Potential for carbon sequestration in European soils: preliminary estimates for five scenarios using...

Global Change Biology (1997) 3, 67-79

Potential for carbon sequestration in European soils: preliminary estimates for five scenarios using results from long-term experiments

PETE S M I T H , DAVID S . P O W L S O N , M A R G A R E T J . G L E N D I N I N G and J O U . S M I T H Soil Science Department, IACR-Rothamsted, Harpenden, Herts AL5 2]Q, UK

Abstract

One of the main options for carbon mitigation identified by the IPCC is the sequestration of carbon in soils. In this paper we use statistical relationships derived from European long-term experiments to explore the potential for carbon sequestration in soils in the European Union. We examine five scenarios, namely (a) the amendment of arable soils with animal manure, (b) the amendment of arable soils with sewage sludge, (c) the incorporation of cereal straw into the soils in which it was grown, (d) the afforestation of surplus arable land through natural woodland regeneration, and (e) extensification of agriculture through ley-arable farming. Our calculations suggest only limited potential to increase soil carbon stocks over the next century by addition of animal manure, sewage sludge or straw (c 15 Tg C y-'), but greater potential through extensification of agriculture (z 40 Tg C y-') or through the afforestation of surplus arable land (= 50 Tg C y-'). We estimate that extensification could increase the total soil carbon stock of the European Union by 17%. Afforestation of 30% of present arable land would increase soil carbon stocks by about 8% over a century and would substitute up to 30 Tg C y-' of fossil fuel carbon if the wood were used as biofuel. However, even the afforestation scenario, with the greatest potential for carbon mitigation, can sequester only 0.8% of annual global anthropogenic COp-carbon. Our figures suggest that, although efforts in temperate agriculture can contribute to global carbon mitigation, the potential is small compared to that available through reducing anthropogenic C 0 2 emissions by halting tropical and sub-tropical deforestation or by reducing fossil fuel burning.

Keywords: carbon sequestration, European Union, GCTE, long-term experiment, soil organic carbon, soil organic matter network (SOMNET)

Received 9 March 1995; revision accepted 20 June 1996

Introduction

The IPCC identifies three main carbon mitigation options for agriculture: (a) reduction of agriculturally related emissions, (b) use of biofuels to replace fossil fuels, and (c) the sequestration of carbon in soils (IPCC 1996). In this paper we concentrate on quantifying the latter option for the land area of the European Union (EUR15).

Agricultural soils contain far less organic matter than do many others. For example, 70-80% of soil organic carbon (SOC) in the UK is contained in peats, mainly in Scotland (Howard et al. 1995) and 35% of the total carbon in European soils is held within high organic matter soils (2 8% organic matter; 0.m.) that cover only 13% of the

Correspondence: Dr Pete Smith, tel + 44/ (0)1582-763133, fax + 44/ (0)1582-760981, e-mail [email protected]

area of the European Union (see Table 1). Many of these high carbon content soils are not used intensively for agriculture, although planting of trees on upland peats is an issue in some areas. It is vital to minimize release of carbon from high organic matter soils through careful management. However, despite the relatively low carbon content of agricultural soils, the greatest potential to increase current soil carbon stocks is through manipulation of agricultural land, particularly cropland.

There are many opportunities for such manipulation within Europe and North America with agricultural overproduction currently dealt with through policies such as set-aside and the Conservation Reserve Program. Some projections suggest that by 2010,20-30% of current cropland in Europe will be surplus to agricultural require-

0 1997 Blackwell Science Ltd. 67

68 P. S M I T H e t a l .

Table 1 Soil organic carbon stocks in the EUR15

%o.m.in top 30 cm of soil

1 2 3 4 5 8

10 14 30

% C in top 30 cm of soil’

0.58 1.16 1.74 2.32 2.90 4.64 5.80 8.12

17.40

Soil bulk density (g ~ m ” ) ~

1.45 1.26 1.15 1.07 1.01 0.88 0.82 0.72 0.51

Carbon (lo6 g ha-’) in top 30 cm of soil

25.23 43.82 59.91 74.37 87.63

122.21 142.09 176.38 268.55

Totals:

% of area of Europe with this 0.m. content3

28.48 10.72 20.87 18.51 10.17 3.84 2.08 3.24 3.87

101.78

Area of EUR15 (lo6 ha)4

Total carbon in that soil type in EUR15 (X 1015 g)

