Interactions between nitrogen deposition, land cover conversion, and climate change determine the contemporary carbon balance of Europe

Abstract

European ecosystems are thought to uptake significant amounts of carbon, but neither the rate nor the contributions of the underlying processes are well known. In the second half of the 20th century, carbon dioxide concentrations have risen by more than 100 ppm, atmospheric nitrogen deposition has more than doubled, and European mean temperatures were increasing by 0.02 °C per year. The extents of forest and grasslands have increase with the respective rates of 5800 km2 yr-1 and 1100 km2 yr-1 as agricultural land has been abandoned at a rate of 7000 km2 yr-1. In this study, we analyze the responses of European land ecosystems to the aforementioned environmental changes using results from four process-based ecosystem models: BIOME-BGC, JULES, ORCHIDEE, and O-CN. All four models suggest that European terrestrial ecosystems sequester carbon at a rate of 100 TgC yr-1 (1980–2007 mean) with strong interannual variability (± 85 TgC yr-1) and a substantial inter-model uncertainty (± 45 TgC yr-1). Decadal budgets suggest that there has been a slight increase in terrestrial net carbon storage from 85 TgC yr-1 in 1980–1989 to 114 TgC yr-1 in 2000–2007. The physiological effect of rising CO2 in combination with nitrogen deposition and forest re-growth have been identified as the important explanatory factors for this net carbon storage. Changes in the growth of woody vegetation are an important contributor to the European carbon sink. Simulated ecosystem responses were more consistent for the two models accounting for terrestrial carbon-nitrogen dynamics than for the two models which only accounted for carbon cycling and the effects of land cover change. Studies of the interactions of carbon-nitrogen dynamics with land use changes are needed to further improve the quantitative understanding of the driving forces of the European land carbon balance.

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Biogeosciences, 7, 2749–2764, 2010
www.biogeosciences.net/7/2749/2010/
doi:10.5194/bg-7-2749-2010
© Author(s) 2010. CC Attribution 3.0 License.
Biogeosciences
Interactions between nitrogen deposition, land cover conversion,
and climate change determine the contemporary carbon balance of
Europe
G. Churkina1,2, S. Zaehle2, J. Hughes3, N. Viovy4, Y. Chen2, M. Jung2, B. W. Heumann5,6, N. Ramankutty5,
M. Heimann2, and C. Jones3
1Leibniz-Centre for Agricultural Landscape Research, Eberswalderstr. 84, 15374 M¨ uncheberg, Germany
2Max-Planck Institute for Biogeochemistry, Hans-Kn¨ oll-Str. 10, 07745 Jena, Germany
3Met Office Hadley Centre, FitzRoy Road, Exeter, Devon, EX1 3PB, UK
4Laboratoire des Sciences du Climat et de l’Environnement, CEA-CNRS-UVSQ, 91191 Gif-sur-Yvette, France
5Department of Geography & Earth System Science Program, McGill University, 805 Sherbrooke St. W., Montreal,
QCH3A2K6, Canada
6Department of Geography, University of North Carolina at Chapel Hill, Saunders Hall, Campus Box 3220, Chapel Hill, NC
27599-3220, USA
Received: 22 February 2010 – Published in Biogeosciences Discuss.: 29 March 2010
Revised: 13 August 2010 – Accepted: 31 August 2010 – Published: 20 September 2010
Abstract. European ecosystems are thought to take up large
amounts of carbon, but neither the rate nor the contribu-
tions of the underlying processes are well known. In the
second half of the 20th century, carbon dioxide concentra-
tions have risen by more that 100ppm, atmospheric nitro-
gen deposition has more than doubled, and European mean
temperatures were increasing by 0.02◦Cyr−1. The extents
of forest and grasslands have increased with the respec-
tive rates of 5800km2yr−1and 1100km2yr−1as agricul-
tural land has been abandoned at a rate of 7000km2yr−1.
In this study, we analyze the responses of European land
ecosystems to the aforementioned environmental changes
using results from four process-based ecosystem models:
BIOME-BGC, JULES, ORCHIDEE, and O-CN. The mod-
els suggest that European ecosystems sequester carbon at
a rate of 56TgCyr−1(mean of four models for 1951–
2000) with strong interannual variability (±88TgCyr−1, av-
erage across models) and substantial inter-model uncertainty
(±39TgCyr−1).
been a continuous increase in the mean net carbon storage of
ecosystems from 85TgCyr−1in 1980s to 108TgCyr−1in
1990s, and to 114TgCyr−1in 2000–2007. The physiologi-
Decadal budgets suggest that there has
Correspondence to: G. Churkina
(galina@churkina.org)
cal effect of rising CO2in combination with nitrogen depo-
sition and forest re-growth have been identified as the impor-
tant explanatory factors for this net carbon storage. Changes
in the growth of woody vegetation are suggested as an im-
portant contributor to the European carbon sink. Simulated
ecosystem responses were more consistent for the two mod-
els accounting for terrestrial carbon-nitrogen dynamics than
for the two models which only accounted for carbon cycling
and the effects of land cover change. Studies of the interac-
tions of carbon-nitrogen dynamics with land use changes are
needed to further improve the quantitative understanding of
the driving forces of the European land carbon balance.
1 Introduction
The contemporary land carbon balance is affected by ecosys-
tem responses to climate variations, land management,
changes in atmospheric composition such as the deposition
of reactive nitrogen or the increase in CO2concentrations,
as well as the interactive effects of these factors in the past
(Schimel et al., 2001). Compilations of various data streams
ranging from eddy-covariance measurements to forest inven-
tories and site-level modeling suggest that European ecosys-
tems are a sink of carbon in the order of 135–205TgCyr−1
(Janssens et al., 2003) or 185–285TgCyr−1(Schulze et al.,
Published by Copernicus Publications on behalf of the European Geosciences Union.

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2750G. Churkina et al.: Contemporary carbon balance of Europe
2009). Schulze at al. (2009) and Janssens et al. (2003) dis-
cuss possible contributions of different vegetation types to
this sink, however, their methodology prevents them from at-
tributing the net carbon balance to various processes ongoing
in the vegetation. The possible driving forces of the land car-
bon uptake in Europe have been discussed in the literature.
These include past trends in climate, atmospheric CO2and
land cover changes (Zaehle et al., 2007), a growing discrep-
ancy between the increase in timber harvests in comparison
to increases to forest biomass (Ciais et al., 2008b), deposition
of reactive nitrogen (Magnani et al., 2007) and the combined
effects of changing environmental conditions and forest re-
growth (Churkina et al., 2007; Vetter et al., 2005).
The land carbon balance, Net Ecosystem Exchange
(NEE), is defined as the difference between the carbon as-
similated by plants through photosynthesis and the carbon
emitted through auto- and heterotrophic respiration, as mea-
suredinsite-levelstudies. Atthelandscapelevel, disturbance
byfireorbiomassharvest, aswellasexportoforganiccarbon
into rivers and seas further modify the NEE and return CO2
to the atmosphere or alter rates and place of organic carbon
decomposition. The resulting Net Biome Productivity (NBP)
is the long-term carbon gain or loss of terrestrial biomass and
soil pools, describes the net storage of carbon in terrestrial
ecosystems, and thereby whether or not a particular region
play a role in reducing or increasing atmospheric CO2levels.
Process-based models allow to integrate the individual driv-
ing forces of the terrestrial carbon balance in different land
cover types into a comprehensive framework.
