselsson et al., 2004a,b; Akselsson and

Westling, 2005). Biological fixation is often considered to

represent only a small contribution to the N budget but DeLuca

et al. (2002) has found fixation in the size of 1.5–

2 kg ha�1 year�1 through a symbiosis between a cyanobacter-

ium (Nostoc sp.) and the feather moss Pleurozium schreberi in

northern Scandinavia and Finland. Based on this the N fixation

was set to a constant value of 1.5 kg ha�1 year�1. Denitrifica-

tion was assumed to be very small and was thus neglected. The

methodology and input data of the N budget calculations are

described more thoroughly in Akselsson and Westling (2005).

2.3. Carbon budgets and carbon–nitrogen interactions

Net C fixation through net photosynthesis constitutes the

input of C to the system while the outputs are soil respiration

losses, leaching of DOC (dissolved organic C) and losses

through harvesting of biomass. C fixation and soil respiration

are, however, difficult to quantify. Since the N budget is easier

to calculate, and since the C and N cycles are closely linked in

organic matter, the N accumulation can be used to approximate

C sequestration (Gundersen et al., 2006). In a N-limited forest

ecosystem, increased input of N, e.g. as deposition, may lead to

increased tree biomass. More C and N are thus bound to the

growing biomass and more C and N will be added to the soil as

above- and below ground litter. The net result of increased N

deposition may thus be increased C and N sequestration, in

both trees and soil. If the C/N ratio in soil, as an average for a

forest rotation, is assumed to be constant, the C sequestration

rate in the soil (DC) can be approximated from the N

accumulation data (DN)—the ‘N balance method’ (Gundersen

et al., 2006):

DC ¼ DNC

N(4)

N accumulation from the N budget calculations described

above was used together with C/N ratios in the organic layer

from a interpolated map (Swedish University of Agricultural

Sciences, 2003) based on data from the Swedish National

Inventory of Forests (Hagglund, 1985).

The assumption in the C budget calculations that the C

and N accumulates at the same C/N ratio as the current bulk

ratio in the forest floor is a rough assumption. It is likely that

C accumulated under high N loads will accumulate with a

lower C/N than the bulk C/N ratio. Then Eq. (4) will give an

upper estimate of the C sequestration rate in soil organic

matter. There is, however, not much evidence for signifi-

cantly decreasing C/N ratios in forest floors due to N

deposition (Kristensen et al., 2004). Accumulation of N in

new organic matter could on the other hand occur at a C/N

ratio higher than the bulk C/N ratio leading to slowly

increasing bulk C/N ratio, but this seem very unlikely under

the currently elevated N deposition, thus we consider the

approximation in Eq. (4) as sufficiently reliable upper

estimate (Akselsson, 2005). Also the N accumulation in the

soil (DN) is uncertain, due to uncertainties in the N fluxes

(Eq. (3)). The uncertainty may be in the order of 1–2 kg

N ha�1 year�1.

3. Policy applications

3.1. Whole-tree harvesting

The interest of whole-tree harvesting is increasing rapidly in

Sweden, since it can provide biofuel which can replace oil as an

energy source and thus counteract the net carbon dioxide

emissions leading to climate change. However, whole-tree

harvesting also means removal of a large amount of nutrients in

branches and needles. The mass balance calculations of N in

Sweden (Fig. 1a–d) show that whole-tree harvesting can

decrease the risk of eutrophication in the south where the N load

is high, but leadsto increased N shortage in the north with low N

load. If the N deposition decreases according to the agreements

in the Gothenburg protocol, i.e. the NOx emissions are cut by

41% and ammonia emissions by 17% from the 1990 level by the

year 2010 (UN/ECE, 1999), whole-tree harvesting will lead to

net losses in most part of Sweden. Accumulation will only

occur in the southwestern part. The results show that N

deposition together with the harvest intensity is decisive for the

future N budget.

Whole-tree harvesting also leads to an increased loss of base

cations and thus depletion of the pools of exchangeable base

cations in the soil (Fig. 2). The budget calculations show that

for the important nutrient potassium (K) more than 3% of

the soil pool is lost every year at 25% of 1625 sites from the

Swedish National Inventory of Forests, when whole-tree

harvesting is applied (Fig. 3). This indicates that there is a

risk of K shortage within one forest rotation, unless

compensatory fertilization is applied. The budget calculations

for Ca and Mg show even higher net losses. The main future

risk with lack of those two base cations concerns the runoff

water quality. When the base cations pool reaches low levels the

leaching of base cations decreases and the risk of acidification

of surface waters thus increases. The result suggests that

compensatory fertilization is necessary in large parts of Sweden

and especially after whole-tree harvesting, to keep the forestry

sustainable.

Fig. 1. N accumulation according to four scenarios: (a) deposition of 1998 and stem harvesting, (b) deposition of 1998 and whole-tree harvesting, (c) decreased

deposition and stem harvesting and (d) decreased deposition and whole-tree harvesting (Akselsson and Westling, 2005).

