carbon turnover and sequestration potential of fodder radish cover crop

Carbon turnover and sequestration potential of fodderradish cover crop

J. K. MUTEGI1 ,2, B. M. PETERSEN

1 & L. J. MUNKHOLM1

1Department of Agroecology, Faculty of Science and Technology, Aarhus University, P.O. Box 50, 8830 Tjele, Denmark, and2Embu University College (a constituent college of the University of Nairobi), P.O. Box 6-60100, Embu, Kenya

Abstract

We studied fodder radish carbon turnover as affected by soil tillage in Foulum, Denmark. Actively

growing fodder radish monoliths from direct-drilled (DD) and conventionally tilled (CT) plots were

extracted and labelled regularly with 14C isotope across their entire growth period. At the end of the

fodder radish growth cycle, labelled biomass was harvested and incorporated into the same monolith.

These monoliths were destructively sampled at biomass incorporation, 4, 8 and 18 months after

incorporation. For each sampling period, soil and root samples were taken at 0- to 10-, 10- to 25-,

and 25- to 45-cm-depth increments for determination of 14C distribution and retention. Carbon-14

declined significantly with increasing soil depth at each sampling for the two tillage practices

(P < 0.05). We further observed significantly higher 14C at 0–10 cm for DD than for CT at 4 and

8 months after biomass incorporation. For the 10–25 cm depth, 14C was significantly higher for CT

than for DD, 4 and 8 months after incorporation. However, despite these depth-specific differences,

cumulative (0–45 cm soil depth) 14C retention was similar for DD and CT treatments for all the

sampling periods. On the basis of a CN-SIM model forecast, we estimated that over a 30-yr period of

continuous autumn fodder radish establishment, at least 4.9 t C/ha fodder radish C with a residence

time of more than 20 yr could be stored in the soil.

Keywords: Isotopes, carbon sequestration, carbon turnover

Introduction

Historically, agricultural soils worldwide have lost more than

50 pg C, but some of this lost C can be recovered through

improved land and crop management and in this way decrease

the atmospheric concentration of CO2 (Smith et al., 2007).

Winter cover/catch crops are increasingly used in temperate

agricultural systems for N management and for breaking pest

cycles (Harper et al., 1995; Kristensen & Thorup-Kristensen,

2004). Catch crops with a large and deep-rooted biomass may

stimulate long-term soil carbon sequestration because root-

derived carbon has a lower turnover rate than shoot-derived

carbon (Rasse et al., 2005). This, added to aboveground

biomass which is eventually incorporated into the soil, can

significantly influence belowground C.

Previous studies indicate that no-till, also referred to as

direct drilling (DD), retains more carbon than conventional

till (CT) in the soil profile (West & Post, 2002; Six et al.,

2004). More recent studies have, however, demonstrated that

the differences between the C sequestration capacity of

reduced till and conventional till are depth specific (Deen &

Kataki, 2003; Murage et al., 2007). Often studies have

indicated disparate biomass decomposition rates between CT

and DD (Six et al., 2004) with actual rates depending on

soil depths and soil types (Deen & Kataki, 2003). For

many catch crops and environments, the amount of C

retained at various times after incorporation remains

unclear, and the overall effect of land management on C

sequestration and climate change also remains an important

research question.

Fodder radish (Raphanus sativus L.) and other closely

related brassica plants are widely grown in Europe (Allison

et al., 1998) and North America (Dean & Weil, 2009) as

either cover crops or catch crops during autumn. Due to the

shortness of an autumn growing period and the cold winter

that follows, catch crops provide a means of reducing the

bare-soil period at a time when the weather precludes other

crops from growing to maturity. In addition to reducing the

potential carbon losses that would result from a prolongedCorrespondence: J. K. Mutegi. E-mail: [email protected]

Received June 2012; accepted after revision January 2013

© 2013 Aarhus University. Journal compilation © 2013 British Society of Soil Science 191

Soil Use and Management, June 2013, 29, 191–198 doi: 10.1111/sum.12038

SoilUseandManagement

fallow period, high above- and belowground biomass fodder

radish yields of up to 2 t/ha (Kristensen & Thorup-

Kristensen, 2004) could significantly boost soil carbon input

upon incorporation. It is therefore logical to expect a

significant C input to the soil from fodder radish, but the

rate of C turnover and long-term implications are unclear.