92.28 34.73 67.62 59.97 32.95 12.44 6.74

10.50 12.54

329.78

2.33 1.52 4.05 4.46 2.89 1.52 0.96 1.85 3.37

22.95

‘Assuming carbon to comprise 58% of 0.m. (Fraters et al. 1993) 2From equation relating SOC to soil bulk density in Howard et al. (1995) 3Calculated by adding percentage areas of all soil types with same percentage 0.m. content (Fraters et al. 1993); note sum of percentage area values is greater than 100% due to rounding errors. 4Based on a total area for the EUR15 of 324016 X lo3 ha (Eurostat 1995) assuming distribution of soils in the EUR15 to be the same as broader Europe from Fraters et al. (1993)

ments (Flaig & Mohr 1994). This presupposes that intensive agriculture will continue on the remaining area: whether this is feasible depends upon other environmental considerations not addressed in this paper but see CEC (1989) or Stiftung (1994) for a discussion. It also assumes that the surplus land in Europe and North America will not be used for the production of food for export.

Recently, a number of studies have been published that use either carbon/ecosystem models and spatial databases (Lee et al. 1993; Donigan et al. 1994) or simple calculations (Gupta & Rao 1994) to estimate the potential for carbon sequestration in various regions. In this paper, we use the results from European long-term experiments to establish relationships between various agricultural practices and SOC content, then use these relationships to examine the potential for carbon sequestration in agricultural soils within the EUR15 over 100 years.

1 To provide preliminary estimates for the potential to sequester carbon in agricultural soils within the European Union through changes in agricultural practices or land-use. 2 To show how data from long-term field experiments can be used to provide a rational basis for such calculations.

Preliminary calculations such as those presented here are intended to identify the changes in practice that have the greatest potential for increasing soil carbon stocks. The next step is to perform more detailed calculations taking account of variations in soil type and climate within a region.

The aims of this paper are:

Materials and methods

The total carbon stock of the EUR15, and of various portions of land within it, were calculated from figures presented in Fraters et al. (1993). Long-term experiments within Europe were selected from the Global Change and Terrestrial Ecosystems (GCTE) Soil Organic Matter Network (SOMNET) database. GCTE SOMNET is a global network of soil organic matter modellers and holders of data from long-term experiments (most over 20 years old) which is a core research programme of GCTE contrib- uting to Focus 3.3.1., ‘Soil Organic Matter’ (Powlson et al. 1997; Smith et al. 1996a, b). Detailed metadata on all participating models and long-term experiments is held for free world-wide-web access at URL http:/ /yacorba.- res.bbsrc.ac.uk/ cgi-bin/ somnet. Only European long- term experiments were used since these best represent the soil types, climatic conditions and agricultural practices common within the EUR15. The long-term experiments were used to derive relationships between various agricultural practices and changes in SOC content of agricultural soils. These relationships were then used to explore five scenarios for sequestering carbon in soils in the EUR15 over a century namely, (i) the amendment of arable soils with animal manure, (ii) the amendment of arable soils with sewage sludge, (iii) the incorporation of cereal straw into the soils in which it was grown, (iv) the afforestation of surplus arable land through natural woodland regeneration, and (v) extensification of agriculture through ley-arable farming. A century is chosen because soils have a finite capacity to increase in SOC

0 1997 Blackwell Science Ltd., Global Change Biology, 3, 67-79

C A R B O N S E Q U E S T R A T I O N I N E U R O P E A N SOILS 69

Table 2 Area and SOC content of soils under different land-use in the EUR15

of which land with of which utilized of which cereal Total EUR15 0.m. content 5%3 agricultural area of which arable cropland

Area (lo3 ha)' 324016 287564 141490 72802 34937 SOC content (P$ 22.95 15.25 7.50 3.86 1.85

'Areas from Eurostat (1995) except where marked; 2Calculated from figures in Table 1; 3Area and SOC content calculated from figures in Table 1

content. After 100 years or so, soils in a temperate climate may approach a new equilibrium, though this is not always the case (Jenkinson 1990).

Results

Soil organic carbon stocks in the EUR15

The figure of 22.95 Pg g) for SOC stock to 30 cm depth for EUR15 was derived as shown in Table 1.

