Estimating carbon inputs to soil in fora (1)

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

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 small- grain cereal (i.e., undersown), thereafter growing for a number of years until the end of the rotation (Fig. 1).

822 CANADIAN JOURNAL OF SOIL SCIENCE

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BOLINDER ET AL. * MODELING SOC DYNAMICS IN FORAGE-BASED CROP ROTATIONS 823

Phase I: Establishment (yr 1)

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

Phase III: End of rotation (e.g., yr 6)

The forage crop is typically

The forage crop is cut and harvested

The regrowth of the forage crop

established with a small-grain

2 to 3 times during the growing season.

after the last cut is ploughed in at the

cereal such as for example barley

end of the growing season in the fall.

that is harvested in the fall.

BG annual C inputs to soil

BG annual C inputs to soil

BG annual C inputs to soil

From ER during the growing season left after the barley crop

From root biomass

From ER during the growing season

(including roots dying in the winter).

from forage roots incorporated when from ER during the growing season.

the soil is ploughed in the fall. Ex. 900 + 585 = 1485 kg C ha –1

Ex. 2298 kg C ha –1

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

in root tissues, e.g., .

For personal use only. The standing root biomass for the forage crop is only

measurements) with the maximum value of measured incorporated at the end of the rotation; in phase II the

root biomass.

below-ground (BG) input to soil for the forage crop is There has been early interest in compiling literature only originating from extra-root (ER) C. The ER-C

data on root biomass for forages and small-grain can be defined as turnover (individual roots dying and

cereal crops (Table 1). The most extensive review on decomposing) and cell sloughing of epidermal root

forages was presented by Troughton (1957), who tissues during the growing season, and soluble com-

reviewed published data for different swards; some pounds released from the roots by exudation (Andre´n

Russian data for grass mixtures and individual species et al. 1989). The amount of ER-C is not included in

was presented by Kononova (1961), while Goedewaagen Can. J. Soil. Sci. Downloaded from pubs.aic.ca by University of Laval on 07/06/15

root biomass estimates by soil coring or excavation. and Schuurman (1950a, b) calculated mean values for To account for this component in the annual C inputs

small-grain cereals. More recently, Bolinder et al. to soil we need a coefficient that multiplies the root

(2007a) summarized a number of Canadian studies biomass by the proportion (%) of ER-C produced

(including some from the United States) for both during a growing season (root respiration is not

cultivated forages and small-grain cereals. It is clear

from these data that root biomass for forages can often sphere). Information on this is often obtained from

included because the CO 2 is returned to the atmo-

be at least three times that of small-grain cereals. The

estimated mean values and relative differences between This can also be quantified using the physical difference

tracer ( 14 C, 13 C) studies, particularly for cereal crops.

these two types of crops have remained fairly similar in in root biomass from successive temporal measure-

earlier and more recent literature surveys. ments. For example, Dahlman and Kucera (1965)

Root biomass measurements obtained under Swedish calculated root turnover by dividing the net annual

conditions are similar and within the range of these significant increment in root biomass (i.e., maximum

observations. For instance, early measurements made minus minimum value determined with sequential

by Torstensson (1938) reported maximum values for by Torstensson (1938) reported maximum values for

ha (e.g., Paustian et al. 1990; Ka¨tterer et al. 1993). There are several literature estimates of ER-C coeffi- cients 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 defini- tion 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

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 assump- tions 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),

and peas, or fodder rape), winter rye (WR), peas (P),

manure) (Table 2). For each 6-yr period manure was applied twice in rotation A and B (equivalent to 4.48 Mg

C ha split in two applications) and once in rotation

C (equivalent to 2.99 Mg C ha ). Each phase of the 6-yr rotations (i.e., year 1 to year 6) was present every

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

Origin of data and

time-period

Mean9Std. Dev

Min. Max. Perennial forages

Bolinder et al. (2007a)

Small-grain cereals

Goedewaagen and Schuurman (1950a, b) x

Bolinder et al. (2007a)

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

824 CANADIAN JOURNAL OF SOIL SCIENCE

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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

Rotation D Year 1

Rotation A

Rotation B

Rotation C

Undersown barley Year 2

Undersown barley

Undersown barley

Undersown barley

Green manure Year 3

Winter rye Year 4

Peas Year 5

Forage

Forage

Winter rye

Root crop Year 6

Forage

Green fodder

Green fodder

Forage

Fodder rape

Root crop

Root crop

rotation cycles were completed from 1963 to 2004. The

(k Y

time period between the initiation of the experiment and to the original calibration (Andre´n and Ka¨tterer 1997; 1962 was used to establish the different phases of each