In the present study, we used the 14CO2 labelling

technique to compare the fate of fodder radish C in DD and

CT after biomass incorporation over an 18-month period.

Furthermore, using the seven-pool SOM model (CN-SIM)

described by Petersen et al. (2005a) in combination with our

18-month time-stepped 14C measurements, we modelled the

turnover trend of fodder radish C over a 35-yr period.

The objective of this study was to evaluate the effect of

tillage on soil profile distribution and fate of fodder radish-

derived C.

Materials and methods

Site description

We conducted this study between the summer of 2008 and

the autumn of 2010 in a 6-yr-old field tillage experiment and

a semi-field facility. The semi-field facility at Foulum

Research Centre (56°30N, 9°35E) in Denmark represents a

smaller-scale outdoor experimental facility close to actual

field plots where experiments under more controlled

conditions can be carried out (Ahmadi et al., 2011; Mutegi

et al., 2011). Foulum soil is a Mollic Luvisol with 9% clay,

13% silt (2–20 lm), 44% fine sand (20–200 lm), 31% coarse

sand (200–2000 lm) and 18 g C/kg in the top 0- to 25-cm

soil layer (Munkholm et al., 2008). The monthly average

rainfall, air temperature and soil temperature (10 cm and

30 cm depth) in Foulum during the study are shown in

Figure 1.

Experimental design

The main experiment was a split-plot with four replicate

blocks, with tillage treatments in each subplot (Hansen

et al., 2010). The tillage practices included DD and

conventional till, 18–20 cm depth. From when tillage was

started in 2002 to the present sampling period, the plots

were managed as cereal rotations with autumn catch crops

undersown during the summer–autumn fallow period.

Residues from cereals and cover crops were always retained

in the plots. The plots were fertilized every year in spring in

accordance with the current standard Danish regulations,

that is, manure applied at a rate of 140 kg total-N/ha and N

fertilizer applied at a rate of 44 kg N/ha. Each block

contained two similar 72.2 9 3 m tillage plots, which were

further subdivided into smaller 20 9 3 m plots that were

allocated to different catch crops. In this study, a fodder

radish catch crop was undersown by surface broadcasting

into spring barley two weeks prior to spring barley harvest.

Neighbouring pairs of DD and CT in three of the four

blocks under the fodder radish catch crop were used for

monolith extraction. Details of monolith extraction and

randomization in the semi-field experiment prior to labelling

are shown below.

Monolith extraction

Sixty intact soil monoliths with actively growing standing

fodder radish seedlings were extracted from three blocks (10

per treatment per block) by random placement and

extraction of stainless steel cylinders (20 cm diameter

9 50 cm height) within each plot between 23 and 25

September 2008. Care was taken to recover a continuous

undisturbed soil monolith to ensure soil structural similarity

between monoliths and the plots from where they were

extracted. The cylinders were then transferred to the adjacent

Months

Apr-09

May-09Ju

n-09Ju

l-09

Aug-09

Sep-09Oct-

10

Nov-10

Dec-10Ja

n-10

Feb-10

Mar-10Apr-1

0

May-10Ju

n-10Ju

l-10

Rai

nfal

l (m

m)

20

40

60

80

100

120

140

Air

tem

pera

ture

°C

–10

0

10

20

30Rainfall (mm) Air temperature °CSoil temp (10 cm depth) Soil temp (30 cm depth)

Figure 1 Average monthly rainfall and

temperature from fodder radish incorporation

in April 2009 to final monolith sampling in

October 2010.

