Estimating carbon inputs to soil in fora

Estimating carbon inputs to soil in forage-based crop
rotations and modeling the effects on soil carbon dynamics
in a Swedish long-term field experiment
M. A. Bolinder1,2, T. Ka¨tterer1, O. Andre´n3, and L. E. Parent2
1

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Swedish University of Agricultural Sciences (SLU), Department of Soil and Environment, Box 7014, S-75 007
Uppsala, Sweden (e-mail: martin.bolinder@slu.se); 2Department of Soils and Agrifood Engineering, Universite´
Laval, 2425 rue de l’Agriculture, G1V 0A6 Que´bec, Canada; and 3oandren.com, Bjo¨rklundav. 3, S-756 46 Uppsala,
Sweden. Received 10 April 2012, accepted 31 July 2012.
Bolinder, M. A., Ka¨tterer, T., Andre´n, O. and Parent, L. E. 2012. Estimating carbon inputs to soil in forage-based crop
rotations and modeling the effects on soil carbon dynamics in a Swedish long-term field experiment. Can. J. Soil Sci. 92:
821
833. There is a need to improve the understanding of soil organic C (SOC) dynamics for forage-based rotations.
A key requisite is accurate estimates of the below-ground (BG) C inputs to soil. We used the Introductory Carbon Balance
Model (ICBM) to investigate the effects of C input assumptions on C balances with data from a 52-yr field experiment in
northern Sweden. The main objective was to validate an approach for estimating annual crop residue C inputs to soil using
the data from a continuous forage-based rotation (A). A rotation with only annual crops and more frequent tillage events
(D) was used to obtain a rough estimate of the effect of tillage on SOC dynamics. The methodology used to estimate annual

crop residue C inputs to soil gave a good fit to data from four out of the six large plots for rotation A. The approximate
effects of more frequent tillage in rotation D increased SOC decomposition rate by about 20%. These results allow us to
have more confidence in predicting SOC balances for forage-based crop rotations. Root biomass measurements used for
calculating BG C inputs were also reviewed, and we show that they have not changed significantly during the past 150 yr.
Key words: Introductory Carbon Balance Model concept, cool temperate climate, roots, carbon sequestration, forage crops
Bolinder, M. A., Ka¨tterer, T., Andre´n, O. et Parent, L. E. 2012. Estimation des apports de carbone au sol dans des rotations
incluant des plantes fourrage`res et mode´lisation de leurs effets sur la dynamique du carbone dans un essai au champ de longue
dure´e en Sue`de. Can. J. Soil Sci. 92: 821
833. Afin d’ame´liorer la compre´hension de la dynamique du C organique du sol
(COS) dans les rotations incluant des plantes fourrage`res, l’e´le´ment cle´ re´side dans de bonnes estimations des apports
annuels de C au sol issus de la partie souterraine (PS). Les stocks de COS d’un essai au champ de longue dure´e dans le nord
de la Sue`de ont permis de ve´rifier diffe´rentes hypothe`ses d’apport de C sur le bilan du COS a` l’aide du mode`le d’introduction
au bilan du carbone (ICBM). L’objectif principal de l’e´tude e´tait de valider une approche pour estimer les apports annuels de
C au sol a` partir des donne´es pour une rotation avec plantes fourrage`res en continu (‘A’). Une rotation incluant seulement
des cultures annuelles et assujettie a` un travail du sol plus fre´quent (‘D’) e´tait utilise´e pour calculer un estime´ approximatif de
l’effet du travail du sol sur la dynamique du COS. La me´thodologie employe´e pour estimer les apports annuels de C au sol
via les re´sidus de culture a permis de faire de bonnes pre´visions sur l’e´volutions des stocks de COS dans quatre des six
grandes parcelles de la rotation ‘A’. L’analyse de la rotation ‘D’ sugge`re qu’un travail du sol plus fre´quent a fait augmenter le
taux de de´composition du COS d’environ 20%. Ces re´sultats nous permettent de faire des pre´dictions du bilan de la COS
avec plus de certitude dans des rotations incluant des plantes fourrage`res. E´galement, une revue de litte´rature sur les mesures
de biomasse racinaire qui sont utilise´es pour calculer les apports annuels de C au sol issus de la PS re´ve`le qu’ils n’ont pas

change´ significativement au cours des 150 dernie`res anne´es.
Mots cle´s: Concept ICBM, climat frais et tempe´re´, racines, se´questration du carbone, plantes fourrage`res

The beneficial effect of forage crops and manure
applications on soil organic matter (SOM) content and
nutrient cycling is not a new observation. Already in the
golden age of Greeks (800 to 200 BC) it was observed
that manure increased crop productivity, and that green
manure crops enriched the soil (Sarton 1959). However,
the theories related to these issues were fairly esoteric
and sometimes controversial (Manlay et al. 2007). The
fact that grasses increase SOM content was highlighted
in several investigations in the beginning of the 20th
century in, for example, Europe and North America [see
references cited in Troughton (1957)]. Since the 1940s,
Can. J. Soil Sci. (2012) 92: 821
833 doi:10.4141/CJSS2012-036

SOM has been firmly recognized in its role in relation to
soil quality and fertility and many other ecosystem
services (Manlay et al. 2007).