Here we provide a comprehensive assessment of the con-
temporary terrestrial carbon balance of Europe and its most
important driving forces. We analyze the relative roles of
rising atmospheric CO2, increasing deposition of nitrogen,
changes in climate, and land cover conversion in increasing
land carbon uptake in Europe between 1950 and 2007, al-
though no single model or simulation is yet able to consider
all of these forcings together. We estimate the evolution of
European carbon balance over the 20th century with three
ecosystem models BIOME-BGC, JULES, and ORCHIDEE
driven with a consistent set of model drivers to allow for
a meaningful comparison across the models. To corrobo-
rate the findings of the only carbon-nitrogen cycle model
used in the study (BIOME-BGC), we present also results
from a fourth ecosystem model (O-CN), which extends the
ORCHIDEE model inter alia by a representation of nitro-
gen dynamics. We then assess the degree of agreement be-
tween the estimates from process-based models with inde-
pendent, data-driven estimates obtained from recent bottom-
up compilation of field studies and from top-down inverse
calculations by atmospheric transport models relying on at-
mospheric measurements of CO2concentrations.
2Materials and methods
2.1Models’ description
In this study, we use four process-based terrestrial ecosystem
models: BIOME-BGC, JULES, ORCHIDEE, as well as its
nitrogen cycle version O-CN to simulate carbon fluxes. All
models simulate gross primary productivity and respiration
independently. The models differ by the number of simulated
ecosystem types as well as by implementation of land use
conversion algorithm. BIOME-BGC and O-CN simulate ni-
trogen cycle and carbon-nitrogen interactions explicitly, but
do not model effects of land cover conversion. JULES and
ORCHIDEE simulate effects of land cover conversion, but
not nitrogen cycle. All models estimate NEE as a difference
between gross photosynthetic uptake and ecosystem respira-
tion. ORCHIDEE and JULES estimate also NBP as a dif-
ference between NEE and harvest. Descriptions of photo-
synthesis, respiration and the water cycle in the models are
summarized in (Vetter et al., 2008). Below we give only a
general overview of the models’ concepts.
BIOME-BGC: BIOME-BGC is a process model describ-
ing the carbon, nitrogen, and water cycles within terrestrial
ecosystems (Running and Gower, 1991; Thornton, 1998).
It has been corroborated for a number of hydrological and
carbon cycle components (Churkina et al., 2003; Thornton
et al., 2002; Churkina and Running, 2000). In this study
BIOME-BGC was parameterized for seven vegetation types
including evergreen needleleaf, evergreen broadleaf, decid-
uous needleleaf, deciduous broadleaf, shrubs, as well as
grass with C3 and C4 type photosynthesis. Ecophysiologi-
cal parameters were estimated using eddy covariance mea-
surements for evergreen needleleaf and broadleaf deciduous
forests (Trusilova et al., 2009) and for C3 grasslands (Tomel-
leri, 2007). General parameters were used for the other veg-
etation types (White et al., 2000). Croplands were simulated
as C3 grasslands which productivity is unlimited by nitrogen
availability. Forest management was not included in these
simulations.
Joint UK Land Environment Simulator (JULES): JULES
is a land-surface model based on the MOSES2 land surface
scheme (Essery et al., 2003) used in the Hadley Centre cli-
mate model HadGEM (Johns et al., 2006), also incorporat-
ing the TRIFFID DGVM (Cox, 2001; Cox et al., 2000). The
model simulates carbon, water and energy fluxes of five plant
functional types: broadleaf and needleleaf forests, C3 and
C4 grasses, and shrubs. In this study conversion of vegeta-
tion types within each grid cell was modeled internally based
on Lotka-Volterra competition equations driven by predicted
rates of photosynthesis.A basic dominance hierarchy is
assumed: trees replace shrubs and grasses, shrubs replace
grasses, croplands displace all natural vegetation.
When shrubs and trees are displaced by land use expan-
sion, theremovedcarboniseitherpassedtothewoodproduct
pool or added to the soil carbon pool following the rules from
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G. Churkina et al.: Contemporary carbon balance of Europe2751
Table 1. Fate of biomass after land cover conversion or harvest in simulations of model including effects of land cover conversion on carbon
cycle (after Zaehle et al., 2007).
Proportion of biomass left on site
Land use type AbovegroundBelowgroundHarvested biomass [%]
Forest
Grassland
Cropland
Pasture
leaves 100 wood 40
Leaves 100
10
50
100
100
100
100
60 (wood only)
0
90 (leaves and grain)
50 leaves
Table 1. Forest was considered harvested only if its fractional
coverage within grid cell decreased due to the expansion of
land use. To represent crops, the carbon fluxes of grasses
were modified in regions dominated by croplands. A frac-
tion of NPP from grass-crops was diverted from the natural
vegetation carbon pools into an external crop harvest pool,
which was considered separate to the wood products pool
from forest clearance. The default methodology in JULES is
to assume no lateral transport of carbon between grid cells.
Carbon from crop harvest is respired back to the atmosphere
in one year. Carbon from wood product pool is respired back
to the atmosphere on timescales of one, ten, and 100 years.
ORCHIDEE: The ORCHIDEE biosphere model describes
the carbon, energy and water fluxes (Krinner et al., 2005;
Viovy, 1996) at a half hourly time-step. Input daily climate
data are converted to half hourly data using a weather gener-
ator. ORCHIDEE differentiates between 12 plant functional
types, including temperate broadleaf evergreen, temperate
and boreal needleleaf evergreen as well as broadleaf decidu-
ousandborealneedleleafdeciduoustrees. Herbaceousplants
are represented either as natural C3 and C4 grasslands, or as
managed C3 and C4 croplands. An improved cropland phe-
nology was applied in the model simulations. Improved crop
phenology mimics the phenology of winter wheat for C3-
crops with an early leaf onset day and a short growing sea-
son, and of maize for C4 crops with a late onset day of leaves,
based on meteorological parameters. ORCHIDEE does not
take into account crop or forest management practices.
After land cover conversion or harvest, carbon was dis-
tributed between different pools following the rules from Ta-
ble 1. Forest was harvested only if its fraction within grid
cell decreased. Harvested biomass of forests is decomposed
within one, ten, or 100 years. Different decomposition times
of harvested biomass reflect life time of forest products. A
fraction of crop biomass was removed to simulate harvest.
Thereafter it was decomposed within one year, which was
the year of harvest. Biomass removed with harvest of forest
or crops stayed in the same grid cell where it was harvested.
O-CN: ORCHIDEE has been advanced by adding a com-
prehensive nitrogen cycle representation as well as revising
the representation of vegetation structure and growth (Za-
ehle and Friend, 2010; Zaehle et al., 2010). Simulations of
this new model only became available after the main study
had been conducted. The model simulations were performed
with a slightly different set of drivers and at a much coarser
spatial resolution (see below). To corroborate the findings
of the only carbon-nitrogen cycle model used in the study
(BIOME-BGC) we present also results from O-CN model
runs.
2.2Models’ environmental drivers
As input drivers ecosystem models require climate variables,
elevation above the sea level, soil texture, soil depth, frac-
tional land use maps, atmospheric CO2concentrations, and
nitrogen deposition (models including nitrogen deposition
only). All models except O-CN used the same maps of eleva-
tion above the sea level, soil texture, soil-depth, atmospheric
CO2concentrations, and climate drivers at 0.25◦×0.25◦spa-
tial resolution. The elevation above sea level, soil texture,
and soil-depth data are described elsewhere (Vetter et al.,
2008). The atmospheric CO2concentrations were based on
the ice core data from (Etheridge et al., 1996) and atmo-
spheric measurements from Mauna Loa (Keeling and Whorf,
2005). The CO2concentrations data covered the time from
1700 until the end of 2007.