© 2013 Aarhus University. Journal compilation © 2013 British Society of Soil Science, Soil Use and Management, 29, 191–198

192 J. K. Mutegi et al.

semi-field facility and exposed to similar agronomic and

environmental conditions as in the field, pending labelling

with 14C isotope.

Prior to labelling, the number of fodder radish plants

growing in each cylinder was thinned to two giving a plant

density of 64 plants/m2. The cylinders were then

systematically randomized into groups of 12 (to be sampled

at any one time) composed of two replicates of each tillage

treatment per block (two DD cylinders 9 three blocks +two CT cylinders 9 three blocks). All the 12-cylinder

groups were then arranged in two rows of six cylinders on

top of a subsoil-filled wooden box and the spaces between

them filled with sand to maintain realistic thermal

properties. A firm mesh was fixed around each cylinder

rising to 50 cm above the cylinder height to retain any

dropping fodder radish leaves within the cylinders where

they had been growing.

Fodder radish labelling equipment

Details of fabrication of the 14CO2 assimilation chamber are

presented in Mutegi et al. (2011). In brief, for instantaneous14C-labelling of all fodder radish monoliths, a transparent,

airtight, 14CO2 assimilation chamber (2.7 9 1.4 9 2

m = 7.56 m3; L 9 W 9 H) was fabricated using a

transparent polycarbonate sheet. The assimilation chamber

was connected to a cooling tower by airtight, flexible,

aluminium tubes (Ø = 6.5 cm) and was equipped with a

temperature control unit, a 14CO2 production unit and a fan

to circulate air between the cooling tower and labelling

chamber and to ensure complete mixing of air in the

chamber. The CO2 concentration monitoring chamber was

developed by connecting a Licor-800 infrared CO2 gas

analyzer to the 14CO2 assimilation chamber throughout the

labelling period. A detector for ambient temperature

connected to an automatic temperature regulation system

made from a commercial air conditioning unit and a

temperature sensor enabled synchronization of external and

internal temperatures of the 14CO2 assimilation chamber.

The airtightness of the assimilation chamber was tested and

confirmed by pumping in standard unlabelled CO2 and

monitoring the CO2 concentration for 24 h in the absence of

growing plants.

Fodder radish labelling

The 14CO2 that was used for labelling was produced by a

reaction of diluted Na2H14CO3 with 1 M H2SO4. The

labelling period was daily between 10 am and 3 pm in the

period from 10 October 2008 to 20 November 2008.

Continuous agitation of the Na2H14CO3/H2SO4 mixture by a

magnetic shaker ensured continued acid-carbonate reaction

and therefore continuous 14CO2 evolution during the

labelling period.

Throughout the labelling period, we monitored the

concentration of 14CO2 in the labelling chamber using a

Licor-800 infrared CO2 gas analyzer. The CO2 gas analyzer

was calibrated regularly for 0 ppm and at 500 ppm using

standard Scott Specialty unlabelled CO2 gas (Scott Specialty

Gases, the Netherlands). A liquid metering pump connected

the Na2H14CO3/H2SO4 interphase and the Licor-800 (high

alarm = 410 ppm; low alarm = 350 ppm), enabling the

pumping of Na2H14CO3 into H2SO4 to stop automatically

when the CO2 concentration in the assimilation chamber

reached 410 ppm and to start when the CO2 concentration

dropped to 350 ppm. The chamber covered the crops for the

4–8 labelling hours, while for the rest of the time plants grew

in open air just like in the field plots. A detailed description

of labelling days throughout the fodder radish growth period

and the amount of 14C used per labelling day are presented

in Mutegi et al. (2011).