It is well established that suitable management
options can sequester carbon through a sustained increase in SOM content, and thereby contribute to the
Abbreviations: AG, above ground; BG, below ground; DM, dry
matter; ER, extra-root; F, forage; GAI, green area index; GF, green
fodder; GM, green manure; ICBM, Introductory Carbon Balance
Model; P, peas; RC, root crops; SOC, soil organic carbon; SOM,
soil organic matter; UB, undersown barley; WR, winter rye
821

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822 CANADIAN JOURNAL OF SOIL SCIENCE

reduction of greenhouse gases in the atmosphere. This
includes strategies such as conversion of cropland
to grasslands, avoided conversion of grasslands, and
improved grassland management [Intergovernmental
Panel on Climate Change (IPCC) 2000, 2006; Lal
2004; Smith 2004]. The Chicago Climate Exchange

(2009) soil organic C change factors (2009) for conversion to grasslands are 0.67 Mg C ha1 yr 1 for most
of the temperate US regions, in track with a metaanalysis of published data by Eagle et al. (2010).
Empirically derived data from long-term field experiments in Canada comparing perennial grass cover
to annual cropping systems resulted in an average
soil organic C stock change factor of about 0.60 Mg C
ha1 yr 1 for forages (VandenBygaart et al. 2010). The
carbon sequestration potential (i.e., increased SOM
content) of converting cropland to grassland in Europe
has been estimated to range from 1.20 to 1.70 Mg C
ha1 yr 1 (Smith et al. 2000; Smith 2004).
Although knowledge has accumulated through time,
modeling SOM dynamics in forage-based crop rotations
as a function of above-ground (AG) plant production
and management for various soil and climatic conditions still remains a challenge (Jensen et al. 1997).
Indeed, the SOM dynamics in forage-based crop rotations can be attributed to several agro-ecosystem
factors. For instance, compared with annual crops there
is a reduced soil disturbance from tillage in perennial
forage-based crop rotations. When a crop is moldboard
plowed, the soil is broken up and more vulnerable to
SOM decomposition as well as water and wind erosion.

Besides, tillage will also directly contribute to movement
of soil, e.g., from upland positions to depressions (Lal
et al. 2007). Some of the surface soil particles, often rich
in SOM, are only redistributed over the landscape.
Others are deposited in depressions or transported
into aquatic ecosystems where a part of the C is lost
to the atmosphere as CO2 through mineralization or
methanogenesis (Lal 2004). It is also well documented
that tillage disrupts soil aggregates. Although all the
mechanisms are not fully understood, this disturbance
usually leads to decomposition of SOM compounds
because of reduced physical protection (Six et al. 2004;
Bronick and Lal 2005). Furthermore, perennial forage
crops have a root system with root turnover and
rhizodeposition during more of the growing season
than roots of annual crops, as well as a higher relative
allocation of C below-ground resulting in a higher root
biomass (Bolinder et al. 2007a). Forage systems (usually
in cattle farms) are also often subjected to frequent
C input from manure applications (including return

of dung during grazing) that contribute to the buildup
of SOM (Paustian et al. 1997; Bolinder et al. 2010).
Most SOM models developed for agro-ecosystems
accounts for variations in specific climatic and soil
conditions through information on daily or monthly
climatic data in association with basic crop and soil
(e.g., texture) properties. These data are used to cal-

culate the effect of soil water content and temperature
on SOM decomposition rates through pedotransfer and
biological response functions. The effect of tillage
intensity (e.g., moldboard plowing versus harrowing)
and frequency (e.g., annual crops only versus annuals
in forage-based rotations) on C evolution in the tilled
layer is included in many SOM models. This is done
by increasing the decay rates of the different SOM
pools (e.g., Century/DAYCENT, Ecosys) or using a
simple scaling of SOM model parameters (e.g., ICBM,
CN-SIM) (Ka¨tterer et al. 2008; Chatskikh et al. 2009).
However, most of the models do not account for the fact

that tillage may temporarily decrease bulk density and
that it can change soil hydraulic properties (Sommer
et al. 2007). The magnitude of the annual C inputs
to soil is one of the crucial factors for SOM modeling (Andre´n et al. 2008). Annual C inputs to soil are
typically calculated from information on AG plant
production and root biomass measurements. However,
the approaches used to estimate these C inputs for
forage crops vary widely and are often poorly described,
but, most importantly, they need to be validated by
long-term field data.
ICBM is a two-pool and five-parameter soil C balance
model that calculates changes in soil C stocks and
has been applied in several temperate regions for
estimating regional C balances (Andre´n et al. 2008;
Borgen et al. 2012; Lokupitiya et al. 2012). ICBM
includes a soil biological activity factor for annual crops,
a scaling parameter which is a multiplier in the firstorder decomposition rates of the two SOM pools
to account for cultivation, and a parameter used to
represent annual C inputs to soil. There is a need
to improve the model parameter estimates for describing

SOM dynamics in the northern areas of temperate
regions that are dominated by grass leys. Our objectives
in this study were: (I) to describe and test an approach
for estimating annual C inputs to soil for forage cropping systems, (II) to calculate a soil biological activity
factor specific for forages, and (III) to refine the scaling
parameter for cultivation. This was done using data
from a 52-yr field experiment in northern Sweden, where
measurements of SOM dynamics in different foragebased crop rotations were available.
MODELING SOM DYNAMICS IN TEMPERATE
REGONS
Annual C Inputs to Soil for Forage-based Crop
Rotations
It is not easy to estimate the annual C inputs to soil
in forage-based crop rotations (or grassland
pasture
systems) and they have been highlighted as a prioritized
research area due to their crucial role in the modeling
of SOM dynamics (e.g., Jensen et al. 1997). The forage
crop is usually established together with a smallgrain cereal (i.e., undersown), thereafter growing for a
number of years until the end of the rotation (Fig. 1).

BOLINDER ET AL. * MODELING SOC DYNAMICS IN FORAGE-BASED CROP ROTATIONS

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Phase I: Establishment (yr 1)
The forage crop is typically
established with a small-grain
cereal such as for example barley
that is harvested in the fall.