2.2.1Climate
Climate variables were from the modified Climate Research
Unit (MCRU) dataset (Chen et al., 2009), which is based on
combination of monthly data from Climate Research Unit
(CRU), Norwich, UK database (CRU, 2007) and daily me-
teorological variables from ECHAM5 and REMO climate
model simulations. ECHAM5 is the 5th generation of the
ECHAM general circulation model driven by changes in
greenhouse gases and aerosols (ECHAM5, 2007) developed
at the Max-Planck Institute for Meteorology, Hamburg, Ger-
many. REMO (REgionalMOdel, Jacob and Podzun, 1997)
is a regional climate model forced at the boundaries with
variables from global 6-hourly NCEP (National Center for
Environmental Prediction) reanalysis data set (Kalnay et al.,
1996) from 1948 until 2007. A more detailed description
of the multi-decadal REMO simulation is given in Feser et
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2752G. Churkina et al.: Contemporary carbon balance of Europe
al. (2001). MCRU data set provides daily climate variables
from 1861 until 2007. For the period 1700–1860 models
used MCRU climate for 1901–1930, repetitively.
2.2.2 Atmospheric nitrogen deposition
Annual estimates of atmospheric nitrogen deposition for
1860–2007 were used to drive simulations of models with
nitrogen cycle. These estimates for Europe were extracted
from a global dataset at 1◦×1◦spatial resolution for 1860–
2030. The global dataset was created using estimates from
the three dimensional atmospheric chemical transport model
TM3 (Rodhe et al., 2002) for 1860–1980 and the mean of an
ensemble of model results (Dentener et al., 2006) for 2000
and 2030. For each grid cell nitrogen deposition was linearly
interpolated for 1980–2000 and 2000–2030. The estimates
included wet and dry depositions of both NOy and NHx.
The depositions of reactive nitrogen between 2000 and 2007
were estimated with a “high emission” scenario which corre-
sponds to the IPCC SRES A2 scenario. The original decadal
model outputs for 1860–1980 as well as for 2000 and 2030
were transformed into time series of annual atmospheric ni-
trogen depositions using linear interpolation between avail-
able data for each grid cell.
2.2.3Land cover
Fractional land use maps with annual time step for 1700–
2000 at 0.25◦for Europe were available for this study. These
maps are a combination of historical croplands/pastures
maps with a vegetation classification map adjusted for car-
bon cycle modeling (SYNMAP, Jung et al., 2006). Historical
cropland/pasture maps were derived using hindcasting tech-
niques similar to those of Ramankutty and Foley (1999), by
combining historical agricultural census data at the national
and subnational levels with remote-sensing cropland/pasture
maps for year 2000 from Ramankutty et al. (2008). In this
version, a much richer historical census database was used
for most of Europe. National-level data from 1961–2000
were obtained for all 36 nations from the Food and Agri-
culture Organization (FAO, http://faostat.fao.org). National-
level data from FAO provides statistics at a country level,
where the smallest spatial unit is one country. Further, we
compiled subnational statistics for 248 administrative units
from various sources. Data for 19 countries, with the ear-
liest data available for 1974 in the best case, were obtained
from EUROSTAT database (http://epp.eurostat.ec.europa.eu)
at the second administrative level of Nomenclature of Ter-
ritorial Units for Statistics (NUTS2).
other individual national reports (Economic Research Ser-
vice, 1975; Committee for the World Atlas of Agriculture,
1969; EUROSTAT, 1993; Norwegian Census of Agriculture,
1991; Bouzaher et al., 1994; Organization for Economic Co-
operation and Development, 1996) were used to fill some
of the gaps in this database. Extrapolation from the ear-
Data from various
liest available data was used to fill other gaps, which oc-
curred mainly during the 1950–1961 period. Annual maps of
land cover were produced by superimposing the fractions of
croplands/pastureswithamapofpotentialvegetationderived
from SYNMAP (potential SYNMAP, see the online materi-
als in Churkina et al., 2009).
Because of grid cell-wise inconsistencies between the crop
and pasture area of historical maps and the crop area of SYN-
MAP, adjustments of the fractions of the natural vegetation
types from SYNMAP are necessary. By using a map of re-
constructed natural vegetation compatible with SYNMAP or
potential SYNMAP, it is possible to account for preferential
conversion of natural vegetation types into crops or pastures
within each grid cell. The fraction of a natural vegetation
type (F) is calculated as:
F =Fpot−x·(Fpot−Fact)
The subscripts “pot” and “act” refer to the fractions of the
vegetation type for potential and actual SYNMAPs respec-
tively. The factor x scales the difference between the frac-
tions of potential and actual vegetation types. It is calcu-
lated as the ratio between the crop (CROPrec) and pasture
(PASTURErec) fractions of historical maps and the crop frac-
tion of SYNMAP (CROPSYNMAP):
x =(CROPrec+PASTURErec)/CROPSYNMAP.
2.2.4Drivers of O-CN
O-CN was driven at a 2◦×2◦spatial resolution with the same
atmospheric CO2and nitrogen deposition data, however, us-
ing observations of the monthly CRU meteorology directly
as input for 1901–2002 (Mitchell et al., 2004). Land cover
was assumed to be constant at 1995 levels (Loveland et al.,
2000).
2.3Models’ simulations
All models were initialized with the assumptions that the
ecosystem carbon stocks and fluxes were in equilibrium in
1700. To achieve this equilibrium, spin-up simulations were
performed with repeated MCRU climate for 1901–1930 and
constant CO2concentrations for 1700. In JULES and OR-
CHIDEE land use maps for 1700 were used in spin-up run.
In BIOME-BGC and O-CN land use maps for 2000 and
1995 respectively as well as pre-industrial nitrogen deposi-
tion were employed. The major reason behind different land
use maps used by models in equilibrium run is consistency
of that map with the map in the first year of transient simu-
lations. If these two maps are not consistent, a model may
simulate abnormal carbon fluxes in the first few decades of
transient simulations, which is a result of abrupt changes in
land use types. Because JULES and ORCHIDEE simulate
changes in land use, they used the year 1700 map for equilib-
rium simulations and annual land use maps from 1700 until
2000 for transient simulations as inputs. As BIOME-BGC
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G. Churkina et al.: Contemporary carbon balance of Europe2753
Table 2. Protocol of transient models’ simulations.
Model
Simulation
Climate
Change
Change in Atmospheric
CO2Concentrations
Change in Atmospheric
Depositions of NOy
and NHxa
Land Coverb
Conversion
Reference
Clim
Clim+CO2
Clim+CO2+LUC
Clim+CO2+N
No
Yes
Yes
Yes
Yes
No
No
Yes
Yes
Yes
No
No
No
No
Yes
No
No
No
Yes
No
aOnly for models including nitrogen cycle.
bOnly for models including effects of land cover conversion.
and O-CN do not incorporate land use changes in their simu-
lations, they used recent land use maps for both equilibrium
and transient simulations.
Three transient simulations were performed with different
combinations of environmental drivers for 1700–2007 (Ta-
ble 2). The first transient simulation, “Reference”, was per-
formed by four models without changes in climate, atmo-
spheric CO2concentrations, nitrogen deposition, and land
cover. In the “Clim” simulation only changes in climate were
implemented in all four models. In “Clim+CO2” simulation
four models were driven by changes in climate and rising at-
mospheric CO2. In “Clim+CO2+LUC” simulation JULES
and ORCHIDEE models were driven by changes in climate,
increasing CO2, and land cover conversion. BIOME-BGC
and O-CN models were driven by changes in climate, ris-
ing CO2, and nitrogen deposition in “Clim+CO2+N” simu-
lation. Because MCRU data set covered only time period
from 1861 until 2007, repeated climate for 1901–1930 was
used for model simulations from 1700 until 1860. After 2000
fractions of cropland/pasture were assumed to be constant
at 2000 values. Transient simulations of ORCHIDEE and
JULES without land use change were performed with land
cover for 1700.