Andre´n et al. 2004). These decomposition rates are rotation. During that period no detailed yield data were

thereafter modified by a soil climate-management para- recorded.

meter (r e ) that summarizes the effect of climate and Thereafter, the DM biomass of all AG plant parts was

tillage on soil biological activity (i.e., r e multiplies the measured every year in each of the 24 plots. A few

first-order decomposition rates of the two SOC pools in missing data in the files were estimated from the closest

ICBM) as described below in more detail. The annual C data point in time and space, and the yield records for

inputs to soil (i) come from above- and below-ground forages were almost complete. The yield of F for each

crop residues and from manure that enters the model harvest (the number of harvests varied from 1 to 2)

through the ‘‘Young’’ (Y) SOC pool. The humification and the yield of GF was measured. Both the grain and

coefficient (h) determines the fraction of these annual straw yields were measured for the small-grain cereals

C inputs to soil (i) that enters the ‘‘Old’’ (O) SOC pool. (i.e., barley, oats and winter rye). The yields of peas

We used the same humification coefficients for crop and vines were measured separately. The yields were

measured for all the RCs, but the vines were determined M the study by Ka¨tterer et al. (2008), which were based

residues (h Y

for carrot and rutabaga only. All small-grain cereal straw on those of Andre´n and Ka¨tterer (1997) who calibrated was left on the plots after harvest, as well as the vines

the model on a long-term site in central Sweden. from peas and RCs. For F grown as GM in rotation D

The total SOC stocks at Offer in 1956 were 8.19 kg the forage was cut at the end of July and cuttings left in

to 25-cm depth (Bolinder et al. 2010). This the plots; thereafter the forage was allowed to grow until

Cm

amount was used as the initial value for each of the plots mid-August when it was plowed under. Since the yield

since only composite soil samples from each replicate was not measured for GM it was calculated from the

were used to determine C concentrations in 1956. These For personal use only.

average AG biomass by using the average of the first

C concentrations were uniform across the site [for more production year of F in rotation A, B and C. No root

details see Bolinder et al. (2010)]. The initial C mass in biomass measurement was made for any crop.

each of the SOC pools was estimated as described by The site was sampled in the fall of 2008 for the 0- to

Ka¨tterer et al. (2008) by assuming that the young pools 25-cm soil layer by taking three soil cores in each of

were in approximate equilibrium with the previous the plots. Dry soil bulk density was calculated, and soil

conditions at the site. For that purpose we used the organic carbon (SOC) was determined by dry combus-

estimated parameter values of r e and i for rotation B tion (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 calcula-

Can. J. Soil. Sci. Downloaded from pubs.aic.ca by University of Laval on 07/06/15 tions are described in detail by Bolinder et al. (2010), where they were applied to the mean values of the ‘Young C pool’

(1–h)k Y r Y

four different rotations. In this paper we made a more detailed analysis with focus on rotation A and D

hk Y r e Y

by calculating the changes in SOC stocks for each

‘Old C pool’

individual plot.

The ICBM Concept and Parameterization The Introductory Carbon Balance Model (ICBM) is

Fig. 2. Structure of the Introductory Carbon Balance Model

and k O state-variables; ‘‘Young’’ (Y) and ‘‘Old’’ (O) SOC pools

a two-compartment first-order kinetic model with two

order decomposition rate constants for the Young and with specific decomposition rates (k Y and k O , respec-

tively) (Fig. 2). The first-order decomposition rates e

management parameter.

826 CANADIAN JOURNAL OF SOIL SCIENCE

to derive initial data for all rotations, since that rota-

Table 3. Mean soil organic carbon (SOC) content and estimated soil

tion reflected the most common previous management

water content at field capacity (u fc ) in the 0 25 cm soil layer in 2008 for

practices. Consequently, the initial mass from crop

each of the four rotations at Offer

residues was 0.378 and that from manure 0.109 kg

u Cm fc . The initial size of the old SOC pool (7.703 kg Cm ) was set to the difference between the total SOC

stocks measured in 1956 and the sum of these two

A 3.18 0.455

B 2.70 fractions. 0.432

C 2.38 0.416 D 2.19 0.406

Estimating the Soil Climate-management

Parameter (r e )

Annual dynamics of GAI is governed by empirical

functions, the amplitude (GAI max ) of which is related to three components: soil water content (r w ), soil tempera-

The soil climate-management factor, r e , is governed by

the mass of harvested products and a crop specific

ture (r T ) and tillage intensity (r c ). These three compo-

parameter related to the length of the vegetation period.

