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ORIGINAL PAPER

Assessing causes of recent organic carbon losses fromcropland soils by means of regional-scaled input balancesfor the case of Flanders (Belgium)

Steven Sleutel Æ Stefaan De Neve ÆGeorges Hofman

Received: 26 July 2006 / Accepted: 11 January 2007 / Published online: 21 February 2007� Springer Science+Business Media B.V. 2007

Abstract Several recent reports on cropland

soil organic carbon (SOC) stock changes

throughout Europe indicate a general continuing

loss of SOC from these soils. As most arable soils

in Europe are not in an equilibrium situation

because of past changes in land-use and man-

agement practices, shifts in both have been

suggested to drive this decline of SOC stocks.

A lack of data has prevented the unambiguous

verification of the contribution of these factors to

SOC loss. First, this study focused on recent

evolutions in management options for SOC

sequestration in Flanders and showed that

despite such practices have increased since

1990, their current contribution is still limited.

Strikingly, their expansion is at odds with the

reported general losses of SOC (–0.48 t OC ha–1

year–1 on average). We used very detailed

datasets of livestock numbers, N-application

rates and cropping surfaces to calculate regional

shifts in input of effective OC from animal

manure application, cereal straw incorporation

and crop residue incorporation which amounted

to –0.094, –0.045 and –0.017 t OC ha–1 year–1,

respectively. Shifts in management were identi-

fied to have potentially brought about but a third

of the recent loss of SOC in the study area,

although for central West-Flanders and the East-

ern border of Flanders larger impacts of manage-

ment were observed. This study suggests other

influences such as land-use change and climate

change to be involved as well. We estimated that

another 10%–45% of the loss of SOC could

potentially be attributed to land-use changes from

grassland to cropland during the 1970–1990 period

and about 10% to the observed temperature

increase. While being a regional-scaled case study,

these findings may be relevant to other European

regions in particular (Denmark, The Netherlands,

North-West Germany, Brittany and the North-

West of France, the Po-valley in Italy and parts of

England), with similar climate and intensity of

agriculture, and where comparable trends in

farming management may well have taken place.

Keywords Soil organic carbon � Carbon

sequestration � Soil management � Animal

manure � Humification coefficient � Land use

Introduction

In the context of climate change cropland pro-

duction is discussed both as a source of green-

house gasses as well as a possible sink for carbon.

Art. 3.4 of the Kyoto Protocol allows carbon

S. Sleutel (&) � S. De Neve � G. HofmanDepartment of Soil Management and Soil Care,Ghent University, Coupure Links 653, 9000 Gent,Belgiume-mail: [email protected]

123

Nutr Cycl Agroecosyst (2007) 78:265–278

DOI 10.1007/s10705-007-9090-x

sequestration due to human-induced agricultural

activities, which have started after 1990, to be

accounted for during the 2008–2012 commitment

period. Every participating country may choose

which additional agricultural management options

it will take into account. Consequently, this also

implies that any possible extra emissions from

these alternative management options will also

have to be reported during the commitment

period. Quantification of their net sequestration

potential will thus be crucial, since selection of

management options with a net negative green-

house gas balance, when compared to the ‘‘busi-

ness as usual’’ scenario, will have negative

consequences on the National greenhouse gas

balance. Numerous previous studies have all

focused on the ‘‘biological’’ sequestration poten-

tial (Vleeshouwers and Verhagen 2002; Freibauer

et al. 2004) which is likely to strongly exceed the

sequestration that may be actually achieved.

Smith et al. (2005b) concluded that in the EU15

as a whole cropland management has not in-

creased SOC sequestration significantly since 1990

nor is it predicted to do so in the near future.

Similarly, the present study focuses on estimating

recent evolutions in management for SOC seques-

tration in the intensively managed cropland

soils of Flanders (Belgium) and uses the current

contribution of such management as a safe mea-

sure of the actually achievable SOC sequestration

potential.

In previous studies (Sleutel et al. 2003, 2006),

we have demonstrated that large-scale SOC stock

losses have recently occurred in Flanders

(Belgium). Both recent changes in OM-input

management as well as past shifts in land-use

were hypothesized to contribute to this decrease

in SOC stock. As is the case for any region in the

world, a lack of data constricted unambiguous

verification of such a hypothesis. For example,

Bellamy et al. (2005) observed similar recent

general SOC stock losses based on large-scale

soil inventories, but a further analysis of possible

causes was indeed hindered by data restrictions.

Sleutel et al. (2006) did however find supporting

indications for West Flanders, i.e., a part of the

study area, which suggest a strong impact of

manure and crop residue inputs on evolution of

SOC levels.

The objectives of the present study were:

(1) to link recent evolutions in manuring and

crop rotations to SOC stock changes for the

whole of Flanders on a very detailed scale,

i.e., at the community level.

(2) to investigate the past and current extent of

practice of a number of management options

for SOC sequestration and evaluate their

future potential.

To do so, we examined very detailed datasets

of livestock numbers, manure application rates,

individual crop surfaces and a unique extensive

SOC stock dataset, which all cover the whole

study area and were available at the community

scale. The research described in this paper uses

Flanders as a case study area to illustrate the

development of regional-scaled OM balances as a

means for explaining SOC stock evolutions.