Field operations and sampling of labelled monoliths

Labelled biomass was incorporated to 18–20 cm depth of

CT monoliths using a hand hoe. For DD monoliths, labelled

biomass was left on the surface to mineralize in situ. The

biomass was incorporated at the end of March 2009. This

implies that for the sampling dates that followed

incorporation, only belowground components could be

sampled. The first sampling was carried out 6 days after

termination of labelling in November 2008 at the last phase

of fodder radish crop growth (fodder radish was killed by

frost 3 days later) and the second at biomass incorporation

phase in spring 2009. Consecutive samplings were thereafter

performed 4, 8 and 18 months after incorporation. The first

sampling is the subject of another paper (Mutegi et al.,

2011). The results presented here are those of samples

collected at incorporation in spring and at 4, 8 and

18 months after biomass incorporation. At every sampling,

12 monoliths (two rows) were destructively sampled by

cutting aboveground biomass at soil level and dividing soils

by depth increments (0–10, 10–25 and 25–45 cm). This depth

partitioning was followed by separation of roots from soil

for each of the three depths by gentle shaking followed by

sieving the soil through a 1-mm sieve and handpicking all

visible roots (macro-roots). While working on barley and

wheat, Jensen (1993) and Swinnen et al. (1994) showed that

the loss of micro-roots during root-washing could lead to a

loss of between 5 and 41% of labelled C depending on the

operator precision in picking micro-roots and the stage of

plant development. We minimized losses of labelled C by not

washing the roots. This was crucial for determination of the

total system carbon losses via biological processes and

carbon retention. After each sampling, the remaining

monoliths were exposed to the same natural conditions and

agronomic practices as the field plots from where they had

been extracted. A summary of field activities between


Long-term implications of cover crops on soil carbon 193

incorporation and final monolith harvest is shown in

Table 1.

Sample preparation and analysis

Soil from each depth was dried at 80 °C for 48 h and mixed

thoroughly to homogenize. A 200 g subsample was then ball

milled to fine powder for analysis. Roots and shoots were

dried at 60 °C to a constant weight and ground to a fine

powder. Total C and N of root and shoot samples was

determined by CNS and Dumas methods, respectively

(Hansen, 1989). Sample analysis to determine 14C activity

was preceded by combustion of 50 lg of soil weighed into

flammable combusto-cones (paper foam-Holland) on a

Model 307 Packard Sample oxidizer for 1 min. Carbon-14

recovery performance of the sample oxidizer was tested

immediately before initiation of sample oxidation and

between every 20 samples. Recoveries of 96% and above

were accepted and used as a correction for calculation of

sample 14C activity. The analysis procedures were repeated

for samples where recoveries were less than 96% up to the

time acceptable recoveries were achieved. The 14CO2 evolved

during combustion was trapped in Carbosorb (Packard) and

flushed into 20-ml scintillation vials using Permafluor as a

rinsing medium. The 14C activity (dpm) in these vials was

determined with a Beckman liquid scintillation counter

(Tri-Carb, Packard) set at a count rate of 5 min per sample.

Two replicates were combusted and analysed for every

sample and an average of the two adopted as the actual

activity of the sample. Table 2 sets out the 14C distribution

shortly after labelling (autumn–winter 2008) and immediately

prior to biomass incorporation in the spring of 2009.

Modelling soil organic matter turnover

To estimate the long-term consequences of C sequestration,

the measurements were supplemented with soil organic

matter modelling. This was performed with the CN-SIM

carbon model (Petersen et al., 2005a). The model utilizes

seven C pools, with first-order decay in daily time steps. One

of the pools is considered inert, the other six pools are

coupled. Two pools are used to simulate microbial biomass,

and the decay for all six active pools is affected by

temperature. For simplicity, the turnover was assumed to

begin from the time of incorporation (March–April 2009).

As there are no parameters available for fodder radish

components, those of species from the same family

(Brassicaceae) were utilized. So for plant tops, parameters

for white cabbage (Brassica oleracea) were used, and for

roots, those for oilseed rape (Brassica napus) were used.

These parameters were adopted from Petersen et al. (2005b).

The top organic matter was assumed to constitute 66.5%

and the root organic matter 33.5%, taken from the average

fodder radish 14C translocation (Mutegi et al., 2011) because

the present study is a continuation of Mutegi et al. (2011).