BG annual C inputs to soil
From root biomass
left after the barley crop
+
from ER during the growing season.

Ex. 900 + 585 = 1485 kg C ha–1

Phase II: Production (e.g., yr 2 to 5)

The forage crop is cut and harvested
2 to 3 times during the growing season.

BG annual C inputs to soil
From ER during the growing season
(including roots dying in the winter).

Ex. 2298 kg C ha–1

823

Phase III: End of rotation (e.g., yr 6)
The regrowth of the forage crop
after the last cut is ploughed in at the
end of the growing season in the fall.

BG annual C inputs to soil
From ER during the growing season
+
from forage roots incorporated when

the soil is ploughed in the fall.
Ex. 2298 + 3535 = 5833kg C ha–1

Fig. 1. Description of the three phases related to the growth of a forage crop in the northern parts of temperate climates
and the annual C inputs to soil associated with the below-ground (BG) component. See the text for explanations with respect to
the extra-root C (ER-C). The examples given for the flux of BG annual C inputs to soil for each phase was calculated from the data
for mean root biomass (Table 1) and using a mean ER-C coefficient of 65%, assuming a C content of 0.45 g g 1 in root tissues, e.g.,
for phase III: C from roots 7855 kg DM45% C3535 kg C ha1 and C from ER 3535 65% ER-C 2298 kg C ha1.

The standing root biomass for the forage crop is only
incorporated at the end of the rotation; in phase II the
below-ground (BG) input to soil for the forage crop is
only originating from extra-root (ER) C. The ER-C
can be defined as turnover (individual roots dying and
decomposing) and cell sloughing of epidermal root
tissues during the growing season, and soluble compounds released from the roots by exudation (Andre´n
et al. 1989). The amount of ER-C is not included in
root biomass estimates by soil coring or excavation.
To account for this component in the annual C inputs
to soil we need a coefficient that multiplies the root
biomass by the proportion (%) of ER-C produced
during a growing season (root respiration is not
included because the CO2 is returned to the atmosphere). Information on this is often obtained from
tracer (14C, 13C) studies, particularly for cereal crops.
This can also be quantified using the physical difference
in root biomass from successive temporal measurements. For example, Dahlman and Kucera (1965)
calculated root turnover by dividing the net annual
significant increment in root biomass (i.e., maximum
minus minimum value determined with sequential

measurements) with the maximum value of measured
root biomass.
There has been early interest in compiling literature
data on root biomass for forages and small-grain
cereal crops (Table 1). The most extensive review on
forages was presented by Troughton (1957), who
reviewed published data for different swards; some
Russian data for grass mixtures and individual species
was presented by Kononova (1961), while Goedewaagen
and Schuurman (1950a, b) calculated mean values for
small-grain cereals. More recently, Bolinder et al.
(2007a) summarized a number of Canadian studies
(including some from the United States) for both
cultivated forages and small-grain cereals. It is clear
from these data that root biomass for forages can often
be at least three times that of small-grain cereals. The
estimated mean values and relative differences between
these two types of crops have remained fairly similar in
earlier and more recent literature surveys.
Root biomass measurements obtained under Swedish
conditions are similar and within the range of these
observations. For instance, early measurements made
by Torstensson (1938) reported maximum values for

824 CANADIAN JOURNAL OF SOIL SCIENCE
Table 1. Summary of data from some literature surveys on quantitative estimates of root biomass (kg of dry matter per ha) for perennial forages and
small-grain cereals covering work conducted from approximately the 1850s to the end of the twentieth century
Literature survey
Perennial forages

Troughton (1957)
Kononova (1961)
Bolinder et al. (2007a)

Mean
Small-grain cereals

Goedewaagen and
Schuurman (1950a, b)x
Bolinder et al. (2007a)

Origin of data and
time-period

N

Mean9Std. Dev

Min.

Max.

Europe 1844
1955z
North-America 1889
1954z
Russia 1939
1949
North America 1969
2003y

82
92
19
70

790897286
702494936
936195166
712794983
7855
22509701

1131
1108
3390
930

47322
25101
18000
20120

1400

3100

175091424
2000

240

7000

Europe 1866
1950

6

North-America 1986
2001y

Mean

73

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z

Calculated from the mean values presented in the appendix of that study.
Calculated from the references cited in Bolinder et al. (2007a) that included mainly data from Canadian studies.
Calculated from the mean values for wheat and winter wheat, barley and winter barley, oats and rye as cited by Troughton (1962).

y

x

pastures :2800 kg dry matter (DM) ha1, while
Paustian et al. (1990) and Ka¨tterer and Andre´n (1999)
reported mean values of about 6000 to 9000 kg DM ha1
for commonly grown forage species. Data for barley and
wheat indicate a mean BG biomass of :1700 kg DM
ha1 (e.g., Paustian et al. 1990; Ka¨tterer et al. 1993).
There are several literature estimates of ER-C coefficients for small-grain cereals and forages (Dahlman
and Kucera 1965; Barber and Martin 1976; Johansson
1992; Gill and Jackson 2000; Kuzyakov and Domanski
2000; Bolinder 2004). For forages most studies have
been conducted on grasslands and pastures, and here
we assume they behave like a forage crop in an arable
system. Furthermore, it is not always clear whether
the coefficients for forages include the exudates, or
the roots that die during the winter in the studies
from the cooler northern regions. Some of these studies
were the result of quite extensive literature surveys.
In particular those of Gill and Jackson (2000) who
estimated root turnover for grasslands using the definition by Dahlman and Kucera (1965), and Kuzyakov
and Domanski (2000) who reviewed tracer studies. The
average ER-C from all the studies on small-grain cereals
was 32% and that of forages 45%. By comparison, in
a long-term (1979
1988) project on agroecosystems,
Ecology of arable land
the role of organisms in N
cycling, Andre´n et al. (1989) concluded that the ER-C
coefficient for these two types of crops would be : 50%
under Swedish conditions. However, common assumptions have been made that the ER-C coefficient for
crops can be as high as 100% (e.g., Rasse et al. 2005).
Consequently, there is a wide range in reported values
(i.e., 32 to 100%). In this study we use the plant C
allocation coefficients proposed by Bolinder et al.
(2007a), where an intermediate ER-C coefficient of
65% is used.
The examples of C fluxes from BG for each of the
phases (Fig. 1) shows that the C input from BG is lowest
for phase I, a little higher for phase II, and that the input
from phase III is usually dominating a typical forage-