Farming management in the study area

Agriculture in the study area Flanders (the

Northern part of Belgium) is very intensive,

characterized by a large use of agricultural inputs

and by a very intensive livestock production, with

a predominance of pig and poultry production.

The agricultural and horticultural holdings in

Flanders are small scaled, with a mean area of

15 ha. Large animal manure applications on both

arable land and grassland are a direct conse-

quence of the high livestock density, and this

manure is mostly applied in the form of slurry

(only about fifth of all N in manure is applied as

farmyard manure). The use of other organic

fertilizers such as composts is very limited.

Table 1 compares the composition of a number

of organic amendments that are commonly

applied in Western European cropland produc-

tion. The European nitrate directive (91/676/

CEE) was practically enforced in Flanders with

the implementation of the Flemish ‘‘Manure

Action Plan’’. Since 1991, the Manure Action

Plan prescribes the type and quantity of manure

that can be applied to field crops and pasture, thus

preventing the excessive rates of manure appli-

cation that often occurred in the past. This caused

an excess of about 2,390,000 t of pig slurry and

266 Nutr Cycl Agroecosyst (2007) 78:265–278

123

356,000 t of poultry slurry in 2003 that had to be

processed in Flanders or exported, which clearly

demonstrates that there remains a large overpro-

duction of manure. The main arable crops are

maize, winter cereals, potatoes, temporary pas-

ture, sugar beet and field grown vegetables

(Table 2), which are non-uniformly distributed

over the whole of Flanders: cattle production is

concentrated in the heavy textured soils in the

North-Eastern Polder area and the North-East

(the Northern Campines). There is a concentra-

tion of pig production in the North of West-

Flanders. Intense production of field-grown veg-

etables is concentrated in the centre of the

province of West-Flanders and on sandy soils in

the South of the Province of Antwerp. The silt

region, which covers the whole Southern border is

characterized by pure arable crop production with

winter cereals, sugar beets and potatoes. During

winter time the soil is often left bare after harvest

in Western European croplands, but the use of

green manures as catch crops and as an effective

erosion control measure is now actively pro-

moted, also in Flanders. Typical green manure

crops include yellow mustard (Sinapis alba),

Italian ray grass (Lolium multiflorum) and Phac-

elia (Phacelia tanacetifolia), among others. They

provide a mean OM input of 2.0 t OM ha–1

(Sleutel et al. 2003). The composition of the most

frequently used green manures in the study area is

given in Table 3 (data taken from Anonymous

2002). Organic farming occupies but a very small

share of the total agricultural land, but there was

a substantial increase in the organically farmed

cropland and grassland during the last decade,

from 296 ha in 1991 to 3470 ha in 2001, of which

some 678 ha is estimated to be cropland (Biofo-

rum 2002). In spite of this exponential increase

over the last years, the sector still merely accounts

for 0.63% of the total Flemish agricultural land,

Table 1 Dry matter (DM), OM, total N and P content and the humification coefficient or fraction of OMeff of commonlyapplied organic fertilizers (Source: Bries et al. 1995; Vanongeval et al. 1995; De Neve et al. 2003)

Organic fertilizer DM content(t DM t–1)

OM content(t OM t–1)

hc (–) N content(kg N t–1)

P content(kg P2O5 t–1)

Slurry (pigs) 0.09 0.06 0.4 6.5 4.0Slurry (cattle) 0.09 0.06 0.4 4.4 1.8Farmyard manure (pigs) 0.23 0.16 0.5 7.5 9.0Farmyard manure (cattle) 0.24 0.14 0.5 5.5 3.5VFG-composta 0.79 0.23 0.87 12.3 3.9Green-compost 0.51 0.18 0.95 9.6 2.9

a VFG vegetable, fruit and garden waste

Table 2 Amount of DM, fraction of effective OM (hc) of crop residues from the main crops in Flanders and 1990 and 2002surfaces of the main arable crops

Crop Plant part Amount(t DM ha–1)

C content(kg C kg–1 DM)

hc (–)b Crop surface (ha)

1990 2002

Potatoe Above ground part 2.1 0.402 0.22 35,682 39,983Grain maize Above ground part 8.0a 0.429 0.22 6,148 45,127Silage maize Stubble 3.4a 0.429 0.22 87,650 115,240Sugar beet Heads + leaves 7.0 0.376 0.21 38,794 36,329Winter wheat Stubble 1.0 0.437 0.31 71,490 65,141

Chaff + straw 4.7 0.421 0.31Winter barley – – – – 30,958 2,344Leeksc Residues 1.7 – 0.23 – –

a Own calculationsb Source: Consulentschap voor Bodemaangelegenheden (1980)c Example of field-grown vegetables

Nutr Cycl Agroecosyst (2007) 78:265–278 267

123

which clearly shows its potential for further

expansion.

Materials and methods

Evaluation of management options for SOC

sequestration

Decomposition of SOM is directly influenced by

physical and chemical soil properties and climatic

and management factors. Higher inputs of OM

imply a larger application of organic manures such

as farmyard manure, compost and wastes as

replacements for mineral fertilizers and slurry.