To perform simulation forecasts, mean air temperatures for

Denmark 1961–1990 (Danish Meteorological Institute) were

utilized for the period starting October 2010 onwards. The

coefficient of determination (R2) or CN-SIM model fit was

96% and highly significant (P < 0.001). This implies that it

could confidently simulate fodder radish carbon decay as a

function of time.

Statistical analysis

All statistical analyses were carried out using PROC

MIXED procedures of SAS (SAS, 2007). We analysed data

on a per sampling basis and then across samplings to reveal

within sampling and between sampling differences. Unless

otherwise stated, differences between treatments, depths and

sampling dates were considered significant at the P < 0.05

probability level.

Results

Effect of tillage and time on soil 14C activity

Soil radioactivity 4, 8 and 18 months after biomass

incorporation decreased with increasing soil depth (P < 0.05)

for both DD and CT treatments (Table 3). For each of the

two treatments, radioactivity decreased with length of time

from the date of incorporation for all the depths, but more

significantly for the 0–10 cm depth (P < 0.01).

Table 1 Field activities in fodder radish plots and monoliths after

fodder radish biomass incorporation

Date

(DD/MM/YY) Activity

24.03.09 Monolith sampling (residue incorporation phase)

(Time 0)

27.03.09 Ploughing to 20 cm for CT and weed spraying

for DD

29.03.09 Spring Barley sown

07.04.09 Mineral N fertilizer NS-24-7 applied (44 kg N/ha)

14.04.09 Manganese applied (3.0 kg/ha)

24.04.09 Slurry applied (140 kg total-N/ha)

30.07.09 Monolith sampling (Time 4 months)

24.08.09 Spring barley harvested

17.11.09 Monolith harvest (Time 8 months)

12.04.10 Ploughing to 20 cm for CT and weed spraying

for DD

15.04.10 Sowing spring barley

23.04.10 Mineral N fertilizer NS-24-7 applied (44 kg N/ha)

28.04.10 Manganese applied (3.0 kg/ha)

28.08.10 Harvest spring barley

01.09.10 Final monolith harvest (Time 18 months)



Effect of time and depth on belowground levels of fodder

radish 14C

As noted earlier, shoot biomass was available only during

incorporation (i.e. time 0 months). For the later samplings,

we tracked 14C dynamics from incorporated biomass, roots

and rhizodeposits in the soil. The Type three test of fixed

effects revealed no tillage effects but a significant effect of

time, that is, months and tillage*months (P < 0.05), on 14C.

The amount of 14C in shoots was twice as high as the

amount of 14C in belowground components (soil + roots)

(P < 0.001) for each of the two tillage treatments at

incorporation (Table 4). Even without accounting for shoot14C, which was slightly higher in CT than in the DD at

incorporation, belowground 14C was 423 and 388 9 10�5 Bq14C/m2 in the CT and DD, respectively, at incorporation,

implying a higher C translocation into the soil in CT than in

DD during the fodder radish crop growth phase. At 4, 8 and

18 months after incorporation, we observed higher 14C in

the 0- to 10-cm layer in the DD treatment than in the CT,

but for the 10–25 cm depth, 14C was ca. twice as high in the

CT as in the DD for all samplings (P < 0.05) after

incorporation. Additionally, across the four samplings,

belowground 14C decreased with increasing depth and

sampling time in the DD treatment. This was, however, not

the case for CT where at 4 months, the highest

concentration of 14C was observed at the 10–25 cm depth

(Table 4). Cumulatively (0–45 cm depth), however, for every

sampling, the total amount of labelled C retained in the soil

was similar for DD and CT (P < 0.05) (Table 4).