For undersown barley and winter rye we used the r T

nents are assumed to be multiplicative, i.e., r e W

function presented by Bolinder et al. (2008). For root previously been based mostly on assumptions and expert

C . Where the r c component (i.e., tillage) has

crops, we used a fixed GAI

opinion (Andre´n et al. 2004; Ka¨tterer et al. 2008). The

max

first two components (r and r ) are estimated with a from potatoes (Fortin 2008). For forage crops, we

soil climate module connected to ICBM. This module simulated leaf area dynamics with logistic functions uses daily standard meteorological data, a soil water

for the two seasons between the cuts. Estimates of model, and commonly used assumptions with respect to

) were based on the relationships between temperature, soil water con-

GAI max

data from Be´langer and Richards (1995) using the tent and biological activity (Andre´n et al. 2004, 2007;

assumption that GAI max did not exceed a value of Bolinder et al. 2007b, 2008). It also uses green area index

10 m 2 m . If not recorded in log-files from the ex- (GAI) dynamics that influence both the r W and r T

perimental station, we assumed reasonable dates for components. The GAI is defined as the projected area of

management operations (sowing, harvesting, plowing all plant parts that are visibly green (Ka¨tterer and

etc.). The typical GAI dynamics of the different crops Andre´n 2008).

contributes to the difference in the soil climate/manage- The drivers for estimating r w are daily precipitation

ment factor between continuous forage rotations versus and potential evapotranspiration data and two soil

rotations with annual crops (Fig. 3). parameters: water content at wilting point (u wp ) and

The simple bucket-model used to calculate daily soil For personal use only.

water content was based on concepts as described in u fc defines the storage capacity of plant-available

at field capacity (u fc ). The difference between u wp and

water. Furthermore, GAI is governing the plant water

requirements for transpiration. Daily climatic data (air temperature, potential evapotranspiration and precipi-

tation) 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 5

from 1961 to 1964 and 2003 to 2005, respectively.

Pedotransfer functions developed from a Swedish soil

GAI

Can. J. Soil. Sci. Downloaded from pubs.aic.ca by University of Laval on 07/06/15 database (model 7; Ka¨tterer et al. 2006) were used for

estimating u fc and u wp from soil texture and C concen-

trations. Since soil C concentrations changed over time

in the treatments, we estimated these parameters for both 1957 and 2008 and calculated intermediate values 1

by linear interpolation. The water content at wilting

3 point did not change with time and was 0.126 m 0 m . 0 50 100 150

3 The water content at field capacity was 0.434 m Julian day m in 1957 and changed depending on changes in soil C

Forage

Root crop

Barley

(Table 3). In 2008, u fc was 12% higher in rotation A

Fig. 3. Representation of typical green area index (GAI)

dynamics for forages, root and barley crops. The GAI is here close to those measured in an adjacent field, i.e., 0.128

than in rotation D. These estimates for u fc and u wp were

defined as the projected area of all plant parts that are visibly and 0.456 m m , respectively (Andersson and Wiklert

green and was estimated with different functions, as described 1977).

in the text.

BOLINDER ET AL. * MODELING SOC DYNAMICS IN FORAGE-BASED CROP ROTATIONS 827 detail by Karlsson et al. (2011) and Fortin et al. (2011).

contributes the most to the total annual C inputs to This model involves parameters for precipitation and

soil from crop residues, while the undersown barley crop interception of precipitation; both of these are

contributes a similar amount in both rotations. The modulated by the GAI. A semi-empirical model was

winter rye, undersown barley, peas and the root crops used for estimating daily soil temperature from air

generally contribute a smaller amount of BG C, temperature and leaf area index as described by Ka¨tterer

compared with the forage crop at the end of rotation and Andre´n (2008). Leaf area index was assumed to

A and the forage crop grown as green manure in

be 80% of GAI [see Ka¨tterer and Andre´n (2008) for

rotation D.

details]. Daily soil water and soil temperature were The annual amount of BG C inputs to soil for the transformed into the activity factors, r w and r T , respec-

three phases related to the continuous forage-based crop tively, and the resulting annual means of the product

rotation A (Fig. 4) was estimated using the Bolinder r w

T calculated for each day were then divided by a et al. (2007a) methodology. Considering the entire constant scaling factor which refers to conditions at the

period from 1957 to 2008, our values for the annual reference sites used for model calibration [see Bolinder