Use of green manures and temporary pasture (ley-

farming) and alternative crop rotations provide

other alternatives for maintaining or increasing

the SOC stock. In order to evaluate the individual

role of these management options in the OM

balance of Flemish cropland soils, the extent of

each one of these management options in Flanders

in the ‘‘current’’ (2002) and baseline (1990) year

were calculated. Additionally, based on these

data, their potential for future SOC sequestration

is calculated. Data sources and assumptions made

are given below.

Decomposability of OM inputs differs strongly

between crops, manures and composts and is

dependant on their composition. Under field

conditions, after one year merely 15% to more

than 50% of this OM will remain in soil unde-

composed. Therefore, to assess and compare the

ability of different management options for SOC

sequestration it is more sensible to calculate with

the effective amount of OC (OCeff) added to the

soil, which is assumed to be the fraction (hc) of

the incorporated OC that remains in the soil after

one year under field conditions (Henin and

Dupuis 1945). For example, results from three

Belgian field experiments on the use of vegeta-

bles-, fruit- and garden waste compost (VFG-

compost) in arable rotations (details shown in

Table 4) were used to compare these calculations

of OCeff inputs to measured SOC sequestration

rates. From the yearly SOC storage in these

experiments a mean SOC storage of 0.083 t OC t–1

applied compost was deducted. The calculated

amount of OCeff per ton VFG-compost (based on

data in Table 1) was 0.101 t OCeff t–1, which

closely matches to this storage, and demonstrates

that the here applied humification coefficient-

based calculation of OCeff input as an estimate of

SOC sequestration is realistic. This approach has

furthermore been applied in recent simulation

models of C sequestration in soils (Vleeshouwers

and Verhagen 2002).

We calculated 1990 and 2002 OCeff inputs from

several management options:

(a) Green Manuring: The average input of

OCeff per ha of green manure can be calculated to

be 0.34 t OCeff ha–1 year–1 (based on Table 3).

The total area of crops after which a green

manure could be sown (cereals, early potatoes

and legumes) currently amounts to some

93,000 ha. In 2002 only 34,342 ha was subsidized

for green manuring by the Flemish Government,

which indicates that but a third of this manage-

ment option’s potential is being used. We here

assumed that virtually no green manure crops

were sown in 1990.

Table 3 Amount of dry matter (DM), C content, C:N ratio and amount of effective OC (OCeff) of the main green manurecrops in the study area

Crop Amount(t DM ha–1)a

C content(kg C kg–1DM)a

C:N (–)a OCeff (t OC ha–1)b

Yellow Mustardc 4.1 0.351 16.0 0.30Italian Ray Grassc 7.2b 0.385 22.6 0.42Phaceliac 4.1 0.364 22.1 0.25Vetchc 3.8 0.456 9.5 0.29Fodder Radishc 4.3b – – 0.34

a Source: Anonymous (2002)b Own calculationsc Yellow Mustard: Sinapis alba; Italian Ray Grass: Lolium multiflorum; Phacelea: Phacelia tanacetifolia; Vetch: Vicia sativa;Fodder Radish: Raphanus sativus subsp. oleiferus


123

(b) Crop rotations: Practically, the effect of

different crop rotations on the SOM balance can

be summarized as the sum of the effects of the

individual crops, which differs mainly by the

amount and composition of their harvest residues.

Table 3 gives the humification coefficients of

harvest residues of the main arable crops in

Flanders. It is, however, difficult to predict future

cropping area changes and it is also impossible to

precisely estimate the fraction of residues which

was incorporated in 1990 and which is incorpo-

rated at present. Regardless of these obstructions,

we assume all crop residues except cereal straw,

to have been incorporated for further calcula-

tions. From Table 2, it can be concluded that a

large decrease in the cropping surfaces of cereal

crops and to a lesser extent of sugar beets were

compensated by a large increase of the acreages

of both grain and silage maize. Taking into

account crop surface area, OM from harvest

residues and hc values, possible changes in input

of OCeff between 1990 and 2002 are calculated.

Cereal straw is mostly removed from the field

and used as bedding material in animal housing,

and will eventually be returned to the field as

farmyard manure. However, no figures are avail-

able for calculating the proportion which is directly

incorporated in the field and which is applied as

farmyard manure. For the calculation of shifts of

the input of OCeff from cereal straw we considered

the hc of straw to be roughly equal in both cases.

(c) Ley farming: The surface of temporary

pasture strongly increased from 38,080 ha in 1990

to 57,262 ha in 2001. From a literature study for

Northern Europe (Katterer and Andren 1999), a

surplus mean effective OC storage of 0.8 t OC

ha–1 year–1 was found in crop rotations with 50%

pasture and this value was also used here.

(d) Organic farming: There was a substantial

increase in the organically farmed cropland and

grassland during the 1990–2002 period. Based on

Baritz et al. (2004), who reported increases in

SOC stocks of 0–1.98 t OC ha–1 year–1 after

conversion to organic farming, calculations for

SOC accumulation under organic farming were

based on an average 1 t OC ha–1 storage rate.