At incorporation, the patterns of 14C accumulation with

depth for the two tillage treatments were similar. These

patterns, however, varied with the type of tillage 4, 8 and

18 months later (Figure 2). In the DD treatment, there was

a significant accumulation of 14C in the top 0–10 cm relative

to other depths across the four sampling periods. In

contrast, the pattern of 14C in CT was characterized by

lower 14C activity at the 0–10 cm soil depth, but a clear

accumulation of 14C in the10- to 25-cm soil layer four and

Table 2 Distribution of 14C (%) in the plant soil system 6 and 100 days after termination of labelling, n = 6, values in brackets represent � SE

(adopted from Mutegi et al., 2011)

Time

Tillage

(%)

Shoot

(%)

Macro-roots

(%)‡ Soil (%)

Total belowground Total activity*

Total belowground

shoot ratioBq m�2 9 10�5

6 days after

labelling

DD 78.7 (1.5) 9.4 (1.4) 11.9 (1.2) 21.3 (1.5) 1920.5a 0.27

CT 78.3 (3.6) 10.3 (3.1) 11.5 (1.5) 21.7 (3.6) 2023.9a 0.29

At incorp† DD 66.8 (8.9) 10.4 (4.1) 22.7 (4.3) 33.2 (7.9) 1161.9b 0.47

CT 64.2(3.2) 6.8 (1.2) 28.9 (2.9) 35.8 (2.9) 1213.3b 0.59

*Total activity = belowground + aboveground activity per unit area †is used to represent distribution of 14C a week prior to actual

incorporation; it also represents 100 days after termination of labelling. ‡Macro-roots: the roots that cannot pass through a 1-mm sieve.

Table 3 Specific activity at soil depths down to 45 cms at 4, 8 and

18 months after fodder radish incorporation

Tillage

0–10 cm 10–25 cm 25–45 cm Average

Bq 14C/g

DD 4 months 327.5Aa 85.7Bb 39.6Ac 150.9A

8 months 299.2Ba 48.4Cb 28.9Ab 125.5AB

18 months 236.3Ca 43.3Cb 24.7Ab 101.5C

CT 4 months 173.0 Da 163.2Aa 35.5Ab 123.9AB

8 months 163.8 Da 145.6Aa 34.5Ab 114.6B

18 months 194.7CDa 101.1Bb 21.3Ac 106.9C

Column values followed by different uppercase letters are

significantly different (P < 0.05); row values followed by different

lower case letters are significantly different.

Table 4 Effect of tillage and time since incorporation on fodder

radish C retention at soil depths down to 45 cm

Tillage

0–10 cm 10–25 cm 25–45 cm Cumulative

Bq 14C/m2 9 10�5

DD 0 months† 241.0a 87.4b 52.9c 1161.9A

4 months 383.1Aa 165.2Bb 114.8Ab 663.1B

8 months 352.6Aa 92.8Cb 82.1Ac 527.6C

18 months 289.9Ba 85.1Cb 72.4Ab 447.5D

CT 0 months† 242.0a 107.4b 73.5c 1213.3A

4 months 207.3CDb 312.5Aa 102.2Ac 622.1B

8 months 191.6Db 278.6Aa 99.5Ac 569.7C

18 months 224.4Ca 191.4Ba 57.4Ab 473.3D

†14C at each of the depths for this row does not include

aboveground biomass, because aboveground and belowground

components were analysed separately at this sampling; cumulative

represents total aboveground + belowground 14C. It is assumed that

either there was no change in aboveground and belowground 14C

within the 10 days or that the change was negligible; Column values

followed by different uppercase letters are significantly different

(P < 0.05); row values followed by different lower case letters are

significantly different (P < 0.05).



8 months after biomass incorporation. However, 18 months

after incorporation, CT had significantly more 14C in the top

0–10 cm relative to other depths (Figure 2).

Carbon turnover

The fodder radish biomass C/N at incorporation was

similar for the two tillage treatments (Mutegi et al., 2011).