based crop rotation. In phase I it is often considered that
the annual C input to soil from BG is calculated as if it
was a small-grain cereal year. The AG C inputs to soil
are naturally more straightforward, easily estimated as a
proportion of the harvested biomass. Recently, a few
simple equations have been proposed for estimating the
BG C inputs to soil from forage-based crop rotations
(e.g., Andre´n et al. 2004; Bolinder et al. 2007a) and there
is a need to validate those equations using data from
long-term field experiments.
The Offer Long-term Field Experiment In
Northern Sweden
The Offer long-term field experiment is located in
the ‘‘North’’ agricultural production region of Sweden
(lat. 63.148N, long. 17.758E) and was initiated in 1956
(Andre´n et al. 2008). It was part of a study that compared
forage yields in four 6-yr forage-based rotations at three
sites (Offer, A˚s and Ro¨ba¨cksdalen). Soil organic carbon
and nitrogen dynamics for these sites were presented by
Bolinder et al. (2010). In this paper we use data for the
Offer site for modeling SOM dynamics; this site was
running for a longer time period than the other two and
had the most detailed crop records and soil sampling
program. A full description of the history of the site, the
rotations, soil sampling and analysis are given in Bolinder
et al. (2010), only a brief description is given here.
The four 6-yr rotations (A, B, C and D) contained
the following crops: undersown barley (UB), forage (F),
green fodder (GF
which was either a mixture of oats
and peas, or fodder rape), winter rye (WR), peas (P),
root crops (RC
potato, carrot or rutabaga), green
manure (GM
the forage crop was grown as green
manure) (Table 2). For each 6-yr period manure was
applied twice in rotation A and B (equivalent to 4.48 Mg
C ha1 split in two applications) and once in rotation
C (equivalent to 2.99 Mg C ha1). Each phase of the
6-yr rotations (i.e., year 1 to year 6) was present every
year and grown in large plots (8 20 m) for a total of
24 plots (i.e., 4 rotations 6-years). Seven complete

BOLINDER ET AL. * MODELING SOC DYNAMICS IN FORAGE-BASED CROP ROTATIONS

825

Table 2. Summary of the four 6-yr rotations for the Offer site in northern Sweden

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Year
Year
Year
Year
Year
Year

1
2
3
4
5
6

Rotation A

Rotation B

Rotation C

Rotation D

Undersown barley
Forage
Forage
Forage
Forage
Forage

Undersown barley
Forage
Forage
Forage
Green fodder
Fodder rape

Undersown barley
Forage
Forage
Winter rye
Green fodder
Root crop

Undersown barley
Green manure
Winter rye
Peas
Root crop
Root crop

rotation cycles were completed from 1963 to 2004. The
time period between the initiation of the experiment and
1962 was used to establish the different phases of each
rotation. During that period no detailed yield data were
recorded.
Thereafter, the DM biomass of all AG plant parts was
measured every year in each of the 24 plots. A few
missing data in the files were estimated from the closest
data point in time and space, and the yield records for
forages were almost complete. The yield of F for each
harvest (the number of harvests varied from 1 to 2)
and the yield of GF was measured. Both the grain and
straw yields were measured for the small-grain cereals
(i.e., barley, oats and winter rye). The yields of peas
and vines were measured separately. The yields were
measured for all the RCs, but the vines were determined
for carrot and rutabaga only. All small-grain cereal straw
was left on the plots after harvest, as well as the vines
from peas and RCs. For F grown as GM in rotation D
the forage was cut at the end of July and cuttings left in
the plots; thereafter the forage was allowed to grow until
mid-August when it was plowed under. Since the yield
was not measured for GM it was calculated from the
average AG biomass by using the average of the first
production year of F in rotation A, B and C. No root
biomass measurement was made for any crop.
The site was sampled in the fall of 2008 for the 0- to
25-cm soil layer by taking three soil cores in each of
the plots. Dry soil bulk density was calculated, and soil
organic carbon (SOC) was determined by dry combustion (LECO CNS 1000), and final SOC stocks were
thereafter calculated for each plot. We also used the
concept of ‘‘equivalent soil mass’’ to estimate the
changes in SOC stocks through time; the calculations are described in detail by Bolinder et al. (2010),
where they were applied to the mean values of the
four different rotations. In this paper we made a
more detailed analysis with focus on rotation A and D
by calculating the changes in SOC stocks for each
individual plot.
The ICBM Concept and Parameterization
The Introductory Carbon Balance Model (ICBM) is
a two-compartment first-order kinetic model with two
state-variables; ‘‘Young’’ (Y) and ‘‘Old’’ (O) SOC pools
with specific decomposition rates (kY and kO, respectively) (Fig. 2). The first-order decomposition rates