The models with land use change used different algo-
rithms for conversion of land use types. ORCHIDEE pre-
scribed annual conversion of vegetation types from the sup-
plied dataset. In JULES, annual conversion of vegetation
types within each grid cell was modeled internally as de-
scribed above and was constrained by the prescribed land use
distributions.
2.4 Evaluation of changes in environmental drivers and
carbon balance estimates
Rates of change for all environmental drivers were calculated
for Europe over the first and second half of the 20th cen-
tury. We divided the 20th century into two periods (before
and after 1950) to reflect different trends in the environmen-
tal drivers. Fast industrial growth in Europe and also in some
other parts of the world was observed after the Second World
Warwhichendedin1945. Countrieswererebuildingtheirin-
dustries and population was growing. Emissions of CO2and
NOxfrom energy production were rising fast. To meet the
dietary needs of growing population, production of synthetic
nitrogen fertilizers started in 1950’s after the discovery of
Haber-Bosch process in 1930’s. Production and application
of fertilizers increased amount of available inorganic nitro-
gen.
We evaluated responses of land ecosystems to environ-
mental drivers for the second half of 20th century, because
we aimed at understanding the roles of different drivers in
theEuropeancarbonsink. Furthermore, moreconfidencecan
be placed into recent trends of land-cover changes because of
the much increased level of documentation in land-use statis-
tics.
The effects of individual drivers on the terrestrial carbon
balance were attributed by comparing the different factorial
experiments. For example, the effect of land-cover conver-
sion was inferred by comparing simulations with climate and
CO2changes to that accounting for these changes in addition
to land-cover changes.
To test statistical significance of model responses to indi-
vidual environmental drivers, we have performed a Welch
t-test of the model outputs, because this test does not re-
quire that the variances of the samples are equal. Welch t-test
was applied to the net ecosystem exchange and carbon stock
changessimulatedwithfactorialexperimentsfor1951–2000.
For each factorial experiment, we tested the hypotheses that
the output from each model was equal to zero and that the
average simulated net ecosystem exchange or stock changes
were similar between the individual models. Statistical sig-
nificance was tested with a threshold value of 0.05.
3 Results
3.1 Rates of change in major environmental drivers
In the 20th century two periods with different rates of
change in major environmental drivers of Europe can
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2754G. Churkina et al.: Contemporary carbon balance of Europe

2
1900
3
4
5
6
7
8
9
10
1910 19201930 19401950 1960 1970 19801990 2000 2010
year
Tg/yr
0.1Tg/yr
0.03Tg/yr



0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
1900191019201930 19401950 196019701980 1990 20002010
year
mln sq km
crop+pasture
forest
grass
-7000 sq km/yr
-6900sq km /yr
10100 sq km/yr
-3200 sq km/yr
5800 sq km/yr
1100 sq km/yr
Land cover conversion
Air temperature
6
1900
6.5
7
7.5
8
8.5
9
9.5
19101920 193019401950 19601970 19801990 2000 2010
year
deg C
0.02 degC/yr
0.01 degC/yr
Atmospheric nitrogen deposition
Atmospheric CO2 concentration
270
290
310
330
350
370
390
1900 1910192019301940 195019601970 1980 1990 20002010
year
ppm
1.11 ppm/yr
0.29 ppm/yr
Fig. 1. Changes in major environmental drivers of carbon balance of Europe over 20th century. Annual values of average air temperature,
atmospheric nitrogen deposition, and land cover conversion are calculated for the whole study domain. Atmospheric CO2concentrations are
calculated as global averages.
be distinguished: a period of slow changes from 1900
to about 1950 and a period of faster changes thereafter
(Fig. 1). Between 1900 and 1950, atmospheric CO2con-
centrations, nitrogen deposition, and temperatures increased
only moderately. Their rates of change were 0.01◦Cyr−1,
0.03TgNyr−1, and 0.29ppmyr−1respectively.
(4.9millionkm2in 1900) and grassland (0.75millionkm2
in 1900) extents declined with the rates of 6900km2yr−1
(0.14%yr−1) and 3200km2yr−1(0.67%yr−1), respectively.
The area of land in agricultural use such as cropland and
pasture (3.8millionkm2in 1900) increased with the rate
of 10100km2yr−1(0.27%yr−1). After 1950, atmospheric
CO2concentrations, nitrogen deposition, and air tempera-
tures increased at rates 2–3 times higher than in the first half
of 20thcentury. Their respective rates were 0.02◦Cyr−1,
0.1TgNyr−1, and 1.11ppmyr−1for the period 1950–2000.
To the contrary, the rate of land cover conversion slowed
down, with the agriculture coverage declining at a rate of
7000km2yr−1(0.18%yr−1). Forest and grassland areas ex-
panded with the rates of 5800km2yr−1(0.12%yr−1) and
1100km2yr−1(0.24%yr−1), respectively.
Overall changes in land cover of Europe (9.32 million
km2) were relatively small over the 20th century. The frac-
tion of European forests shrank by approximately 3% before
1950 and expanded thereafter by the same amount. Propor-
tion of agricultural land increased by 5% in the first half of
20th century and dropped by 4% after 1950. Changes in the
Forest
fractions of grasslands were in the order of 1–2% over the
first and the second halves of the 20th century.
3.2 Carbon balance and its attribution
Until approximately 1960, the average carbon balance of Eu-
ropean terrestrial ecosystems estimated by all models was
close to zero (Fig. 2). From the 1960–70’s onwards, Eu-
ropean ecosystems were sequestering atmospheric CO2 at
an average rate of 85TgCyr−1in 1980’s, 108TgCyr−1in
1990’s, and 114TgCyr−1in 2000–2007. These estimates
are subject to considerable interannual variability (69, 79,
and 54TgCyr−1respectively; Table 3, Fig. 2), as was also
shown earlier (Zaehle et al., 2007; Vetter et al., 2008).
The respective rates of changes in atmospheric CO2 con-
centrations were 1.38ppmyr−1in 1980’s, 1.39ppmyr−1in
1990’s, and 1.71ppmyr−1in 2000–2007. The respective
rates of changes in nitrogen deposition were 0.06TgNyr−1,
0.09TgNyr−1, and 0.09TgNyr−1. In the second half of
the 20th century European ecosystems sequesterd carbon
at a rate of 56TgCyr−1(mean of four models for 1951–
2000) with strong interannual variability (±88TgCyr−1, av-
erage across models) and substantial inter-model uncertainty
(±39TgCyr−1). Below we describe the effect of environ-
mentaldriversonlandcarbonuptakeafter1950andthepools
in which carbon was accumulating.
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G. Churkina et al.: Contemporary carbon balance of Europe2755
Table 3. Mean carbon balance of Europe from different land-based data compilation and model simulations. The inter-model uncertainty is
quantified as the standard deviation of the modeled average carbon uptake for a given period. The inter-annual variability is the inter-model
mean of the variability of the annual means for each model.