Monthly 14C change across the 18 month period was

plotted together with the modeled decay function

(Figure 3a). Four months after incorporation, DD and CT

treatments lost 43 and 49%, respectively, of 14C that was

available at incorporation (Table 4). Eight months from the

date of incorporation, DD had lost 55%, while CT had lost

53% of the carbon available at incorporation. Total carbon

losses 18 months after incorporation were similar for DD

and CT at 61.4 and 60.6%, respectively. As noted in

Mutegi et al. (2011), at incorporation the total fodder

radish contribution to belowground C was 162.4 and

169.1 g C/m2 for DD and CT, respectively. This implies

that for DD, the remaining fodder radish C in the soil was

ca. 93, 74 and 63 g C/m2 at 4, 8 and 18 months,

respectively, while for CT, it was 87, 79 and 67 g C/m2.

Longer-term projections as forecasted by the CN-SIM

model showed a rapid initial decline in fodder radish

carbon followed by a slow decline phase (Figure 3b). The

CN-SIM model results also indicate that 30 yr after

incorporation, ca. 8–10% of original fodder radish C could

still be traced at the 0–45 cm soil depth (Figure 3b).

Discussions

Effect of tillage and sampling depth on soil C sequestration

Although fodder radish is one of the highest-yielding catch

crops in Western Europe (Thorup-Kristensen, 2001;

Kristensen & Thorup-Kristensen, 2004), very little, if any,

work has been carried out on its effects on carbon dynamics.

The observed decline in 14C with increasing depth can be

linked to the decline in roots with increasing depth (Mutegi

et al., 2011). Such a decline of roots with depth is usually

also linked to a decline in rhizodeposition with depth

(Petersen et al., 2005a). During incorporation, more 14C was

available in CT than in DD. This could be explained by the

higher fodder radish biomass under CT than DD (Mutegi

et al., 2011), a scenario that in this specific experiment

has been associated with the development of a tillage pan at

10–30 cm depth in the DD plots (Munkholm et al., 2008).

Such tillage pans presumably constrain root growth,

reducing both belowground and aboveground biomass

production and hence carbon capture and translocation.

Bq C m–2 x 10–5

0 100 200 300 400S

oil d

epth

(cm

)

0

10

20

30

40

50

DDCT

Bq C m–2 x 10–5

0 100 200 300 400

4 months8 months18 months

Bq C m–2 x 10–5

0 100 200 300 400

Before Incorp DD CT

Figure 2 Patterns of 14C variation with depth at incorporation, four, eight and 18 months after incorporation.

Time (years)2009 2010 2011

Per

cent

(%

) ch

ange

20

40

60

80

100DD CT Trend

Time (years)2005 2010 2015 2020 2025 2030 2035 2040

Per

cent

(%

) ch

ange

0

20

40

60

80

100

(a)

(b)

Figure 3 Short term observed and modelled (a) and long term

modelled (b) decay trend of C after incorporation (model fit = 96%;

P < 0.001).



A review by West & Post (2002) concluded that carbon

accumulation is higher in no-till than in conventional till.

Baker et al. (2007), however, showed that where such

analysis to be extended beyond the plough layer (0–20 cm),

the situation could change. Our results for the samplings 4, 8

and 18 months after biomass incorporation are an indicator

of this as well. While there was a tendency for 14C to be

higher in CT than DD at incorporation, the results 4, 8 and

18 months after incorporation showed greater similarities for

CT and DD cumulatively across the 0–45 cm soil depth.

However, for the DD, more than 60% of cumulative 14C

could be found in the top 0–10 cm at incorporation eight

and 18 months after fodder radish biomass incorporation,

while ca. 25% could be found in the 10- to 25-cm layer and

10% was at 25–45 cm depth. In contrast, ca. 60% of 14C in

CT was in the 10- to 25-cm layer, while 30% was in the top

0–10 cm and the rest (ca. 10%) was in the 25- to 45-cm layer

4 and 8 months after incorporation. Eighteen months after

incorporation, however, more 14C was in the top 0–10 cm

than in the 10–25 and 25–45 cm for CT, too. Presumably,

soil inversion during tillage explains the shifts in profile

distribution of 14C in the CT plots. It would be expected

that at incorporation, biomass would be buried to 20 cm

depth, but during subsequent soil tillage, the already

decomposed and decomposing forms of such material would

be moved to the surface layer by inversion.