BG C input were 1.0290.27 Mg C ha yr for the et al. (2008) for details]. All calculations were conducted

undersown barley, 1.4390.40 for the forage crop grown for each experimental plot.

in a production year and 3.4490.93 Mg C ha yr for the forage crop grown at the end of rotation A. This

Estimating Annual C Inputs to Soil (i) is lower than the example of values presented in Fig. 1 We used the plant C allocation coefficients as described

yr , respectively) and in detail by Bolinder et al. (2007a) to estimate the annual

(1.49, 2.30 and 5.83 Mg C ha

is explained by the fact that the estimates of BG input is

C inputs to soil from BG for the undersown barley, specific to the AG productivity at the Offer site. forage and green manure crops. The coefficients were

The Bolinder et al. (2007a) methodology also includes based on a review on plant shoot-to-root ratios for these

specific assumptions about the BG allocation based crop types. We considered that 40% of the total annual

on the values that were reviewed for Canada. However, AG production for forage was returned to the soil, i.e.,

the relative differences in BG C inputs to soil between as litter fall and harvest losses during the growing season

the three phases we obtained with our approach are plus the regrowth after the last cut dying during the

similar to those from a broader perspective, as presented winter. For the crop types in rotation D that were not

in Fig. 1.

addressed in Bolinder et al. (2007a), we used data from a The two main driving variables used to estimate the Swedish field study and calculated BG input for winter

effects of climate on SOC decomposition are air tem- rye with a shoot- to root-ratio of 13.5 (Ka¨tterer et al.

perature and total precipitation (Fig. 6). They are used to 1993). The BG input for peas was based on data from

calculate the multiplicative soil-temperature (r T ) and Wichern et al. (2007). The shoot refers to the total AG

soil-moisture (r W ) factor that results in the soil-climate For personal use only.

material (i.e., grain, straw, vines etc.). We used a fixed parameter r e (Fig. 7). In the ICBM, annual average estimate (0.12 kg DM m ) of BG input for the potato

values for r e were used when simulating changes in SOC crop (Carter et al. 2003) and for the other two root crops

stocks through time. However, the r e parameter is (i.e., carrot and rutabaga). We assumed that the C

calculated using daily time-steps for the climatic input content of all plant parts was 0.45g g and the ER-C

data because it has been shown that this allows a more coefficient was set to 0.65.

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 r e

RESULTS AND DISCUSSION value for Offer was 0.88 for rotation A and 0.92 for Annual C Inputs to Soil and Climate Factors

rotation D (Table 4). Because the total steady-state The average annual C inputs to soil from crop residues

C mass is linear in response to the ICBM r e and i Can. J. Soil. Sci. Downloaded from pubs.aic.ca by University of Laval on 07/06/15

(1957 to 2008) were similar between rotations A and D, parameters (Andre´n and Ka¨tterer 1997). This means that with 0.280 kg C m

it would be necessary to have an annual C input to soil m

yr

for rotation A and 0.276 kg C

yr for rotation D. The continuous forage-based about 5% higher (i.e., 0.92/0.88) for rotation D in order rotation A also received manure applications equivalent

to reach the same steady-state soil C mass as that for to 0.075 kg C m

yr

and the total annual C input to

rotation A.

soil was therefore higher than that for rotation D. The The first-order decomposition rates for the ‘‘young’’ BG C inputs were higher in rotation A (0.170 kg C m

(k Y ) and ‘‘old’’ (k O ) SOC pools in ICBM are multiplied yr ) compared with those of rotation D (0.126 kg C

by the soil-climate parameter. Consequently, r e also m

yr ). affects the inter-annual variations in decay rates There were inter-annual variations in the total

(Fig. 7). Other components of the ecosystem, both and BG annual C inputs to soil in the rotations,

biotic and abiotic related factors (e.g., soil structure both between crop types and for a given crop type

and earthworm activity), also play a role in temporal (Figs. 4 and 5). The forage crop grown at the end of

variations in SOC decay rates, but they are less well rotation A and the green manure crop in rotation D

documented and orders of magnitude more difficult to

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 Predicting Final SOC Stocks with ICBM reasonable assumptions are made in most SOC models.

The predicted final SOC stocks with the ICBM for

The inter-annual range in r e for the Offer site was about

rotation A were close to those measured for four of the

a factor 2, with an average minimum and maximum six plots (plot no. 2, 3, 5 and 6), for which the deviation value of 0.66 and 1.15, respectively.

from measured final SOC stocks was less than 5%

For personal use only.

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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).