Net Carbon Uptake
TgCyr−1
Uncertainty
TgCyr−1
Area
106km2
Time
Period
Modeled (this study,
Clim+CO2+LUC/N)
56
100
85
108
114
157
30
235
111
±39 inter-model difference; ±88 inter-annual variability
±45 inter-model difference; ±85 inter-annual variability
±26 inter-model difference; ±69 inter-annual variability
±74 inter-model difference; ±79 inter-annual variability
±19 inter-model difference; ±54 inter-annual variability
70–230 inter-model range
−45–106 inter-annual variability range
±50
±280
9.321951–2000
1980–2007
1980–1989
1990–1999
2000–2007
1980–2005
1990–1999
2000–2005
unspecified
Modeled (Vetter et al., 2008)
Modeled (Zaehle et al., 2007)
(Schulze et al., 2009)
(Janssens et al., 2003)
9.32
3.7
9.29
10.4
−600
−400
−200
0
200
400
−600
−400
−200
0
200
400
Net Land Atmosphere Flux [Tg C yr−1]
1900 19201940 196019802000
year [A.D.]
Ecosystem models: mean
Ecosystem models: range
Ecosystem data compilation
Atmospheric models: mean and uncertainty
Fig. 2. European net land-atmosphere carbon flux in 20th century
estimated with ecosystem models. Red line is an average value es-
timated with three ecosystem models (BIOME-BGC, ORCHIDEE,
and JULES). The gray shaded area depicts the models’ range. Mod-
eled carbon balance at the end of the 20th century is lower than the
mean value of carbon balance from the ecosystem model compila-
tion (blue square) or the inverse estimations of atmospheric models
(blue dot) (Schulze et al., 2009) reported as an average for 2000–
2005.
3.2.1Effects of environmental drivers
The overall effect of rising carbon dioxide, changing climate,
as well as land cover conversion or rising nitrogen on the av-
erage net carbon uptake for 1950–2000 was positive in all
models, implying a net storage of 25–94TgCyr−1. All mod-
els indicated that European terrestrial ecosystem sequestered
carbon as a result of the interactions between several envi-
ronmental factors (Fig. 3). The total simulated carbon sink in
Europe was significantly different from zero and similar be-
tween JULES, BIOME-BGC, and O-CN models – at least if
one considers only the European-wide mean values and their
interannual variability. ORCHIDEE simulated response of
NEE to all factors, which was not significantly different from
zero.
In BIOME-BGC and O-CN, which account for carbon-
nitrogen dynamics, the European carbon sink was the result
of CO2fertilization and nitrogen deposition, as well as inter-
actions between these factors. CO2fertilization of ecosys-
tems was the most important factor responsible for the net
carbon uptake simulated in models only accounting for the
carbon cycle. This CO2fertilization effect more than offset
the negative effects of land cover conversion on terrestrial
carbon storage as simulated by ORCHIDEE and strength-
ened the positive effect of land cover conversion in JULES.
Changes in climate (mostly due to rising temperatures) re-
sulted in a land carbon source in the two models including
for carbon cycle only, due to high soil carbon losses (see
Fig. 4). Responses of net carbon flux to changes in climate
were significantly different from zero in simulations with
JULES and ORCHIDEE, but not in BIOME-BGC and O-
CN simulations. Climate had hardly any effect on the carbon
balance in models accounting for carbon-nitrogen dynamics
(Fig. 3). Models with carbon-nitrogen dynamics simulated
very small net carbon uptake (3–6TgCyr−1), while mod-
els with carbon cycle only suggest a net carbon source (30–
80TgCyr−1) during 1950–2000. Changes in climate during
this time period were mostly associated with rising temper-
atures, which lead to increases in both CO2release as well
as soil nitrogen mineralization from accelerated soil organic
matter decomposition. Because nitrogen is the major limit-
ing nutrient for plant growth in high and mid latitudes, more
available nitrogen in soil enhances plant CO2uptake, which
compensates for the higher CO2emissions from soil decom-
position. This is exactly the case in the responses of models
with carbon-nitrogen dynamics to changes in climate. Mod-
els with carbon cycle only lack this compensation mecha-
nism and respond to rising temperatures with increased CO2
release only.
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2756G. Churkina et al.: Contemporary carbon balance of Europe
Climate CO2
LCC
N−depositionAll Factors
1951−2000
−300
−200
−100
0
100
200
300
−300
−200
−100
0
100
200
300
Land Atmosphere Flux [Tg C yr−1]
Land Atmosphere Flux [Tg C yr−1]
JULES
ORCHIDEE
BIOME−BGC
OCN
Fig. 3. Changes in European land-atmosphere carbon fluxes over
the period 1951–2000 attributed to climatic changes, increases in
atmospheric CO2, land-cover changes (LCC), nitrogen deposition,
and three factors combined. Land-atmosphere carbon flux was esti-
mated with four ecosystem models such as JULES, ORCHIDEE,
BIOME-BGC, and O-CN. Two models including N cycle (solid
shaded bars) show better agreement than the models including land
cover conversion (bars with diagonal stripes). Each bar depicts the
annual change in terrestrial C storage averaged over 1951–2000,
with the error bars denoting the standard deviation of the change.
Increasing CO2enhanced net land carbon uptake and it
was significantly different from zero in all models.
response of land ecosystems to rising CO2 was consid-
erably larger in models without nitrogen dynamics (110–
230TgCyr−1) than in models with nitrogen dynamics (30–
65TgCyr−1). Land cover conversion had opposite net ef-
fects on the carbon balance simulated by ORCHIDEE and
JULES. Land cover changes in JULES led to a small net car-
bon uptake (15TgCyr−1), which was not significantly dif-
ferent from zero. In ORCHIDEE land ecosystems responded
with the substantial net release of carbon (115TgCyr−1).
Nitrogen deposition enhanced ecosystem net carbon uptake
between 30 and 37TgCyr−1. Both models with carbon-
nitrogen dynamcs had consistent responses to rising nitrogen
deposition.
The
3.2.2Changes in carbon pools
In land ecosystems, carbon can accumulate in soil or in veg-
etation or in both. Our study points to changes in the growth
of woody vegetation as an important contributor to the Eu-
ropean carbon sink (Fig. 4). Three out of four models re-
ported that vegetation accumulated most of the additional
carbon. Model simulations of ORCHIDEE, BIOME-BGC,
and O-CN indicated that carbon was accumulated in both
vegetation and soil. In these three models vegetation was
a stronger sink (20, 60, 60TgCyr−1respectively) than soil
(10, 5, 25TgCyr−1respectively). JULES simulations indi-
cated that soil stored all additional carbon (100TgCyr−1),
while vegetation was a small carbon source (4TgCyr−1).
Δ C [Tg C yr−1]
−150
−100
−50
0
50
100
a)
JUL
ORCBGCOCN
Δ C [Tg C yr−1]
−50
0
50
100
150
200
250
b)
JUL
ORC BGCOCN
Δ C [Tg C yr−1]
−150
−100
−50
0
50
100
c)
JUL
ORCBGCOCN
Δ C [Tg C yr−1]
−150
−100
−50
0
50
100
d)
JUL
ORCBGC OCN
Δ C [Tg C yr−1]
−150
−100
−50
0
50
100
JUL
ORCBGCOCN
e)
Veg CSoil CTotal C
Fig. 4. Changes in soil and vegetation carbon pools in response
to climate (a), atmospheric carbon dioxide concentration (b), land
cover change (c), atmospheric nitrogen deposition (d), and three
factors together (e). Each bar represents annual change in carbon
stock averaged over 1951–2000. Carbon stocks are estimated with
JULES, ORCHIDEE, BIOME-BGC, and O-CN.