The current IPCC carbon sequestration estimates for

agricultural land are based on top soil models (IPCC, 2007).

With over 50% of North American farmers using no-till and

an increasing use of no-till in Europe, conclusions made on

the basis of topsoil values and extrapolated across similar

soil types in North America and Europe could lead to

serious overestimations of C sequestration via no-till. Our

present results therefore highlight the need for revision of

previous C sequestration models to include lower depths to

avoid depth-specific artefacts.

Short and long-term fate of fodder radish carbon

An initial rapid turnover of fodder radish carbon was

displayed by the two tillage treatments within the first 2 yr

of biomass incorporation followed by a slower turnover

phase (Figure 3a). A similar pattern of rapid initial

mineralization of plant residue followed by a slower turnover

phase was observed by Jenkinson & Rayner (1977) after

incorporating ryegrass. Despite the different C/N ratio of

these grass species from that of fodder radish, this pattern of

turnover is consistent with the pattern we observed in our

fodder radish trials (Figure 3). Additionally, using soils

differing in level of organic matter Jenkinson & Rayner

(1977) demonstrated that retention of C from biomass is not

affected by the amount of organic matter already in the soil.

The results we observed here are therefore applicable to

fodder radish cover crops established on sandy soils under

similar climatic conditions, irrespective of the initial soil

organic matter level.

High and fast initial mineralization was expected in our

present study because the fodder radish biomass has a low

C/N ratio of ca. 14 (Mafongoya et al., 1998). This high and

fast initial mineralization is the result of a fast turnover of

the most easily decomposable components of plant material,

such as free amino acids, amino sugars, carbohydrates and

some cell constituents (Watkins & Barraclough, 1996). As

the more easily decomposable components of the plant

biomass deplete, the more slowly degradable (resistant) plant

matter such as cell walls and structural components become

relatively more important, hence slowing the rate of biomass

turnover (Watkins & Barraclough, 1996). On the basis of the

CN-SIM model results, we estimated that ca. 8–10% of

original fodder radish C remained in the soil for up to 30 yr

after biomass incorporation. Combining these CN-SIM

model turnover results with Jenkinson & Rayner (1977)

observations, we estimate that after 30 yr of continuous

fodder radish establishment as an autumn catch crop, at

least 4.9 t C/ha fodder radish C with a residence time of

more than 20 yr in the soil could be sequestered.

Conclusions

The role of agricultural land management in accumulation

and sequestration of atmospheric carbon has been a subject of

considerable debate in discussions relating to soil health and

climate change. The results presented here add some insights

into the fate of soil organic carbon in agricultural systems.

Overall, we have demonstrated that for these sandy soils, the

total C retention and sequestration potential of fodder radish

is not affected by tillage, but there are depth-specific

differences. Estimating carbon inputs without taking account

of lower depths is therefore a potential source of errors. There

has been very little, if any, previous investigations about the

carbon sequestration potential of autumn-winter established

fodder radish. These results suggest that this management

practice could store ca. 4.9 t C/ha with a residence time of

about 20 yr in the soil over a 30-yr period. This amount is not

large enough to influence the global concentration of CO2, but

if adopted over large areas in similar agroecologies, it may

present a great opportunity for offsetting the contribution of

agriculture to atmospheric carbon accumulation. Additionally,

such retained carbon has a potential for improving soil health

through enhancement of soil organic matter.

Acknowledgements

The Danish Ministry for Food, Agriculture and Fisheries

funded this study. We gratefully acknowledge the technical

support received from Karin Dyrberg, Stig Rasmussen,

Bodil Christensen, David Croft, Jonathan Maurier, Paw D.



Rasmussen and Lene Skovmose in data collection and

analysis. Dr. Kristian Kristensen gave good guidance on

statistical analysis while Margit Schacht played a crucial role

in English editing.

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