(e) Compost application: Up to 268,000 t

green-waste compost and VFG-compost have

been produced in Flanders in 2002 (VLACO

2003). However, merely 6% of all produced

compost was applied to cropland in 2002 and

none in 1990. This demonstrates that there is still

a large potential for expansion of this manage-

ment option given that its potential application is

not constrained by resource availability.

(f) Animal manure: Because of environmental

constraints, including the European nitrate direc-

tive (91/676/CEE), which sets limits for nutrient

application, the potential for SOC sequestration

by means of manure application is restricted. As

the future application of animal slurries can only

be lower compared to the pre-1990 situation,

future SOC sequestration by extra slurry amend-

ment will be impossible. Based on livestock

distribution figures (VLM 2003), excretion and

C:N values, we calculated productions of OC in

animal manure of 814 kt OC in 1990 and 770 kt

OC in 2002. We assumed that all animal manure

was applied on agricultural land in 1990. Accord-

ing to the Flemish Land Agency (VLM 2003) on

Table 4 Results of three different field experiments regarding the effect of VFG compost on the SOC content (differencein SOC% with a control plot receiving only mineral fertilizer: D SOC%; annual difference in SOC stock: D SOC stock)

Soil texture(USDA)

Crop rotation VFG dose(t ha–1 year–1)

Duration(year)

D SOC%(%OC)

D SOC stock/toncomposta

(t OC t–1 ha–1 year–1)

Referenceb

Silt Arable rotation 13.3 4 0.14 0.11 1Silt-loam Continuous maize 22.5 4 0.11 0.05 2Silt-loam Arable rotation 15.0 6 0.2 0.10 3Silt-loam Arable rotation 45.0 6 0.4 0.06 3

a Top 30 cm soil layerb 1: Sleutel et al. (2002); 2: Nevens and Reheul (2003); 3: Deproost and Elsen (2003)


123

average 212 kg N ha–1 year–1 from animal manure

was applied on agricultural land in 2002. This

amount corresponds with about 0.9 t OC

ha–1 year-1 resulting in a total input of 514 kt

OC year–1 in 2002.

Regional scale assessment of the relation

between SOC stock changes and OM-input

management

In a previous study (Sleutel et al. 2003), we

concluded that general losses of SOC had

occurred in cropland soils for the whole of

Flanders during the 1990s. As no clear causes

could be identified for this loss of SOC, we here

investigated the evolution of the OM input from

these options in more detail. With detailed

datasets of crop surfaces, livestock numbers and

manure application rates at our disposal a spa-

tially explicit approach was possible. The differ-

ence in average OCeff input per ha cropland

between 2000 and 1990 was calculated at the

community scale, which is the finest scale at which

data of crop surfaces and animal numbers are

available for the study area. Average application

rates of the OCeff input were calculated using

livestock density and crop area data (NIS 1990,

2000). The average inputs to cropland of OCeff

from crop residues, animal manure and cereal

straw per community were calculated separately:

The difference in OCeff input from crop resi-

dues (in t OCeff ha–1 year–1) of the 10 most

cultivated crops was calculated per community as:

DOCeff;crops ¼P10

i¼1 Si;2000 � IOCeffi

S2000

�P10

i¼1 Si;1990 � IOCeffi

S1990

with Si is the surface area occupied by crop i, in

2000 or 1990; IOCeffiis the input of OCeff per ha of

crop i (in kg OCeff ha–1 year–1) and S2000 and S1990

the cropland surfaces for that community for 2000

and 1990, respectively.

All manure was assumed to be incorporated in

1990, but for 2000 the actual proportion of

manure applied to agricultural land was based

on figures from the Flemish Land Agency (VLM

2005). Animal manure application/production

ratios of <1 and >1 imply manure exports and

imports, respectively, by means of inter-commu-

nity transports (Fig. 1). The difference in OCeff

input from animal manure per community

between 2000 and 1990 was calculated as follows

(in t OCeff ha–1 year–1):

DOCeff;manure

¼

APRðNcattle;2000ccattle

þNpigs;2000cpigs

þNpoultry;2000cpoultryÞ�

ðNcattle;1990ccattle

þNpigs;1990cpigs

þNpolutry;1990cpolutry

2

666666666664

3

777777777775

,

S1990�2000

with APR the 2000 manure production/applica-

tion ratio for a given community (APR is

assumed equal to 1 in 1990), N the 1990 and

2000 livestock numbers (NIS, 1990, 2000), ci is the

amount of effective OC produced per animal

per year and S1990–2000 is the average crop-

land + grassland surface for the 1990–2000 peri-

od. For the annual production of OCeff per

animal, we used the same calculations as Sleutel

et al. (2006) based on livestock distribution and

excretion figures of VLM (2003), being

ccattle = 168; cpigs = 25 and cpoultry = 1.7 kg OCeff

animal–1 year–1.