The differences in ecosystem pools accumulating carbon
stem from differences in ecosystem responses to individ-
ual environmental drivers. In the model simulations with
changing climate only, soil carbon pool was significantly af-
fected in all models (Fig. 4). In JULES and ORCHIDEE,
rising temperatures led to soil carbon releases that averaged
at 37 and 65TgCyr−1respectively over the period 1951–
2000. Soil carbon loss was not compensated for by small
vegetation carbon gain in JULES (7TgCyr−1). Soil car-
bon loss was enhanced by vegetation carbon loss in OR-
CHIDEE, BIOME-BGC, and O-CN accounted for the feed-
back of increasing temperatures on increased nitrogen min-
eralization and therefore improved plant nutrition. Therefore
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G. Churkina et al.: Contemporary carbon balance of Europe2757
these models simulated a stronger vegetation carbon gain (13
and 3TgCyr−1respectively) and weaker soil carbon loss
(10 and 5TgCyr−1respectively), resulting in approximately
zero carbon balance. Changes in vegetation carbon stocks
were not statistically significantly different in JULES and
BIOME-BGC simulations for climate change experiment.
Changes in vegetation stocks of all other model combina-
tions were statistically different from each other and from
zero. Changes in total carbon stocks simulated by BIOME-
BGC were not statistically significant from zero for climate
change scenarios, respectively.
All models agreed that rising atmospheric CO2concen-
trations lead to carbon accumulation in both soil and vege-
tation. In JULES and ORHIDEE less carbon was accumu-
lated in vegetation (44 and 107TgCyr−1respectively) than
in soil (65 and 120TgCyr−1respectively). The increase in
carbon storage in response to CO2outweighed the decrease
due to climate change in all models as previously found in
JULES by Harrison et al. (2008). Conversely, BIOME-BGC
and O-CN simulated higher increase in carbon storage in
vegetation (21 and 40TgCyr−1respectively) than in the soil
(8TgCyr−1). These differences are related to various rep-
resentations of tree mortality and soil carbon turnover rates
in the models. Changes in total carbon stocks in this exper-
iment have been significantly different from zero and from
each other for individual models.
In the simulations with land cover conversion, both
JULES and ORCHIDEE simulated a similar rate of carbon
loss due to the removal of vegetation (55 and 70TgCyr−1,
respectively). The models disagreed strongly on the fate of
carbon in the soil pool. While JULES simulated substantial
soil carbon gain (70TgCyr−1), which more than offset the
rate of carbon lost by vegetation removal, ORCHIDEE sim-
ulated net soil carbon losses (45TgCyr−1). Changes in to-
tal carbon stocks simulated by JULES were not significantly
different from zero in response to land use changes.
3.2.3Spatial patterns of NEE and NBP (1951–2000)
The models generate substantially different patterns of net
carbon sources and sinks when averaged over 50 years
(Fig. 5). BIOME-BGC shows rather homogeneous carbon
sink over Europe with several patches of high carbon sink
where high nitrogen deposition rates overlap with forests.
JULES and ORCHIDEE have very heterogeneous distribu-
tions of carbon sources and sinks as a result of various re-
sponses to the land cover conversion and harvest decomposi-
tion in these model simulations. If we do not account for the
effect of land-cover conversions in these models, the NEE
patterns are rather homogeneous in both models and simi-
lar to the estimates of BIOME-BGC (suppl. Fig. S1), even
though absolute NEE values in these models are substantially
higher than in BIOME-BGC.
The aforementioned uncertainties in the simulation of har-
vest as well as in where and how fast carbon from harvested
biomass is released back to the atmosphere, cause different
patterns of NBP (Fig. 6). ORCHIDEE simulates a large car-
bon source in Eastern Europe, which patterns seem to coin-
cide with areas of continuing agricultural expansion.
4Discussion
4.1Carbon balance of European ecosystems
Previous observation-based (Schulze et al., 2009; Janssens
et al., 2003) and modeling studies (Vetter et al., 2008; Za-
ehle et al., 2007) suggest a substantial net carbon uptake in
European ecosystems over the last decades. Although the
comparable estimates of land-atmosphere carbon fluxes from
this study confirm ecosystem carbon uptake, our estimates
are lower, than those based on extrapolated field studies and
previous model estimates (Table 3). The most recent bottom-
up estimate of the European net carbon uptake from data
compilation is 235±50TgCyr−1for 2000–2005 (Schulze
et al., 2009). This estimate is higher than our estimate of
114TgCyr−1for 2000–2007 as well as the previous estimate
of111±280TgCyr−1fromdatacompilationforunspecified
period (Janssens et al., 2003). High carbon uptake estimated
for forests and grasslands as well as almost negligible carbon
source for croplands (33TgCyr−1) are the possible reasons
for high carbon uptake estimated by (Schulze et al., 2009).
Croplands emitted 300TgCyr−1in the previous compilation
report (Janssens et al., 2003).
Estimates of the European carbon balance based on the in-
version of atmospheric CO2measurements by atmospheric
transport modeling give a wide range of the net carbon stor-
age rates. The most recently published estimate of land-
atmosphere flux from atmospheric inversions averages at
−313±342TgCyr−1(Schulze et al., 2009), with the uncer-
taintyestimatebeingthequadraticsumofthespreadbetween
individual inversions and the uncertainties in each inverse es-
timate. TheearlierinverseestimatessummarizedbyJanssens
et al. (2003) are at a comparable level, suggesting a net car-
bon uptake of 290TgCyr−1(80–560TgCyr−1). While there
isasubstantialdifferenceof213TgCyr−1betweentheatmo-
spheric and ground-based modeled estimates of mean carbon
NEE for 2000–2007, the ground-based model based estimate
easily falls into the range of the atmospheric inversions.
Based on the results of seven vegetation models, Vetter
et al. (2008) estimated the net carbon balance of European
ecosystems to be between 70 and 230TgCyr−1. This car-
bon balance was calculated as a mean of annual NEP for
1980–2005 from models’ simulations driven by REMO cli-
mate data and did not include effects of land cover conver-
sion, but nitrogen deposition in BIOME-BGC model. Zaehle
et al. (2007) used one vegetation model (LPJ) to estimate
the effect of land-cover changes, climate, and CO2on the
Western European terrestrial carbon balance. The resulted
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2758G. Churkina et al.: Contemporary carbon balance of Europe

ORCHIDEE -NBP
JULES- NBP
BIOME-BGC - NEE
Fig. 5. Cumulative net land-atmosphere fluxes (kgC per 50 years, sum over 1951–2000) estimated with three ecosystem models. NBP for
JULES and ORCHIDEE refer to a simulation accounting for climatic changes, increases in atmospheric CO2concentrations, and land use
changes (Clim+CO2+LUC). The NEE simulated by BIOME-BGC is driven by climatic changes, increases in CO2and atmospheric nitrogen
deposition (Clim+CO2+N). Areas colored in different shades of red and yellow are carbon sink. Areas colored in different shades of blue
are carbon source.
net carbon uptake of 30TgCyr−1is compatible with our es-
timates given the substantially smaller spatial domain.
4.2Attribution
What are the processes driving the European land net car-
bon uptake? Previous observation-based studies analyzed
contributions of land cover types or forest processes to this
sink. Schulze at al. (2009) and Janssens et al. (2003) consis-
tently suggest that there is a net carbon uptake in the Euro-
pean ecosystems mostly because of carbon gains in forests
and grassland soils, which are partly offset by losses of car-
bon from croplands and peat soils. Schulze at al. (2009)
also quantify land use change as an additional carbon sink
(discussed below). As we have discussed the effect of cli-
mate and climate variability in a previous paper (Vetter et
al., 2008), here we focus our discussion on the effects of the
other transient changes in driving forces of the terrestrial car-
bon balance.