We assume that the majority of soils under arable agriculture in the EUR15 will have 0.m. contents < 5% (SOC contents s 2.9%) and that those with 0.m. contents

8% (SOC contents 3 4.64%) will not usually be under arable agriculture. Current agricultural land-use in the EUR15 and the corresponding carbon pool sizes found in soils under these land-uses are given in Table 2.

Having derived baseline figures for SOC stocks in the EUR15, various scenarios were tested to examine their potential to sequester carbon in soils. Potential environmental side-effects such as increased risk of nitrate leaching or trace gas fluxes from the soil were ignored as were the economic and policy implications for the changes in agricultural practice and land-use envisaged in the scenarios. Only the effect on soil carbon was considered. Our aim was to establish an upper limit for carbon sequestration that could be obtained under ideal conditions using agronomically realistic practices.

Scenario (i): Amendment of arable soils with animal manure

In this scenario, all arable soils are amended with organic manure at realistic application rates, i.e 5, 10 or 15 t ha- ' y-', in order to increase SOC content and to reduce, to an extent, the amount of inorganic fertilizers required. It is assumed that the current disposal of animal manure to arable land has not decreased the capacity of soils to continue to accumulate SOC. Long-term experiments suggest that this is not an unreasonable assumption. For example, after 144 years of very large annual applications of animal manure (35 t ha-' y-'), SOC content is still increasing on the Broadbalk Wheat Experiment (Jenkin-

son 1990). It is assumed that if animal manure is not incorporated into arable soil much of the carbon will be lost from the terrestrial pool through more rapid decomposition. Furthermore, addition to soil under long- term grassland is regarded as a less desirable option as such soils are likely to already have a high carbon content, so the potential for further increase is more limited than in arable soils. We ignore all possible problems (economic, logistical, etc.) of moving manure from where it is produced to where it is spread. We also take no account of any changes in farming practice that would be required or of the relative merits of alternative uses of manure such as biogas production.

Fourteen long-term field experiments (17 treatments) were identified in Europe that measure changes in SOC after amendment with animal manures or inorganic fertilizers. Table 3 presents details of these experiments which are all in arable rotations or monoculture.

Values for percentage change in SOC per year were derived by dividing the percentage difference in SOC between manure and inorganic only fertilizer treatments by the duration over which the difference was measured. These yearly percentage changes in SOC content were then plotted against the amount of organic manure added per year (Fig. 1).

Despite very different soil types, rotations, climatic conditions and durations of experiment, a highly significant linear relationship was found (f = 0.4851, t15 =

3.76, P [two-tailed] = 0.0017). This relationship was then used to explore scenarios involving the amendment of arable land with organic manure. From the regression equation, amendments of soil with animal manures at 5, 10 and 15 t ha-' y-' increase SOC contents by 0.136,0.326 and 0.516% y-', respectively. Table 4 shows the changes that would occur if arable soils were amended with the three rates of animal manure.

An amendment rate of 10 t ha-' animal manure to all arable soils would increase total SOC stocks in Europe by 5.5% over 100 years (Table 4) and would also leave 93.9 x lo6 t of animal manure plus all slurry currently produced in EUR15 to be used for other agricultural purposes (on grassland); see footnote 1, Table 4.


v 0

Tab

le 3

Cha

nges

in

SOC

in l

ong-

term

exp

erim

ents

con

cern

ing

orga

nic

vs. i

norg

anic

fer

tiliz

atio

n 'j

v)

Diff

eren

ce (

%)

from

z - +I

Ref

eren

ce

inor

gani

c on

ly

-

Rat

e pe

r y

SOC

to 3

0 cm

N

o. y

ears

Tr

eatm

ent

(t ha

-' y-

') (t

ha-')

' Ex

perim

ent

Cro

pRot

atio

n ~

90

38

144

100

123 30

I12 31

49

70

70

25

75

47

Di

2

(199

6); S

mith

&

?