Differences in the average yearly input of

OCeff in cereal straw to cropland, either by direct

incorporation or through farmyard manure appli-

cation, was calculated indirectly based on the

surfaces area occupied by wheat and barley in

1990 and 2000:

DOCeff cereals ¼ðSwheat;2000 þ Sbarley;2000Þ � IOCeff straw

S2000

� ðSwheat;1990þSbarley;1990Þ � IOCeff straw

S1990

with Swheat and Sbarley the wheat and barley surfaces

in 1990 and 2000, IOCeffstraw, the average yearly

production of OCeff per hectare cereal crop being

613 kg OCeff ha–1 year–1 and S1990 and S2000 the

cropland surfaces in 1990 and 2000, respectively.


123

Over 190,000 SOC measurements (0–24 cm)

have been made in Flemish cropland soils in the

1989–2000 period. These SOC data were

grouped in three-year periods (1989–1991,

1992–1994, 1995–1997, 1998–2000) and were

available as means plus standard deviation per

community. This large dataset was used to

calculate SOC stocks and their evolution with

time, without any need for data-extrapolation.

As the data were only available to us as means

per community, normal linear regression of the

SOC contents against time yielded few signifi-

cant regressions (for merely 18% of all commu-

nities) due to a lack of data points (at most 4 per

community). To increase the number of data

points in each regression, the communities were

grouped in 27 groups based on their dominant

soil textural class and their spatial location

(Sleutel et al. 2003) (Fig. 2). On average a loss

of –0.48 t OC ha–1 year–1 was calculated, which

is considerable. Linear regression analysis was

used to relate shifts in OCeff input to this

negative evolution in cropland SOC stocks

during the 1990–2000 period at the spatial scale

of these community groups.

Results and discussion

An overview of the estimated shifts in input of

OCeff which have occurred during the 1990s from

management options is given in Table 5. Further

details on the calculations per management

option are discussed below.

(a) Green manure: Assuming that no green

manure crops were sown in 1990, the expansion of

the use of green manures since 1990 yielded an

increased input of 11.7 kt OCeff year–1 in 2002. If

green manuring were adopted to the entire

maximum land area of 93,000 ha, the input of

OCeff from green manure would increase by

about 31.6 kt C OCeff year–1.

(b) Crop rotations: The decrease in acreages of

winter barley, winter wheat and sugar beets

resulted in a shift in OCeff input from their crop

residues (excluding cereal straw) of –5.9 kt

Fig. 1 Manure N-Application/Manure N-Production ratioper community for 2000

Fig. 2 Map of clusteredgroups of communitiesbased on soil texture andspatial location


123

OCeff year–1. On the other hand, there was a

surplus in OCeff input + 39.1 kt OCeff year–1 due

to the increased acreages of potato and grain and

silage maize and potato. Both evolutions resulted

in a net surplus of OC input of 33.1 kt OCeff year–1

in 2002 when compared to 1990. The substantial

decrease in cereal crops between 1990 and 2002

resulted in a decreased input of 21.5 kt OCeff

year–1, either via straw incorporation or farmyard

manure application.

(c) Ley farming: Theoretically, assuming that

grass constitutes 50% of the crop rotation in all

temporary pastures in Flanders, the increase in

the temporary pasture acreage resulted in an

extra storage of 15.3 kt OC–1 year–1. The assump-

tion of 50% grass may, however, be overoptimis-

tic for Flanders, in which case the SOC storage

would be overestimated.

(d) Organic Farming: With an average increase

in the SOC accumulation rate of 1 t OC ha–1 year–1

under organic farming compared to conventional

farming, the substantial increase in the organi-

cally farmed cropland and grassland during the

1990–2002 period represents a surplus sequestra-

tion of 0.7 kt OC year–1.

(e) Compost application: Considering that at

present 6% of all produced compost is applied to

cropland and using a hc of 0.87, the actual surplus

input of OCeff in 2002 (compared to a 0%

application in 1990) is 1.8 kt OCeff year–1.

(f) Animal Manure: Assuming all slurry was

applied to agricultural land in 1990 and a reduced

application to production ratio for 2002, 240 kt

OC year–1 less in animal manure was applied in

2002 compared to 1990. Using a hc of 0.4 for

slurry (Table 1), this amount corresponds with

96 kt OCeff year–1. Assuming half of all manure is

spread on cropland soils (the other half would be

spread on grassland), yearly some 48 kt OCeff less

is applied to Flemish cropland compared to 1990.

In summary, surplus 2002 OM inputs from

green manuring, crop residues, temporary pas-

tures, organic farming and compost application

resulted in a total net SOC sequestration of

62.6 kt OC year–1, additional to 1990, in the study

area. Particularly ley farming and recent changes

in crop rotations have a large contribution to this

sequestration. However, this additional SOC

sequestration seems to have been insufficient for

maintaining SOC stocks. As these figures provide

the current net SOC sequestration from these

management options, any economic, social, polit-

ical and institutional constraints for the imple-

mentation of SOC sequestration measures are

irrelevant to our estimates, which is not the case

for potential SOC sequestration estimates. It

should be noted, however, that the Kyoto Proto-

col states that verifiability of biospheric sinks

should be taken into account and measuring

change in SOC is essential when considering

SOC sequestration under the Kyoto Protocol.