Large differences in responses of the carbon balance to
changes in single drivers between models (Fig. 3) are related
to inclusion/omission of carbon-nitrogen interactions in the
models and differences in modeled processes related to land
use change. Simulated ecosystem responses were more con-
sistent for the two models accounting for carbon-nitrogen
dynamics than for the two models including carbon cycle
and the effects of land use change. The latter is because
the carbon–nitrogen interactions are easier to upscale than
the carbon- land-use change interactions. Below we discuss
modeled responses of land-atmosphere carbon flux to indi-
vidual environmental factors and their plausibility in detail.
4.2.1Atmospheric CO2concentrations
Because very few studies (Hamilton et al., 2002) have at-
tempted to quantify the effect of elevated CO2on NEE, we
focus the discussion on the response of NPP, which is the pri-
mary cause for changes of the modeled net carbon uptake in
response to increase in atmospheric CO2. This comparison is
challenging because experimental designs of field and model
experiments are not the same. In the model experiment ter-
restrial ecosystems have been exposed to continuously ris-
ing CO2over 200 years. Field experiments impose a 6–10
years “step” increase of CO2to at least 200ppm above re-
cent ambient levels of atmospheric CO2. The modeled re-
sponse of European NPP to 100ppm increase in atmospheric
CO2concentrations (from 287ppm in 1870 to 387ppm in
2007) was 10% (BIOME-BGC), 20% (O-CN and JULES),
and 36% (ORCHIDEE). Above-ground dry matter produc-
tion increased 20% on average for 29 C3 species grown in
six different Free Air CO2Enrichment (FACE) experiments
(Ainsworth and Long, 2005). The response of the above-
ground dry matter production reported in field studies ranges
from 10% for C3 grasses to 28% for forests to 190–200ppm
increase of CO2(from ambient CO2concentration in 1990’s
of360ppmtomaximum550–600ppmofCO2). Treesgrown
under nutrient limitations had an insignificant 14% stimula-
tion in above-ground biomass accumulation (Ainsworth and
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G. Churkina et al.: Contemporary carbon balance of Europe2759


BIOME-BGC
JULES
ORCHIDEE
Fig. 6. Cumulative land-atmosphere fluxes (kgC per 50 years, sum over 1951–2000) resulted from land use conversion and rising nitrogen
deposition. These fluxes are calculated as difference between NEE from two model experiments Clim+CO2+LUC and Clim+CO2for
ORCHIDEE and JULES and as difference between Clim+CO2+N and Clim+CO2for BIOME-BGC. Areas colored in different shades of
red are carbon sink; areas colored in different shades of blue are carbon source.
Long, 2005). Field studies suggest that plant growth re-
sponse to elevated CO2likely slows over time, probably be-
cause of reduced nitrogen availability (Hungate et al., 2006).
The latter effect explains weaker response to increasing CO2
in BIOME-BGC or O-CN than in ORCHIDEE or JULES.
4.2.2 Atmospheric nitrogen deposition
Soil nitrogen status as well as frequency and intensity of ni-
trogen additions play important roles in the ecosystem’s re-
sponse to increased deposition of atmospheric nitrogen. In
field experiments 1–1000kgNha−1is added one-two times
per year. These frequencies and magnitudes of nitrogen ad-
dition cannot be directly compared to the gradual increase in
nitrogen deposition from atmosphere (2–28kgNha−1yr−1)
in “undisturbed” ecosystem such as those represented in our
numerical simulations. Between 1860 and 2007 the modeled
European NPP increased on average by 7.5% in BIOME-
BGC and by 16% in O-CN in response to the rise in average
nitrogen deposition rate of 5kgNha−1yr−1. This response
seems plausible given the evidence from field studies. Obser-
vational studies suggest that aboveground NPP increased by
approximately 28–29% in the cross-biome analyses of terres-
trial plants (LeBauer and Treseder, 2008; Elser et al., 2007)
in fertilizer experiments. Taking into account uncertainties
about the response of belowground NPP which might com-
pensate for the increase in aboveground growth, this estimate
gives the upper bound of the likely response of total net pri-
mary production as simulated by the models. The average
biomass response to low nitrogen additions of approximately
10–50kgNha−1yr−1was considerably weaker for woody
than for herbaceous plants or 24.6% and 50% respectively
(Xia and Wan, 2008). In opposite, modeled NPP response
of herbaceous plants (6%) to nitrogen additions was weaker
than for deciduous forests (10%), but stronger than for conif-
erous forests (3%) in the BIOME-BGC model. Differences
in forest responses can be explained by different average
rates of nitrogen deposition over coniferous and broadleaf
forests (4 and 11kgN/ha/yr respectively). Modeled NPP re-
sponse of herbaceous vegetation is underestimated, because
average nitrogen depositions over herbaceous vegetation and
deciduous forest were comparable (6–10kgCha−1yr−1and
11kgCha−1yr−1). This underestimation is most likely re-
sponsible for lower European NPP response to rising nitro-
gen deposition in BIOME-BGC model as compared to O-
CN.
A number of recent studies (Magnani et al., 2007; Sut-
ton et al., 2008; de Vries et al., 2009) analyzed the response
of net carbon storage in forest ecosystems to nitrogen depo-
sition using manipulation experiments and other streams of
data. The most recent study (de Vries et al., 2009) reports
the response of 5–75kgC/kgN for both forests and heath-
lands after discounting for potential interaction effects due
to concomitant changes in other environmental factors. The
mean responses of BIOME-BGC (43kgC/kg N) and O-CN
(38kgC/kg N) fall well into this range.
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2760 G. Churkina et al.: Contemporary carbon balance of Europe
4.2.3Land cover conversion
The estimates of changes in the net land-atmosphere carbon
flux associated with land cover conversion and management
depend on the accuracy of the estimates of past land-use
changes, the simulated biomass of the vegetation which is
replaced, and on the ecosystem response to the change in
land use type. Consistent data of historical land use changes
are very sparse at a continental scale, so that substantial un-
certainty is inherent in any backward projection of land-use
patterns (Ramankutty and Foley, 1999; Hurtt et al., 2006).
These data typically only record net changes in a land-cover
type within a region. They do not specify co-occurring de-
and reforestation of this region. Because these two paral-
lel processes determine the net carbon flux due to land-use
changes, our model results provide a low bound of the effect
of land-use changes on the carbon balance.
In 1950–2000 conversions of croplands into forests and
grasslands prevailed in Europe (Fig. 1). Based on meta-
analysis of experimental studies Guo and Gifford (2002) sug-
gested increase in soil carbon after changes from crop to
pasture or to forest plantation (+18–19%) and from crop to
forest (+54%). The change in soil carbon stock as a result
of land cover conversion depends on the sizes of the soil
carbon pools of the land cover types at equilibrium. Crop-
land soils have lower carbon stocks than grasslands, whereas
forest and grassland soils have similar pool sizes (Guo and
Gifford, 2002). Although JULES and ORCHIDEE models
are able to simulate such differences, models had substan-
tially different responses to land cover conversion (Fig. 3).
JULES simulated a small net carbon uptake in land ecosys-
tems (15TgCyr−1). In ORCHIDEE ecosystems responded
with a substantial net release of carbon (115TgCyr−1). Us-
ing data compilation Schulze et al. (2009) estimated a net
sinkof60TgCyr−1associatedwithsoilcarbongainandloss
following land use change in Europe. This discrepancy re-
sults from uncertainty in the fate of residuals remaining in
the ecosystem upon conversion and in the representation of
the differences between soil carbon stocks of different land-
use types in the model. The amount of carbon which enters
the soil carbon pools as residuals increases the potential for
longer-term increases in soil organic matter with time. We
suggest that improved understanding of the fate of carbon is
response to land-use change is a key requirement for future
research.