(kY 0.8 and kO 0.007) are held ‘‘constant’’ according
to the original calibration (Andre´n and Ka¨tterer 1997;
Andre´n et al. 2004). These decomposition rates are
thereafter modified by a soil climate-management parameter (re) that summarizes the effect of climate and
tillage on soil biological activity (i.e., re multiplies the
first-order decomposition rates of the two SOC pools in
ICBM) as described below in more detail. The annual C
inputs to soil (i) come from above- and below-ground
crop residues and from manure that enters the model
through the ‘‘Young’’ (Y) SOC pool. The humification
coefficient (h) determines the fraction of these annual
C inputs to soil (i) that enters the ‘‘Old’’ (O) SOC pool.
We used the same humification coefficients for crop
residues (hY 0.125) and manure (hM 0.310) as in
the study by Ka¨tterer et al. (2008), which were based
on those of Andre´n and Ka¨tterer (1997) who calibrated
the model on a long-term site in central Sweden.
The total SOC stocks at Offer in 1956 were 8.19 kg
C m 2 to 25-cm depth (Bolinder et al. 2010). This
amount was used as the initial value for each of the plots
since only composite soil samples from each replicate
were used to determine C concentrations in 1956. These
C concentrations were uniform across the site [for more
details see Bolinder et al. (2010)]. The initial C mass in
each of the SOC pools was estimated as described by
Ka¨tterer et al. (2008) by assuming that the young pools
were in approximate equilibrium with the previous
conditions at the site. For that purpose we used the
estimated parameter values of re and i for rotation B
i

‘Young C pool’
Y

(1–h)kYreY

hkYreY
‘Old C pool’
O
koreO

Fig. 2. Structure of the Introductory Carbon Balance Model
(ICBM): i annual C inputs to soil, kY and kO firstorder decomposition rate constants for the Young and
Old SOC pools, h humification coefficient, re soil climatemanagement parameter.

826 CANADIAN JOURNAL OF SOIL SCIENCE

Estimating the Soil Climate-management
Parameter (re)
The soil climate-management factor, re, is governed by
three components: soil water content (rw), soil temperature (rT) and tillage intensity (rc). These three components are assumed to be multiplicative, i.e., re rW 
rT rC. Where the rc component (i.e., tillage) has
previously been based mostly on assumptions and expert
opinion (Andre´n et al. 2004; Ka¨tterer et al. 2008). The
first two components (rW and rT) are estimated with a
soil climate module connected to ICBM. This module
uses daily standard meteorological data, a soil water
model, and commonly used assumptions with respect to
the relationships between temperature, soil water content and biological activity (Andre´n et al. 2004, 2007;
Bolinder et al. 2007b, 2008). It also uses green area index
(GAI) dynamics that influence both the rW and rT
components. The GAI is defined as the projected area of
all plant parts that are visibly green (Ka¨tterer and
Andre´n 2008).
The drivers for estimating rw are daily precipitation
and potential evapotranspiration data and two soil
parameters: water content at wilting point (uwp) and
at field capacity (ufc). The difference between uwp and
ufc defines the storage capacity of plant-available
water. Furthermore, GAI is governing the plant water
requirements for transpiration. Daily climatic data (air
temperature, potential evapotranspiration and precipitation) were taken from the Sundsvall airport situated
about 80 km from the experimental site. Daily values
were available for the period from 1961 to 2005. For the
periods 1957 to 1960 and 2006 to 2008 we used records
from 1961 to 1964 and 2003 to 2005, respectively.
Pedotransfer functions developed from a Swedish soil
database (model 7; Ka¨tterer et al. 2006) were used for
estimating ufc and uwp from soil texture and C concentrations. Since soil C concentrations changed over time
in the treatments, we estimated these parameters for
both 1957 and 2008 and calculated intermediate values
by linear interpolation. The water content at wilting
point did not change with time and was 0.126 m3 m 3.
The water content at field capacity was 0.434 m3 m 3
in 1957 and changed depending on changes in soil C
(Table 3). In 2008, ufc was 12% higher in rotation A
than in rotation D. These estimates for ufc and uwp were
close to those measured in an adjacent field, i.e., 0.128
and 0.456 m3 m 3, respectively (Andersson and Wiklert
1977).

Table 3. Mean soil organic carbon (SOC) content and estimated soil
water content at field capacity (ufc) in the 0 25 cm soil layer in 2008 for
each of the four rotations at Offer




Rotation

SOC

ufc

A
B
C
D

(%)
3.18
2.70
2.38
2.19

(m3 m 3)
0.455
0.432
0.416
0.406

Annual dynamics of GAI is governed by empirical
functions, the amplitude (GAImax) of which is related to
the mass of harvested products and a crop specific
parameter related to the length of the vegetation period.
For undersown barley and winter rye we used the
function presented by Bolinder et al. (2008). For root
crops, we used a fixed GAImax (5.6) according to data
from potatoes (Fortin 2008). For forage crops, we
simulated leaf area dynamics with logistic functions
for the two seasons between the cuts. Estimates of
GAImax ( 1.8 DM yield in Mg ha1) were based on
data from Be´langer and Richards (1995) using the
assumption that GAImax did not exceed a value of
10 m2 m2. If not recorded in log-files from the experimental station, we assumed reasonable dates for
management operations (sowing, harvesting, plowing
etc.). The typical GAI dynamics of the different crops
contributes to the difference in the soil climate/management factor between continuous forage rotations versus
rotations with annual crops (Fig. 3).
The simple bucket-model used to calculate daily soil
water content was based on concepts as described in
8
7
6

5
GAI

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to derive initial data for all rotations, since that rotation reflected the most common previous management
practices. Consequently, the initial mass from crop
residues was 0.378 and that from manure 0.109 kg
C m 2. The initial size of the old SOC pool (7.703 kg
C m 2) was set to the difference between the total SOC
stocks measured in 1956 and the sum of these two
fractions.