Kor

sche

ns &

Miil

ler

Pow

lson

(19

96)

Klir

(19

96);

Smith

&

Pow

lson

(19

96)

Jenk

inso

n (1

990)

a

Bad

Lau

chst

adt,

Ger

man

y Su

gar

beds

prin

g ba

rley/

po

tato

es/w

inte

r w

heat

30

t ha

-' 2y

-' 20

t h

a-'

2y-'

Inor

gani

c on

ly

21 t

ha-'

2y-'

Inor

gani

c on

ly

35 t

ha-

' y-

' In

orga

nic

only

13.5

t ha

-' y-

' 9

t ha

-' y-

' In

orga

nic

only

35 t

ha-'

y-'

Inor

gani

c on

ly

50 t

ha-'

y-'

25 t

ha-

' y-

' In

orga

nic

only

10 t

ha-'

y-'

Inor

gani

c on

ly

9.54

t ha

-' 2y

-' In

orga

nic

only

17.6

t ha

-' y-

' In

orga

nic

only

30 t

ha-

' y-

' In

orga

nic

only

3 30

t h

a-'

y-'

Inor

gani

c on

ly3

30 t

ha-

' 2y

-' In

orga

nic

only

12

t ha

-' y-

l In

orga

nic

only

30 t

ha-'

3 y-

' In

orga

nic

only

15

87.1

10

80

.2

0 71

.7

10.5

62

.6

0 55

.8

35

99.1

0

49.6

13.5

57

.2

9 52

.2

0 52

.2

35

113.

2 0

32.9

50

72.5

25

56

.9

0 42

. I

10

48.7

0

40.6

4.78

74

.9

0 53

.0

17.6

72

.0

0 52

.8

30

36.9

0

27.7

30

46

.9

0 34

.1

15

28.8

0

24.2

12

65

.5

0 49

.1

10

38.1

0

34.8

21.5

11

.9

12.2

100 9.

6 0

244.

5

72.2

35

.2

20.0

41.2

36.4

33.0

37.6

19.2

33.3

9.8

Prah

a-R

uzyn

E,

Cze

ch R

epub

lic

Bro

adba

lk. U

K

Suga

r be

et/s

prin

g w

heat

(si

nce

1966

)

Con

tinuo

us w

heat

Ask

ov, D

enm

ark

Win

ter

cere

als/

root

cro

ps/s

prin

g ce

real

s/cl

over

+ gr

ass

Chr

iste

nsen

(19

96)

Hoo

sfie

ld, U

K

Con

tinuo

us b

arle

y Je

nkin

son

& J

ohns

ton

(197

7)

John

ston

(19

75)

Woh

urn

Mar

ket G

arde

n,

UK

V

ario

us v

eget

able

cro

ps

Deh

Cra

in, F

ranc

e W

heat

/sug

ar b

eet

Hou

ot &

Cha

usso

d (1

995)

Witt

er e

t al

. (19

93)

Ulta

na. S

wed

en

Ara

ble

only

Con

tinuo

us w

heat

& b

arle

y2

Wob

urn

Stac

kyar

d,

UK

(18

77-1

926)

Skie

mie

wic

e, P

olan

d

Mat

tingl

y et

al.

(197

5)

Con

tinuo

us p

otat

oes

Mer

cik

(199

3

Skie

rnie

wic

e, P

olan

d)

Con

tinuo

us r

ye

Mer

cik

( 199

3)

4 m

aize

/bar

ley/

pota

toes

/bar

ley

Schn

iede

r (1

976)

Th

yrow

Nut

rient

D

efic

ienc

y, G

erm

any

Hal

le, G

erm

any

Con

tinuo

us r

ye t

o 19

61; a

rabl

e ro

tatio

n si

nce

3 co

urse

ara

ble

rota

tion

Gar

z &

Hag

edom

( 1

990)

Bos

ch &

Am

herg

er

(198

3)

Wei

hens

teph

an, G

erm

any

6 W m

'Con

verte

d to

t h

a-'

to 3

0 cm

fro

m S

OC

at g

iven

dep

th o

r fr

om S

OC

con

tent

(%

) us

ing

soil

bulk

den

sitie

s gi

ven

in r

efer

ence

or

in G

CT

E S

OM

NE

T d

atab

ase

(Sm

ith e

t al.

1996

a). W

here

un

avai

labl

e, b

ulk

dens

ity o

f 1.