BOLINDER ET AL. * MODELING SOC DYNAMICS IN FORAGE-BASED CROP ROTATIONS 829

Mean annual temperature

Mean annual total precipitation

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

rotation A and plot no. 3 in rotation D. Consequently, the six plots (plot no. 1, 4 and 6) was also predicted with

ICBM over-estimated the final SOC stocks for these two less than 5% deviation, and two plots (plot nos. 2 and 5)

plots. ICBM under-estimated the final SOC stocks for with about 10% deviation from measured final SOC

plot no. 1 in rotation A since the measured final SOC stocks. The final measured SOC stocks were somewhat

stocks for this plot were high. We have no particular lower than the overall mean values for plot no. 4 in

explanation why the measured final SOC stock values for

For personal use only.

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Fig. 7. The mean annual soil climate-management parameter (r e ) 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., r e W

T ).

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 ). The values for the annual crop residue C inputs to soil (i) and the soil climate/management factor (r e ) are annual averages for the time

period 1957 to 2008. The scaling factor for cultivation (r C ) was optimized for each plot Plot no. rotation A

1 2 3 4 5 6 Mean Measured z

0.290 0.280 r e 0.88 0.88 0.87 0.88 0.88 0.87 0.88

Plot no. rotation D

1 2 3 4 5 6 Mean Measured

7.2 6.8 6.3 7.6 6.7 6.9 6.9 ICBM z

0.256 0.276 r e 0.93 0.92 0.92 0.91 0.92 0.92 0.92 Optimized y r C 1.05 1.30 1.57 0.95 1.27 1.10 1.21

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 r C 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 We also predicted final SOC stocks considering the values. The mean absolute deviation from measured

equivalent soil mass concept for each of the plots. This values for the six large plots for rotation A was 6.9%

concept applied on SOC stock dynamics accounts for and that for rotation D 7.5%. These overall predictions

changes in soil bulk density due, for example, to tillage made with ICBM are within the range of commonly

and manure applications. Although the application of observed results with SOC models applied to data from

manure resulted in lower bulk density for rotation A, in long-term experiments (e.g., Bolinder et al. 2006).

this study this effect was not crucial and the equivalent The methodology used to estimate annual crop

soil depth was on average 26.5 cm (Bolinder et al. 2010). residue C inputs to soil performed reasonably well.

Values for each plot ranged from 25.1 to 28 cm (data not Annual C inputs to soil were only under-estimated by

shown) and this has a fairly small effect on the predicted 4% (data not shown) when optimizing the annual C

SOC stocks and the optimized i parameter. The effect of For personal use only.

inputs to soil from the undersown barley and forage using equivalent soil depth for rotation D was also small crops for rotation A [i.e., by multiplying the total annual

(i.e., equivalent depth was 23 cm). crop residue C inputs to soil (i) by a constant in order to

For rotation D, the annual C inputs to soil for the force the ICBM predicted value to match the measured

undersown barley and green manure (i.e., the forage value for each plot]. This exercise of course implies that

crop grown as green manure) followed the Bolinder the reference values for the other four parameters

et al. (2007a) methodology, and as discussed above gave remained unchanged. The objective of this study was

reasonable estimations. For the 2 yr of root crops in that to examine the performance used to estimate the i

rotation we used a fixed estimate for BG C inputs. If we parameter for rotation A in such a scenario.

consider that annual C inputs to soil for the other two The data used to derive the relative plant C allocation

crops in that 6-yr rotation (i.e., winter rye and peas) coefficients for forage crops and undersown barley used

were well estimated and using the default parameter Can. J. Soil. Sci. Downloaded from pubs.aic.ca by University of Laval on 07/06/15

in this study originated from a review of field measure- settings for ICBM, then we can have a ‘‘rough estimate’’ ments of shoot and root C allocation. Most of the field

for the cultivation factor (r C ). This resulted in a mean studies that were reviewed considered mainly the 0-to

optimized r C value of 1.21, indicating that cultivation 18-cm, 0- to 20-cm or 0- to 30-cm depths [see Bolinder

would have accelerated the decomposition by 21% in et al. (2007a) for more details]. Only a limited number of

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

of the data included 45-cm depth, but it was found that rotation (i.e., 5 yr of annual crops as compared with the the root biomass measured in the deepest sampling layer

continuous forage rotation A). In previous ICBM applications, the relative difference in decomposition

biomass. Therefore, we considered that the estimate rates between an annual crop or a root crop versus of BG C input with this methodology was representative

a perennial forage crop has used guestimates for r C of for the 0- to 25-cm depth, which was the sampling depth

10 and 30%, respectively (Andre´n et al. 2004; Ka¨tterer used to estimate the SOC stocks in this study.

et al. 2008). Of course, the additional effect attributed to

BOLINDER ET AL. * MODELING SOC DYNAMICS IN FORAGE-BASED CROP ROTATIONS 831 r C could well be assigned to the other components of the

range. These results improve our confidence in using the ICBM approach (e.g., decay rates, SOC pool sizes, etc.).