ORCHIDEE suggests that vegetation is the major store of
extra carbon, while the results of JULES imply that carbon
storage in soils dominates. Model simulations give differ-
ent answers because of different model algorithms. In OR-
CHIDEE, forests re-grow on abandoned agricultural areas
following the land cover conversion prescribed in this study.
Regrowing forests first accumulate carbon in wood, which
only later propagates into increases in soil carbon due to the
reduced carbon export in forests relative to highly managed
cropland ecosystems. Therefore, the vegetation pool con-
tributes stronger to the carbon storage in the ORCHIDEE
model. In JULES, while net conversion rates were used as
given from in the modeling protocol, the internally simu-
lated vegetation dynamics (see Methods section) resulted in
a different fate of the land from cropland abandonment. In
JULES agricultural contraction after 1950 implies grassland
replace cropland with a longer timescale for eventual succes-
sion by forest. The major difference between these two veg-
etation types in JULES is that crops are regularly harvested,
while grasslands are not. Once grassland replaces cropland
all aboveground carbon enters soil pool after litter fall and
soil carbon increases in this model, whereas the vegetation
carbon pool hardly changes. Next generation vegetation dy-
namics models which explicitly treat disturbance, succession
and age-structure are required to more realistically quantify
this response.
4.3 Uncertainties in the modeled carbon balance
The present study provides the latest of various model as-
sessments in which we addressed various uncertainties in
modeled components of carbon balance. We have previ-
ously assessed ability of the models to replicate the ecosys-
tem responses to regional gradients in climate (Jung et al.,
2007a)andlargescaleclimateanomalies(Vetteretal., 2008),
identifying biases of individual models and key uncertain-
ties in regional scale carbon balance from input drivers (Jung
et al., 2007b). Among input drivers climate data has been
identified as the dominant source of uncertainty (Jung et
al., 2007b). The ecosystem models systematically under-
estimated the decrease in gross photosynthetic uptake from
temperate to boreal forest sites (Jung et al., 2007a). This
underestimation was attributed to insufficiently accounting
for nitrogen limitation that acts mainly on leaf area and thus
light absorption. An experiment combining ecosystem mod-
els with atmospheric transport models indicated that mod-
eled CO2concentrations were biased at the measurements
stations, which are located in large agricultural regions of
Hungary (Hegyhatsal station) and the Netherlands (Cabaw
station)(U.Karstens, personalcommunication, 2009). These
biases point to difficulties in capturing the heterogeneity of
agricultural landscapes and in modeling cropland carbon dy-
namics influenced by land management.
Change in land management could be the other reason be-
hind carbon balance change, which we did not account for
in this study. Discrepancy between timber harvest and for-
est productivity has been discussed as a possible reason be-
hind carbon accumulation in European ecosystems. Analy-
sis of forest inventories (Nabuurs et al., 2003; Ciais et al.,
2008b) points to forests as a potential sink of carbon. These
studies suggest that the slow increase in timber harvests in
comparison to the rapid increase in forest ecosystem pro-
ductivity is the key driving factor behind this trend, causing
forest tree biomass in the EU-15 plus Norway and Switzer-
land to accumulate 2.3Pg of carbon between 1950 and 2000.
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G. Churkina et al.: Contemporary carbon balance of Europe2761
NPP of needleaf forests from three ecosystem models used
in this study was lower in than from forest inventory based
model (Tupek et al., 2010), which included forest age and
management effects on NPP. This comparison indicates that
forest carbon uptake may have been underestimated in this
study. A modeling study analyzing forest inventories, for-
est use statistics, climatic changes, and CO2increases sug-
gests that the effects of changes in forest management, forest
area and age-structure explain 50% of the increase in forest
biomass for EU-25 (Zaehle et al., 2006). Although past land-
management changes further affect soil carbon inputs or soil
carbon turnover times and their effect is poorly quantified for
large regions (Zaehle et al., 2007). Model simulations show
that alternative assumptions about crop management and the
fate of residues notably alter the soil carbon stock and tra-
jectories following conversion (Bondeau et al., 2007; Smith
et al., 2005, 2006). These factors offset temperature-related
soil carbon losses from simulated in this study. Because of a
lack of data to parameterize land management changes over
time, these effects have not been considered in the present
study.
The interactions between individual driving forces of car-
bon balance complicate accurate attribution of the simulated
trends to these forces. For instance, compensating effect
of increasing nitrogen availability in soil due to enhanced
atmospheric nitrogen deposition versus decreased nitrogen
availability in soil due to fertilizing effect of CO2on plant
growth, as discussed by Churkina et al. (2009) and Zaehle et
al. (2010). None of the models in this study can yet repre-
sent all of the forcings we consider, and so we are not able to
quantify the effect of possible interactions between nitrogen
limitation and land-use change. Disentangling these effects
requires multi-factorial model simulations able to respond to
the non-linear factor interactions. For instance by running
a simulation with only the factor of interest varying or by
calculating the difference between two simulations, one with
all factors and a second, in which all but the factor of inter-
ests vary with time. Doing so has not been feasible within
the present analyses. Hence, our results allow identifying the
relativeimportanceofthedrivingforces, buttheabsoluteval-
ues need to be treated with caution. Interaction effects may
shift individual contribution up or down by a few percent,
depending on the way the individual contribution has been
calculated.
Here we did not investigate uncertainties related to the
lateral carbon fluxes driven by soil erosion (Quinton et al.,
2010) or by relocation of forest and crop harvests (Ciais et
al., 2008a). None of the models here was capable to simulate
soil erosion. JULES and ORCHIDEE simulated forest and
crop harvest and their decomposition. None of them how-
ever simulated harvest relocation here.
5 Conclusions
The model projections consistently suggest that the Euro-
pean ecosystems acted as net carbon storage during the pe-
riod 1951–2007. The models suggest a slight increase in
the decadal mean uptake from 85TgCyr−1in the 1980s to
114TgCyr−1in the 2000s. Increases in atmospheric CO2
enhanced the carbon uptake across all models. Models ac-
counting for carbon-nitrogen dynamics consistently simu-
lated a weaker response to increases in atmospheric CO2
than models without nitrogen cycle included. The positive
response of net carbon uptake to increased atmospheric in-
put of reactive nitrogen only partly compensated the differ-
ence in uptake between two model types. In 1951–2000 land
cover conversion increased European carbon stock in veg-
etation. Large uncertainty exists however in its impact on
soil carbon because of uncertainties in the fate of soil car-
bon upon conversion. It is likely that changes in manage-
ment may have affected these trends, potentially even over-
riding the effect of land cover conversion. Closing this gap
requires advancing the existing modeling approaches as well
as collecting and harmonizing the information about land
management regimes at the scale of the European continent.
Only then it will be possible to determine whether the conse-
quences of management explain the remaining difference be-
tweenthemodelbased-estimatesofourstudy, estimatesfrom
ground-based data compilation (235±50TgCyr−1), and at-
mospheric observations (313±342TgCyr−1) (Schulze et
al., 2009).
Supplementary material related to this
article is available online at:
http://www.biogeosciences.net/7/2749/2010/
bg-7-2749-2010-supplement.pdf.
Acknowledgements. This article benefited from the work done
undertheCARBOEUROPE
505572) funded by the European Commission. The contribution
of John Hughes and Chris Johns was also supported by the
Joint DECC/Defra Met Office Hadley Centre Climate Program
(GA01101). The authors are grateful for helpful discussions with
Christian R¨ odenbeck and Ute Karstens.
project (No. GOCE-CT-2003-
The service charges for this open access publication
have been covered by the Max Planck Society.
Edited by: U. Seibt
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Page 14

2762G. Churkina et al.: Contemporary carbon balance of Europe
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