4
3
2
1
0

0

50

100

150

Forage

200
250
Julian day
Root crop

300

350

400

Barley

Fig. 3. Representation of typical green area index (GAI)
dynamics for forages, root and barley crops. The GAI is here
defined as the projected area of all plant parts that are visibly
green and was estimated with different functions, as described
in the text.

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BOLINDER ET AL. * MODELING SOC DYNAMICS IN FORAGE-BASED CROP ROTATIONS

detail by Karlsson et al. (2011) and Fortin et al. (2011).
This model involves parameters for precipitation and
crop interception of precipitation; both of these are
modulated by the GAI. A semi-empirical model was
used for estimating daily soil temperature from air
temperature and leaf area index as described by Ka¨tterer
and Andre´n (2008). Leaf area index was assumed to
be 80% of GAI [see Ka¨tterer and Andre´n (2008) for
details]. Daily soil water and soil temperature were
transformed into the activity factors, rw and rT, respectively, and the resulting annual means of the product
rw rT calculated for each day were then divided by a
constant scaling factor which refers to conditions at the
reference sites used for model calibration [see Bolinder
et al. (2008) for details]. All calculations were conducted
for each experimental plot.

Estimating Annual C Inputs to Soil (i)
We used the plant C allocation coefficients as described
in detail by Bolinder et al. (2007a) to estimate the annual
C inputs to soil from BG for the undersown barley,
forage and green manure crops. The coefficients were
based on a review on plant shoot-to-root ratios for these
crop types. We considered that 40% of the total annual
AG production for forage was returned to the soil, i.e.,
as litter fall and harvest losses during the growing season
plus the regrowth after the last cut dying during the
winter. For the crop types in rotation D that were not
addressed in Bolinder et al. (2007a), we used data from a
Swedish field study and calculated BG input for winter
rye with a shoot- to root-ratio of 13.5 (Ka¨tterer et al.
1993). The BG input for peas was based on data from
Wichern et al. (2007). The shoot refers to the total AG
material (i.e., grain, straw, vines etc.). We used a fixed
estimate (0.12 kg DM m 2) of BG input for the potato
crop (Carter et al. 2003) and for the other two root crops
(i.e., carrot and rutabaga). We assumed that the C
content of all plant parts was 0.45g g
1 and the ER-C
coefficient was set to 0.65.
RESULTS AND DISCUSSION
Annual C Inputs to Soil and Climate Factors
The average annual C inputs to soil from crop residues
(1957 to 2008) were similar between rotations A and D,
with 0.280 kg C m 2 yr 1 for rotation A and 0.276 kg C
m 2 yr1 for rotation D. The continuous forage-based
rotation A also received manure applications equivalent
to 0.075 kg C m 2 yr 1 and the total annual C input to
soil was therefore higher than that for rotation D. The
BG C inputs were higher in rotation A (0.170 kg C m 2
yr 1) compared with those of rotation D (0.126 kg C
m 2 yr 1).
There were inter-annual variations in the total
and BG annual C inputs to soil in the rotations,
both between crop types and for a given crop type
(Figs. 4 and 5). The forage crop grown at the end of
rotation A and the green manure crop in rotation D

827

contributes the most to the total annual C inputs to
soil from crop residues, while the undersown barley
contributes a similar amount in both rotations. The
winter rye, undersown barley, peas and the root crops
generally contribute a smaller amount of BG C,
compared with the forage crop at the end of rotation
A and the forage crop grown as green manure in
rotation D.
The annual amount of BG C inputs to soil for the
three phases related to the continuous forage-based crop
rotation A (Fig. 4) was estimated using the Bolinder
et al. (2007a) methodology. Considering the entire
period from 1957 to 2008, our values for the annual
BG C input were 1.0290.27 Mg C ha1 yr 1 for the
undersown barley, 1.4390.40 for the forage crop grown
in a production year and 3.4490.93 Mg C ha1 yr 1
for the forage crop grown at the end of rotation A. This
is lower than the example of values presented in Fig. 1
(1.49, 2.30 and 5.83 Mg C ha 1 yr 1, respectively) and
is explained by the fact that the estimates of BG input is
specific to the AG productivity at the Offer site.
The Bolinder et al. (2007a) methodology also includes
specific assumptions about the BG allocation based
on the values that were reviewed for Canada. However,
the relative differences in BG C inputs to soil between
the three phases we obtained with our approach are
similar to those from a broader perspective, as presented
in Fig. 1.
The two main driving variables used to estimate the
effects of climate on SOC decomposition are air temperature and total precipitation (Fig. 6). They are used to
calculate the multiplicative soil-temperature (rT) and
soil-moisture (rW) factor that results in the soil-climate
parameter re (Fig. 7). In the ICBM, annual average
values for re were used when simulating changes in SOC
stocks through time. However, the re parameter is
calculated using daily time-steps for the climatic input
data because it has been shown that this allows a more
accurate estimate for temperate regions (Fortin et al.
2011). Considering the average value of the six plots for
each rotation over the time period 1957 to 2008, the re
value for Offer was 0.88 for rotation A and 0.92 for
rotation D (Table 4). Because the total steady-state
C mass is linear in response to the ICBM re and i
parameters (Andre´n and Ka¨tterer 1997). This means that
it would be necessary to have an annual C input to soil
about 5% higher (i.e., 0.92/0.88) for rotation D in order
to reach the same steady-state soil C mass as that for
rotation A.
The first-order decomposition rates for the ‘‘young’’
(kY) and ‘‘old’’ (kO) SOC pools in ICBM are multiplied
by the soil-climate parameter. Consequently, re also
affects the inter-annual variations in decay rates
(Fig. 7). Other components of the ecosystem, both
biotic and abiotic related factors (e.g., soil structure
and earthworm activity), also play a role in temporal
variations in SOC decay rates, but they are less well
documented and orders of magnitude more difficult to

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828 CANADIAN JOURNAL OF SOIL SCIENCE

Fig. 4. Total and below-ground (BG) annual C inputs to soil from crop residues in one of the six large plots for rotation A during
1963 to 1986 (time period when the most detailed yield data were recorded). The phases of this 6-yr crop rotation from 1963 is as
follows: 4 yr of forage in production (F-PY), 1 yr of forage in the end of rotation (F-EOR) and 1 yr of undersown barley (UB).

quantify compared with the effect of climate for which
reasonable assumptions are made in most SOC models.
The inter-annual range in re for the Offer site was about
a factor 2, with an average minimum and maximum
value of 0.66 and 1.15, respectively.