ICBM and other models to predict SOC balances for For instance, in another study on SOC dynamics in

forage-based crop rotations in cool, temperate agricul-

a long-term Swedish field experiment, Ka¨tterer et al.

tural regions.

(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

ACKNOWLEDGMENTS residues. That study comprised more detailed data

This work was funded by the Swedish Farmer’s records for soil C changes and was dominated by spring

Foundation for Agricultural Research within the project cereals. However, if we apply these findings to the

‘‘The impact of perennial leys in crop rotations on current study, by calculating a weighted average of h Y

soil carbon balances’’. Additional financial support was for the above- and below-ground crop residues for

also provided by the NSERC project CRDPJ-385199 rotation A and D, then the simulated relative differences

on ecosystem services and collaborative potato farms. between the treatments decreases. With these assump-

We acknowledge Lars Ericson and Kent Dryler who

provided archived yield records. Thanks to Mireille 21% to about 10% since the below-ground C input was

tions, the effect attributed to r C would decrease from

Vigneault who kindly prepared illustrations for the relatively higher in A than in rotation D.

conceptual Figure 1.

Furthermore, it is difficult to use data from long-term field experiments to examine the effect of manage-

Andersson, S. and Wiklert, P. 1977. Studier av markprofiler ment and land use change on SOC dynamics, since they

i svenska a˚kerjordar. Del III. Norrbottens-, Va¨sterbottens-, were often not designed for that purpose. This implies

Va¨sternorrlands och Ja¨mtlands la¨n. Swedish University that it is rather problematic to define ‘‘clear-cut’’ effects

of Agricultural Sciences, Department of Soil Sciences, and there are several non-trivial potential error sources.

Division of Agricultural Hydrotechnics. Report 104. [Online] However, they constitute one of the best sources we have Available: http://pub-epsilon.slu.se/1976/01/andersson_wiklert_

090908.pdf.

to generate hypotheses and bring models into line. In

Andre´n, O. and Ka¨tterer, T. 1997.

experiments like this, the effects of tillage and crop types Carbon Balance Model for exploration of soil carbon in the different rotations are unavoidably confounded.

The initial plant communities of the experimental site Andre´n, O., Kihara, J., Bationo, A., Vanlauwe, B. and Ka¨tterer, they replace may also be a problem (DuPont et al. 2010).

T. 2007. Soil climate and decomposer activity in Sub-Saharan However, this latter aspect may have been less of a

Afrika estimated from standard weather station data: A simple problem in this study because the management of Offer

climate index for soil carbon balance calculations. Ambio 36: prior to initiation involved similar cropping systems used

in the four rotations, i.e., forages and annual crops such Andre´n, O., Ka¨tterer, T., Karlsson, T. and Eriksson, J. 2008.

For personal use only. as small-grain cereals (Bolinder et al. 2010). with preliminary projections. Nutr. Cycl. Agroecosyst. 81: It is recognized that management-induced changes on

SOC stocks are limited to the approximate depth of the Andre´n, O., Ka¨tterer, T. and Karlssson, T. 2004. ICBM regional model for estimations of dynamics of agricultural

(IPCC 2006). Furthermore, the SOC stock changes for grasses appear to decrease with greater depths, reaching

Andre´n, O., Lindberg, T., Bostro¨m, U., Clarholm, M., Hansson,

a modest level beyond the 30-cm depth (Liebig et al. A-C., Johansson, G., Lagerlo¨f, J., Paustian, K., Persson, J., 2010). However, there is a considerable gap in the under-

Pettersson, R., Schnurer, J., Sohlenius, B. and Wivstad, M. standing of SOC dynamics in the whole soil profile, and

1989. Chapter 5. Organic carbon and nitrogen flows. In O. modeling of SOC dynamics from this perspective should Andre´n, T. Lindberg, K. Paustian, and T. Rosswall, eds.

be improved (e.g., Ga¨rdena¨s et al. 2011). Can. J. Soil. Sci. Downloaded from pubs.aic.ca by University of Laval on 07/06/15

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