Results presented by Smith (2004) demonstrate

that it will not be possible to measure changes in

SOC over the 5-year Kyoto Commitment period

unless the considered management alternative

increases OM inputs by at least 20% and unless

an elaborate sampling regime is maintained. For

green manuring this 20% limit is barely met, for

Table 5 Input of OCeff in 1990 and 2002 and potential extra SOC storage resulting from individual agricultural manage-ment options

Management option Input of OCeff (kt OC year–1)

1990 2002 SOC storagea

Green manuring 0 11.7 +11.7Crop rotations—harvest residues 74.3 107.4 +33.1Cereal straw—directly or in FYM 62.9 41.4 –21.5Ley farming 31.5 45.8 +15.3Organic farming – – +0.7Compost application 0 1.8 +1.8Animal manure 162.9 114.8 –48.0Balance –8.5

a Assuming the estimated surplus in OCeff, i.e., the part of added OC which remains in the soil after one year under fieldconditions, to equal the net amount of SOC stored


123

temporary pastures perspectives are fair, but for

any other options changes will be much less than

20%.

Regional scale assessment of the relation

between SOC stock changes and OM

management

Differences among communities in the average

input of OCeff from animal manure applied per

ha cropland between 2000 and 1990 varied

between –0.44 and +0.33 t OCeff ha–1 year–1

(mean: –0.094 ± 0.171 t OCeff ha–1 y–1). Overall

there were significant decreases in manure appli-

cation in the sandy and sandy loam soils in the

West and North-East of Flanders (Fig. 3(a)). As

a cause of dense pig production in West-Flanders

and poultry and cattle production in the North-

East, there was a surplus manure production in

2000, which could not be applied integrally to

agricultural land anymore (Fig. 1) as was still the

case in 1990. This reduction in manure applica-

tion brought about the decrease in OCeff input in

these areas. Differences in the yearly amount of

OCeff from incorporated harvest residues be-

tween 2000 and 1990 were relatively small and

varied between –0.06 and +0.02 t OCeff ha–1

year–1. On average there was a slight decrease

(mean: –0.017 ± 0.017 t OCeff ha–1 year–1) spread

over the whole study area (Fig. 3(b)). Decreases

in the acreages and harvest indices of cereal

crops resulted in an average decreased input of

OCeff from cereal straw (mean: –0.045 ± 0.017 t

OCeff ha–1 year–1). Since cereal crops are mainly

grown in the Silt region in the South of Flanders,

decreases in OCeff input from cereal straw

application through incorporation or through

farmyard manure application were mostly re-

stricted to that area (Fig. 3(c)).

The total differences in the amount of OCeff

applied per ha cropland from animal manure

application, crop residue incorporation and cereal

straw (either by direct incorporation or through

farmyard manure application) between 2000 and

1990 varied between –0.47 and +0.29 t OCeff

ha–1 year–1 among communities (mean: –0.157 ±

0.169 t OCeff ha–1 year–1). Decreases in OCeff

input occurred from central to Western Flanders

and along the Northern and Eastern borders

(Fig. 3(d)). The average rate of change per

community of the SOC stock (DSOC) between

2000 and 1990 amounted –0.48 t OC ha–1 year–1

(Sleutel et al. 2003). A weighted linear regression

against the change in SOC stock per group of

communities calculated by Sleutel et al. (2003)

yielded no significant relation with the changes in

the average OCeff input from crop residues or

cereal straw. This result could be expected given

the small magnitude of these changes in OCeff.

Group 13 was not included in this regression

analysis because the estimated SOC stock de-

crease in that group (–1.78 t OC ha–1 y–1) was

unrealistically high. There was a significant

positive relation (P = 0.1) between DSOC and

the total change in input from OCeff:

DSOC = 0.47 DOCeff – 0.36. Although this rela-

tionship was weak (R2 = 0.14), this suggests that

the observed SOC stock changes are indeed

directly related to shifts in management (Fig. 4).

For some areas such as central West-Flanders

(groups 18, 19, 20, 22; see Fig. 2) and the Eastern

border of Flanders (groups 1, 2; see Fig. 2) DOCeff

can explain about 70%–100% and 40% of DSOC,

respectively, of which most can be attributed to

reductions in manure applications. In a recent

study (Sleutel et al. 2006), we investigated losses

of SOM in West-Flemish cropland soils more

closely. A clear relationship was found between

calculated shifts in OCeff input from manure and

a measured loss of SOC in the period 1990–2003,

which was based on another independent dataset

of SOC generated by an additional soil survey.

Although no significant regression was found

between DSOC and DOCeff from animal manure

for the study area as a whole, these results point

at a strong connection between the SOM balance

and manure management for central West-Flan-

ders and the Eastern border of Flanders.

In contrast, for the Southern silt belt (groups 5,

7, 8, 9, 10, 23, 27; see Fig. 2) lower manure

applications cannot explain the observed recent

losses in SOC. Historically, most of the traditional

mixed farms with livestock and arable crops in

this region were replaced by specialized arable

farms. Sleutel et al. (2003) indeed found the very

low OC stocks in these soils to be correlated to

the livestock density for groups 8, 7 and 9.