Predicting Final SOC Stocks with ICBM
The predicted final SOC stocks with the ICBM for
rotation A were close to those measured for four of the
six plots (plot no. 2, 3, 5 and 6), for which the deviation
from measured final SOC stocks was less than 5%

Fig. 5. Total and below-ground (BG) annual C inputs to soil from crop residues in one of the six large plots for rotation D
during the period 1963 to 1986. The phases of this 6-yr crop rotation from 1963 is as follows: 1 yr green manure (GM), 1 yr of winter
rye (WR), 1 yr of peas (P), 2 yr of root crops (RC) and undersown barley (UB).

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6.0

1200

5.0

1000

4.0

800

3.0

600

2.0

400

1.0

200
Mean annual temperature

829

(mm)

(°C)

BOLINDER ET AL. * MODELING SOC DYNAMICS IN FORAGE-BASED CROP ROTATIONS

Mean annual total precipitation
0

0.0
1957

1982

2008

Year

Fig. 6. Mean annual temperature and total precipitation during 1957 to 2008 for the Sundsvall weather station.

(Table 4). For rotation D, final SOC stocks for three of
the six plots (plot no. 1, 4 and 6) was also predicted with
less than 5% deviation, and two plots (plot nos. 2 and 5)
with about 10% deviation from measured final SOC
stocks. The final measured SOC stocks were somewhat
lower than the overall mean values for plot no. 4 in

rotation A and plot no. 3 in rotation D. Consequently,
ICBM over-estimated the final SOC stocks for these two
plots. ICBM under-estimated the final SOC stocks for
plot no. 1 in rotation A since the measured final SOC
stocks for this plot were high. We have no particular
explanation why the measured final SOC stock values for

Fig. 7. The mean annual soil climate-management parameter (re) for two of the large plots (one for rotation A and one for rotation
D) during the period 1957 to 2008 only including the effects of soil water content and temperature (i.e., re rW rT).

830 CANADIAN JOURNAL OF SOIL SCIENCE
Table 4. Measured and ICBM predicted final soil organic carbon (SOC) stocks in 2008 for the Offer site for each of the six large plots of rotation A and
D (kg C m 2). The values for the annual crop residue C inputs to soil (i) and the soil climate/management factor (re) are annual averages for the time
period 1957 to 2008. The scaling factor for cultivation (rC) was optimized for each plot
Plot no. rotation A

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Measured
ICBMz
i
re

Measured
ICBMz
i
re
Optimizedy rC

1

2

3

4

5

6

Mean

10.1
8.5
0.277
0.88

8.8
8.7
0.268
0.88

8.6
8.8
0.304
0.87

7.4
8.5
0.271
0.88

8.6
8.4
0.269
0.88

9.0
8.6
0.290
0.87

8.8
8.6
0.280
0.88

1

2

5

6

Mean

7.2
7.3
0.259
0.93
1.05

6.8
7.5
0.291
0.92
1.30

6.7
7.2
0.265
0.92
1.27

6.9
7.1
0.256
0.92
1.10

6.9
7.4
0.276
0.92
1.21

Plot no. rotation D
3
4
6.3
7.6
0.296
0.92
1.57

7.6
7.5
0.288
0.91
0.95

z

Final SOC stocks with ICBM were predicted using the annual C inputs to soil and climate-factor estimated for each plot between 1957 to 2008 (see
the text for details).
y
The optimized values represent the estimated constant used to multiply the rC parameter in order to force the ICBM predicted value to match the
measured value for each plot.

these three latter plots diverged from the overall mean
values. The mean absolute deviation from measured
values for the six large plots for rotation A was 6.9%
and that for rotation D 7.5%. These overall predictions
made with ICBM are within the range of commonly
observed results with SOC models applied to data from
long-term experiments (e.g., Bolinder et al. 2006).
The methodology used to estimate annual crop
residue C inputs to soil performed reasonably well.
Annual C inputs to soil were only under-estimated by
4% (data not shown) when optimizing the annual C
inputs to soil from the undersown barley and forage
crops for rotation A [i.e., by multiplying the total annual
crop residue C inputs to soil (i) by a constant in order to
force the ICBM predicted value to match the measured
value for each plot]. This exercise of course implies that
the reference values for the other four parameters
remained unchanged. The objective of this study was
to examine the performance used to estimate the i
parameter for rotation A in such a scenario.
The data used to derive the relative plant C allocation
coefficients for forage crops and undersown barley used
in this study originated from a review of field measurements of shoot and root C allocation. Most of the field
studies that were reviewed considered mainly the 0-to
18-cm, 0- to 20-cm or 0- to 30-cm depths [see Bolinder
et al. (2007a) for more details]. Only a limited number of
data considered a deeper sampling depth such as 0
60
or 0
90 cm (i.e., less than 10% of the data). About 20%
of the data included 45-cm depth, but it was found that
the root biomass measured in the deepest sampling layer
(i.e., 30
45 cm) represented only 10% of the total root
biomass. Therefore, we considered that the estimate
of BG C input with this methodology was representative
for the 0- to 25-cm depth, which was the sampling depth
used to estimate the SOC stocks in this study.