C. Akselsson et al. / Forest Ecology and Management 238 (2007) 167–174170

3.2. N fertilization

N fertilization has a long tradition in Sweden due to its

potential economical benefits. It has decreased during recent

decades (Sedin, 2003) but the present demand for high

production may increase its use. The National Board of

Forestry, together with the Swedish Environmental Protection

Fig. 2. (a) K accumulation at stem harvesting and (b) wh

Agency, has published recommendations as to where, when and

how N fertilization should be applied in order to prevent

acidification and eutrophication of aquatic and terrestrial

ecosystems (National Board of Forestry, 1991). Although the

present N retention in growing forest is high, and the

contribution from forest soils to N eutrophication of marine

environments is low and mainly restricted to the N leaching

ole-tree harvesting (modified from Akselsson (2005)).

Fig. 3. (a) K pool in soil (Akselsson, 2005) and (b) yearly change of K at whole-tree harvesting.

Fig. 4. (a) Division of Sweden into three parts with different recommendations regarding N fertilization (National Board of Forestry, 1991). (b) N accumulation in

forest soils according to the base scenario (Akselsson and Westling, 2005).

C. Akselsson et al. / Forest Ecology and Management 238 (2007) 167–174 171

Fig. 5. C sequestration in forest soils in Sweden calculated with the ‘‘N balance

method’’ (Akselsson, 2005).

C. Akselsson et al. / Forest Ecology and Management 238 (2007) 167–174172

from clearcuts, there are indications that growing forests in

southwestern Sweden may be close to their retention capacities.

N leaching from growing forests has been observed on several

forest sites in southern Sweden, where the N deposition has

been high for many decades and the C/N ratios in the soil

organic matter are generally low (Hallgren Larsson et al., 1995;

Nohrstedt et al., 1996; Nilsson et al., 1998; Andersson et al.,

2002; Uggla et al., 2004).

The N budget calculations (Fig. 1) show that both N

deposition and harvesting intensity are of great importance for

the N budget and the results from the calculations are thus

important as part of the decision support. The results of the N

budget calculations showed that in the southwestern parts of

Sweden the N accumulation rates were up to 14 kg ha�1 year�1

according to the base scenario assuming the deposition of 1998

and stem harvesting only. Continued accumulation of this

magnitude must be regarded as an obvious risk of future N

leaching on a more regular basis also from growing forests. In

southeastern Sweden, with N accumulation rates of 4–

8 kg ha�1 year�1 assuming the deposition of 1998 and stem

harvesting, there is also a risk of N leaching. N fertilization

would increase this risk substantially. In the scenario with

decreased deposition and whole-tree harvesting N fertilization

will, however, be required in large areas in Sweden in order to

counteract the net losses and maintain the production capacity.

It may seem strange to work hard on decreasing the N

deposition, and then apply N fertilization in the forests to

maintain the growth. However, as can be seen in the maps, the

decreased deposition leads to a reduced risk of N leaching in the

areas with high N load, and through fertilization N can be added

where and when it is needed.

The recommendations from the National Board of Forestry

and the Swedish Environmental Protection Agency divide

Sweden into three parts (Fig. 4a). In the southernmost part of

Sweden N fertilization is not recommended at all, in central

areas the maximum recommended amount is 300 kg ha�1

during a rotation period and in the northern parts the

corresponding figure is 600 kg ha�1. The division of Sweden

into three regions is based on the N status in forest ecosystems

in different parts of Sweden. These general recommendations

are further refined based on other aspects on the stand level such

as soil type, productivity class, soil depth and biodiversity

values. An update of the recommendations is planned in the

immediate future by the National Board of Forestry (H.

Samuelsson, personal communication) and the results of the

study of N budgets constitute one of several important inputs in

the new recommendations. The results of the N budget study

suggest that the differences between southwestern Sweden and

southeastern Sweden are large (Fig. 4b), and such a division

should thus be made. It is important when formulating

recommendations not only to consider the estimated accumula-

tion/net loss, but also to consider the historical deposition. In

southeastern Sweden, with high historical deposition, a great

deal of N has accumulated in the soil, and even if the

accumulation were to cease, N fertilization may lead to a risk of

N leaching since the soil N stores are large. Results on research

on the effects of high N load on vegetation, other nutrient

budgets and sensitivity to pathogens due to changes in nutrient

availability (Tainter and Baker, 1996) are other important

aspects that should be considered.

3.3. The forest as a carbon sink or an energy source

The potential of forest ecosystems as C sinks is appealing

from a climate change point of view. Both the standing biomass

and the forest soils can act as C sinks. The soil C sequestration

estimations performed in Sweden with the ‘N balance method’

(Eq. (4)) showed that the total C sequestration rate in Swedish

forest soils is on an average 100 kg ha�1 year�1, which means

totally 2 million metric tonnes year�1 (Fig. 5). This can be

compared with the yearly Swedish CO2 emission, carrying

15 million metric tonnes C year�1 (Jernbacker, 2003; data from

2001).