Additionally, Lettens et al. (2005) argued that in


123

the Southern silt belt typical high application

rates of synthetic fertilizer, often in combination

with the CaCO3-rich byproducts from the sugar

refinery could have lead to an accelerated SOC

decomposition. However, considering the long

time since the onset of these practices (starting

Fig. 3 Calculated shifts in theannual input of effective OCper hectare (OCeff input)between 1990 and 2000 from(a) animal manure application,(b) harvest residueincorporation, (c) cereal straw(directly incorporated orapplied in farmyard manure)and (d) total shift in annualOCeff input


123

mainly after WWII), manure and fertilizer man-

agement fail to explain recent SOC stock changes.

Considerable decreases in OCeff input from cereal

straw (due to a substantial decrease in the cereal

crop acreage during the 1990s) are mainly

restricted to the Southern silt belt (Fig. 3(c)),

and therefore their impact is likely to have

affected the OM balance of soils in that region

only. The difference in DOCeff could however

only explain about 16% of the DSOC for these

Southern silt silty soils.

When simply comparing the average magni-

tudes of the changes in the total OCeff input

(–0.157 t OCeff ha–1 year–1) between 1990 and

2000 to the DSOC (–0.48 t OC ha–1 year–1), one

can observe that the recent shifts in the OM

inputs in these soils could explain but a third of

the observed decrease in SOC stocks in Flanders.

Therefore, other possible explanations should be

considered for the loss of SOC from these arable

soils.

(i) Increasing air temperature speeds up decom-

position and tends to increase the loss of

SOC. By model simulation, Smith et al.

(2005a) predicted cropland SOC stocks to

fall across Europe by a mean of 3.9 t OC ha–1

by 2080 using different global climate mod-

els, which amounts to an average annual loss

of 0.05 t OC ha–1 year–1. Bellamy et al.

(2005) observed much larger recent losses

of SOC (on average –1.25 t OC ha–1 year–1

(0–15 cm depth layer)) in soils across

England and Wales between 1978 and 2003.

Since these losses generally occurred irre-

spective of soil type and of land-use type, i.e.,

in both agricultural, woodland, heath and

bogland soils, they suggested there is no

apparent single factor other than climate

change that could explain this evolution in

non-agricultural soils. There was an increase

of air temperature by about +0.5�C in

England and Wales over the survey period,

which is comparable to the +0.6�C increase

in temperature between 1980 and 2000 in

Belgium. Sleutel et al. (2006) previously

estimated this increase of air temperature

to be accountable for a loss of about –0.05 t

OC ha–1 year–1 or 10% of the observed

DSOC in Flanders, which matches the pro-

jections of Smith et al. (2005a). If these

estimates are acceptable, recent increases in

temperature alone clearly cannot explain the

bulk of the losses observed here and other

factors should be explored.

(ii) There was a substantial decrease in the

permanent pasture area between 1970 and

1990 of 101300 ha (NIS 1970, 1980, 1990).

Since land use changes cause the SOC

content to evolve towards a new equilibrium

SOC content, recent land-use changes may

influence measurement results of SOC stock

changes (Janssens et al. 2003). For instance,

if SOC is measured in land that has recently

been converted from grassland to cropland,

the C content as assessed for cropland will

be exceptionally high, since the SOC

content is not yet in equilibrium with the

new land-use. Janssens et al. (2003) there-

fore suggested recently observed losses of

SOC in European agricultural lands to be

the legacy of conversion of grassland to

cropland during the past 20–30 years. Part

of the cropland soils in 1990 certainly were

former pastures which had been brought

into cultivation during the two preceding

decades. Data are lacking to accurately

estimate this fraction of arable land that

Fig. 4 Relation between the difference in the averageannual input of effective OC between 1990 and 2000(DOCeff input) per group of communities and the mea-sured average change in SOC level (DSOC) between 1990and 2000


123

was previously converted from grassland,

but assuming 50% of all recently tilled

pastures to have been converted into crop-

land would be a safe estimate. Extensive

literature is available on SOC changes

resulting from land-use change. Guo and

Gifford (2002) analyzed research results

from 74 publications on the evolution of

SOC stocks after land-use change in a meta

analysis and reported that conversion of

pasture into arable land causes a decrease of

the SOC stock by 58% on average within a

period of 30 years. Alternatively, Lettens

et al. (2005) estimated losses of SOC of 31%

following such a conversion for Belgian

soils. The loss of SOM with cultivation is

usually exponential, with losses being rapid

during the first 10–20 years, then slower,

with a new equilibrium finally approached in

50–60 years (Arrouays et al. 1995); how-

ever, the time scale varies with climate and

soil type. Soussanna et al. (2004) fitted an

exponential model to a chronosequence of

SOC data after tilling a pasture soil in

France: C(t) = Cc–(Cc–C1)�e–kt (with C(t)

the SOC content at time t (years after

conversion); Cc is the equilibrium SOC

content under cropland and C1 is the SOC

content at the time of tilling of the pasture

soil). Lettens et al. (2005) estimated the

average SOC stock in Flemish grassland

soils in 1960 and 1990 to be 71 and 93 t OC

ha–1, respectively. Assuming this increase in

SOC stock between these two dates to have

followed a linear path, the average grassland

SOC content (i.e., C1) for the 1970–1975,

1975–1980, 1980–1985 and 1985–1990 time

periods were 79.7, 83.3, 87.1 and 90.7 t

OC ha–1 (0–30 cm), respectively. Calculat-

ing with the model used by Soussana et al.