We also predicted final SOC stocks considering the
equivalent soil mass concept for each of the plots. This
concept applied on SOC stock dynamics accounts for
changes in soil bulk density due, for example, to tillage
and manure applications. Although the application of
manure resulted in lower bulk density for rotation A, in
this study this effect was not crucial and the equivalent
soil depth was on average 26.5 cm (Bolinder et al. 2010).
Values for each plot ranged from 25.1 to 28 cm (data not
shown) and this has a fairly small effect on the predicted
SOC stocks and the optimized i parameter. The effect of
using equivalent soil depth for rotation D was also small
(i.e., equivalent depth was 23 cm).
For rotation D, the annual C inputs to soil for the
undersown barley and green manure (i.e., the forage
crop grown as green manure) followed the Bolinder
et al. (2007a) methodology, and as discussed above gave
reasonable estimations. For the 2 yr of root crops in that
rotation we used a fixed estimate for BG C inputs. If we
consider that annual C inputs to soil for the other two
crops in that 6-yr rotation (i.e., winter rye and peas)
were well estimated and using the default parameter
settings for ICBM, then we can have a ‘‘rough estimate’’
for the cultivation factor (rC). This resulted in a mean
optimized rC value of 1.21, indicating that cultivation
would have accelerated the decomposition by 21% in
rotation D (Table 4). This assumes that the constant
obtained in the optimization accounts for the effect
of tillage that occurred in the last 5 yr of that 6-yr
rotation (i.e., 5 yr of annual crops as compared with the
continuous forage rotation A). In previous ICBM
applications, the relative difference in decomposition
rates between an annual crop or a root crop versus
a perennial forage crop has used guestimates for rC of
10 and 30%, respectively (Andre´n et al. 2004; Ka¨tterer
et al. 2008). Of course, the additional effect attributed to

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BOLINDER ET AL. * MODELING SOC DYNAMICS IN FORAGE-BASED CROP ROTATIONS

rC could well be assigned to the other components of the
ICBM approach (e.g., decay rates, SOC pool sizes, etc.).
For instance, in another study on SOC dynamics in
a long-term Swedish field experiment, Ka¨tterer et al.
(2011) found that root-derived C likely contributes
about two times more to relatively stable soil C pools
(h twice as high) than the same amount of AG crop
residues. That study comprised more detailed data
records for soil C changes and was dominated by spring
cereals. However, if we apply these findings to the
current study, by calculating a weighted average of hY
for the above- and below-ground crop residues for
rotation A and D, then the simulated relative differences
between the treatments decreases. With these assumptions, the effect attributed to rC would decrease from
21% to about 10% since the below-ground C input was
relatively higher in A than in rotation D.
Furthermore, it is difficult to use data from long-term
field experiments to examine the effect of management and land use change on SOC dynamics, since they
were often not designed for that purpose. This implies
that it is rather problematic to define ‘‘clear-cut’’ effects
and there are several non-trivial potential error sources.
However, they constitute one of the best sources we have
to generate hypotheses and bring models into line. In
experiments like this, the effects of tillage and crop types
in the different rotations are unavoidably confounded.
The initial plant communities of the experimental site
they replace may also be a problem (DuPont et al. 2010).
However, this latter aspect may have been less of a
problem in this study because the management of Offer
prior to initiation involved similar cropping systems used
in the four rotations, i.e., forages and annual crops such
as small-grain cereals (Bolinder et al. 2010).
It is recognized that management-induced changes on
SOC stocks are limited to the approximate depth of the
plow layer (0
20 or 0
30 cm) in most agricultural soils
(IPCC 2006). Furthermore, the SOC stock changes for
grasses appear to decrease with greater depths, reaching
a modest level beyond the 30-cm depth (Liebig et al.
2010). However, there is a considerable gap in the understanding of SOC dynamics in the whole soil profile, and
modeling of SOC dynamics from this perspective should
be improved (e.g., Ga¨rdena¨s et al. 2011).
CONCLUSIONS
The use of the continuous forage rotation (A) to validate
the Bolinder et al. (2007a) methodology for estimating
annual C inputs to soil for forage crops was relatively
straightforward, and it was shown that it worked fairly
well when used within the ICBM concept. The analysis
of recent and historical root biomass measurements
for forages that is the basis for estimating the below
ground C inputs to soil indicates that the estimates
have remained relatively constant for at least the past
150 yr. The use of rotation D with respect to the
approximate cultivation factor confirms that previous
assumptions we have made are within a reasonable

831

range. These results improve our confidence in using the
ICBM and other models to predict SOC balances for
forage-based crop rotations in cool, temperate agricultural regions.
ACKNOWLEDGMENTS
This work was funded by the Swedish Farmer’s
Foundation for Agricultural Research within the project
‘‘The impact of perennial leys in crop rotations on
soil carbon balances’’. Additional financial support was
also provided by the NSERC project CRDPJ-385199
on ecosystem services and collaborative potato farms.
We acknowledge Lars Ericson and Kent Dryler who
provided archived yield records. Thanks to Mireille
Vigneault who kindly prepared illustrations for the
conceptual Figure 1.
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i svenska a˚kerjordar. Del III. Norrbottens-, Va¨sterbottens-,
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1236.
Andre´n, O., Kihara, J., Bationo, A., Vanlauwe, B. and Ka¨tterer,
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Andre´n, O., Ka¨tterer, T., Karlsson, T. and Eriksson, J. 2008.
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