If harvest statistics from the 1990s are applied (National

Board of Forestry, 2000), the C sequestration in increasing

standing biomass can be estimated to be 3–4 times higher than

the C sequestration in the soil. However, C sequestration in

forest ecosystems only leads to a temporary reduction in

national net CO2 emissions, and cannot be seen as a sustainable

solution, but rather as a method of delaying emissions. A

continuous increase in C sequestration requires a continuous

increase in standing biomass. High sequestration rates require

C. Akselsson et al. / Forest Ecology and Management 238 (2007) 167–174 173

limited harvesting and N inputs to the ecosystem, which also

means that soil accumulation of C occurs together with a

substantial accumulation of N in the soil organic matter.

Biofuel (slash) from whole-tree harvesting can substitute

3.6 million metric tonnes of oil year�1 (Akselsson, 2005).

Combustion of this amount of oil leads to emissions of

11 million metric tonnes of CO2. This can be compared with the

total annual CO2 emissions in Sweden, 56 million metric

tonnes (data from 2001; Jernbacker, 2003). Presently 10–20%

of the biofuel potential associated with slash is used in Sweden,

according to calculations based on the results from the present

study and data in Gustafsson et al. (2002). By employing

whole-tree harvesting the amount of branch and needle litter is

decreased, which has a negative impact on the C sequestration

in soil, and thus partly counteracts the decrease in net emissions

resulting from the replacement of fossil fuel.

A simple sensitivity analysis was performed in order to get a

quantification of the uncertainties. This was conducted by

calculating one ‘‘highest estimate’’ and one ‘‘lowest estimate’’

of the average C budget for Sweden using DN � 2 kg

N ha�1 year�1 and C/N ratios for the accumulating organic

matter varying from the current average ratio (30) down to half

of this ratio. This calculation led to an interval in accumulation

from 20 to 150 kg C ha�1 year�1, representing a total amount

of 0.5–3.5 million metric tonnes year�1 in Swedish forest.

Although this is a relatively wide span, it does not affect the

overall conclusions.

4. Integrating science and policy

Applied research aimed at decision support must fulfil

specific demands. The results must be easy to understand and

interpret and it must be possible to translate them into authority

recommendations or laws. In order to satisfy these demands a

suitable method must be found for investigating the issue in

question, there must be continuous communication between

representatives of all the parties involved, and, finally, the

results must be presented in an easily understandable way.

The budget calculations are intuitive and the results are

reliable and easy to understand. Several assumptions, limita-

tions and uncertainties are associated with the calculations

(Akselsson, 2005), but they are easy to survey, which simplifies

the interpretation. This makes the results suitable as a basis for

policy decisions. From a policy point of view it is important that

the results used for decision support give a clear picture. Maps

are a very useful and powerful means of presenting results.

Continuous communication with authority representatives

increases the probability that the research will be useful.

Scenario analysis is a good platform for this kind of

cooperation, since it has the potential of connecting policies

with effects, and research with decision-making. In the present

study, scenarios were formulated through a dialogue between

researchers and authority representatives during regular meet-

ings. It was also important to consult experts in different fields,

e.g. representatives from the forest sector, to ensure the

relevance of the scenarios and the reliability of the input data.

By performing the process in this way the participation of

interested parties, and thus also confidence in the policies, will

increase.

5. Conclusions

The results from the budget calculations can provide

scientific data and information that can be visualized on maps.

Such maps may provide a good basis for informed decisions.

Scenario analysis provides a good platform, since it has the

potential of connecting policies with effects, and research with

decision-making. Continuous cooperation with stakeholders is

important for optimizing the integration of science and policy

and it increases the chances of success in the process of creating

useful decision support.

Large-scale whole-tree harvesting has the potential to

substantially decrease the present carbon dioxide emissions

from fossil fuels, if the branches, tops and needles are used as a

biofuel replacing fossil fuels. However, it will cause net losses

of N and base cations in large parts of Sweden, which means

that forestry will not be sustainable unless nutrients are added

through compensatory fertilization. To prevent net losses

following whole-tree harvesting, compensatory fertilization of

base cations would be required in almost the whole country,

whereas N fertilization would be needed only in areas with low

present and historical N deposition, mainly in northern

Sweden.

N fertilization is controversial since it increases forest

production and thus also C sequestration in the forest in an N-

limited system, at least on short term, but at the same time it

leads to risks of increased eutrophication and acidification of

terrestrial and aquatic ecosystems. Current recommendations

for N fertilization can be improved in the southern part of

Sweden by applying the geographical gradient in the N budget

calculations (northwest to southeast).

C sequestration in Swedish forest soils is not an effective or

sustainable way to decrease the net carbon dioxide emissions.

The long-term capacity is low, the utilization of biomass must

be limited and a high accumulation of N is required, increasing

the risk of eutrophication and acidification.

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