(2004), an average (from Guo and Gifford

2002; Lettens et al., 2005) loss of SOC of

44% (hence Cc = 0.56 C1) and the by Sous-

sana et al. (2004) estimated k = 0.07 year–1,

we calculated the evolution of the SOC

content of previous grassland soils after

conversion to cropland (Fig. 5a) during

these four time periods. Assuming 20%–

80% (50% on average) of these former

grasslands to have been converted to crop-

lands, –0.05 to –0.22 t OC ha–1 year–1 (i.e., a

total of –503 kt OC on average (Fig. 5b))

was lost during the 1990s from cropland soils

as a consequence of these past shifts in land-

use. In conclusion, changes in land-use could

explain about 10%–45% of the measured

loss of SOC of –0.48 t OC ha–1 year–1, which

is considerable.

The sum of the individual estimates, i.e., shifts

in OM-input management during 1990s: 33%;

temperature increase: 10%; recent (1970–1990)

land use changes: 10%–45%, shows that these still

do not account for the entire loss of SOC.

Particularly large uncertainty still resides on the

impact of temperature and land-use changes.

Although Bellamy et al. (2005) put climate

variation forward as a surprisingly large potential

contribution to release of CO2 from soils, our

current understanding of the sensitivity of respi-

ration to warming shows temperature alone to be

a weak driver (Schulze and Freibauer 2005).

Accurate lab-experiments are much needed to

isolate the impact of both shifts in air temperature

and atmospheric CO2 as well as alterations in

precipitation amounts and patterns. Concerning

the impact of former land-use changes, surface

data are lacking to unequivocally assess its full

impact for our study area, as was also the case in

the study by Bellamy et al. (2005) for England

Fig. 5 (a) Estimated evolution of the SOC content inprevious grassland soils after conversion to croplandduring the 1970–1975, 1975–1980, 1980–1985, 1985–1990periods. (b) Total loss of SOC during the 1990s from tilledprevious grassland soils assuming 50% was converted intocropland


123

and Wales. Still, an important result from the

approach used here is that it suggests the impact

of former land-use changes to be even larger than

of recent changes in agricultural management.

Since large-scale conversions of grassland to

cropland have been general over Europe since

the 1960s with a stabilization in the early 1990s

(Freibauer et al. 2004), similar considerable losses

of SOC are likely to be taking place in croplands

of other European countries as well. To our

knowledge, for the first time different potential

causes of loss of SOC from very intensively

managed cropland soils were investigated

together into such detail on a regional scale.

Our findings of negative SOM balances in inten-

sively managed croplands due to recent changes

in crop rotations and in animal manure produc-

tion, are relevant to other European regions in

particular (Denmark, The Netherlands, North-

West Germany, Brittany and the North-West of

France, the Po-valley in Italy and parts

of England), with similar climate and intensity

of agriculture, and where comparable trends in

farming management may well have taken place.

Similar SOM input balance based calculations

could be used to quantify the relative importance

of shifts in agricultural management for changes

in cropland SOC stocks for these regions as well.

Conclusions

Surplus post–1990 inputs of OCeff from green

manuring, crop residue incorporation, temporary

pastures, organic farming and compost applica-

tion seem to have been insufficient for maintain-

ing or increasing SOC stocks in Flemish cropland.

Furthermore, these surpluses are at odds with the

recently observed general SOC stock decreases.

The post–1990 expanse of these management

options, makes them however, eligible for SOC

sequestration which is accountable under the

Kyoto Protocol’s art. 3.4. Negative shifts during

the 1990s in OM inputs from animal manure

application, crop residue and cereal straw incor-

poration combined were identified to be related

to the observed SOC losses on a regional scale.

Previous suggestions that changes in management

are the single main driver for the observed SOC

stock losses in Flanders appear to be invalid, since

these changes in OM input were found to

contribute only a third of the loss of SOC for

the whole of Flanders. Differing trends were,

however, seen in specific regions. The spatially

explicit approach did show that, in contrast to

Flanders as a whole, for central West-Flanders

and the Eastern border of Flanders management

played a dominant role in the recent SOC stock

losses.

There were indications for a small contribution

to the SOC losses by recent temperature increases

and a large contribution of pre–1990 land-use

changes. Uncertainty associated with the impact

of recent land-use changes does, however, remain

unquantified. Still, with the substantial amount of

management data at our disposal, the present

study is the first one to distinguish and quantify

individual causes for observed SOC stock changes

on a regional scale.

Acknowledgement The corresponding author is financedby a post-doctoral grant of the Research Foundation—Flanders (FWO).

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