3 g

CII

-~ as

sum

ed. 2

Mea

n of

bot

h co

ntin

uous

cro

ps 3

Farm

yard

man

ure

and

NPK

bot

h ad

ded

with

cal

cium

C A R B O N S E Q U E S T R A T I O N I N E U R O P E A N S O I L S 71

u 1.5- .-

gb 3 I - 5 9 - 0.5- 9

0. s

8 2 2.51

Animal manure added (t ha-' y -')

Fig. 1 yearly percentage change in SOC content (difference between animal manure only treatment and inorganic fertilizer only treatment) with increasing yearly additions of animal manure. W = Plotted points (from Table 3), - = Linear regression trend line following y = 0.038~ - 0.0538.

" '

0 10 20 30 40 50 60 70

Sewage Sludge added (t ha"y")

Fig. 2 yearly percentage change in SOC content (difference between sewage sludge treatment and inorganic fertilizer only treatment) with increasing yearly additions of sewage sludge. W = Plotted points (from Table 5), - = Linear regression trend line following y = 0.129~ + 1.8062.

Scenario (ii): Amendment of arable soils with sewage sludge

In this scenario, arable soils are amended with sewage sludge at realistic application rates in order to increase SOC content. The rates chosen are 1 t ha-' y-' (the lowest of the maximum recommended application rates for sewage sludge [Sweden] in the EUR15; Williams 1988) and 2.59 t ha-' y-' (the mean of the maximum recommended application rates in the EUR15; Williams 1988). These values are based upon rates considered to be environmentally safe with respect to heavy metals and organic pollutants (Sauerbeck 1987; Williams 1988; Dirkzwager & L'Hermite 1989; Hall et al. 1992). In 1984, 37% of sewage sludge was recycled to agricultural land in the European Union (Bowden 1987). Our calculations

assume 100% recycling to arable land over the next century.

Three long-term field experiments (five treatments) were identified in Europe that measure changes in SOC after amendment with sewage sludge or inorganic fertilizers. Table 5 presents details of these experiments.

Values for percentage change in SOC per year were derived and plotted against the amount of sewage sludge added per year (Fig. 2).

On a limited range of soil types and climatic conditions, a highly significant linear relationship was found (9 =

0.9754, t3 = 10.91, P [two-tailed] = 0.0004). Although the lowest data point used in deriving the regression was for a rate of 6.5 t ha-' y-', the relationship was assumed to remain constant to rates of 1 t ha-' y-'. This relationship was then used to explore scenarios involving the amendment of arable land with sewage sludge. From the regression equation, amendments of soil with sewage sludge at 1 and 2.59 t ha-' y-' increase SOC contents by 1.96 and 2.14% y-', respectively. Table 6 shows the changes that would occur if arable soils were amended with sewage sludge.

An application rate of 1 t ha-' y-' covers a larger area of land and increases total SOC content (3.7% over 100 years) more than does the higher application to a smaller area. The lower rate is also likely to be environmentally safer.

Scenario (iii): Incorporation of cereal straw into the soils in which it was grown

In this scenario, cereal straw is ploughed back into the soils in which it was grown. The rate of straw incorporation is 5.07 t ha-' y-l to all cereal land (see footnote 1, Table 8). Since some (unquantified) amount of cereal straw is used for animal bedding, and this will continue to be required, the rates of incorporation used in this scenario are unlikely to be achievable in reality. It is our intention here to examine the maximum potential for carbon sequestration.

Eight long-term field experiments (10 treatments) were identified in Europe that measure changes in SOC after incorporation of cereal straw. Table 7 presents details of these experiments.

Values for percentage change in SOC per year were derived and plotted against the amount of straw incorporated per year (Fig. 3).

Despite a wide range of soil types with very different rotations and climatic conditions, a significant linear relationship was found (? = 0.4143, t8 = 2.38, P [two- tailed] = 0.0413). This relationship was then used to explore the scenario of incorporating all cereal straw into cereal land. From the regression equation, incorporation of 5.07 t ha-' y-' increases SOC content by 0.76% y-'.


72 P. SMITH et al.

Table 4 Amendment of arable soils with animal manure

Increase in SOC Increase in SOC in % increase in SOC Amendment rate % of arable land inarable land over total EUR15 soil in total EUR15 over Yearly increase (t ha-' y-') covered' 100 years over 100 years (Pg) 100 years (Pg) in SOC (Tg y-')

5 10 15

> 100 0.53 > 100 1.26

75.26 1.50

22.95 + 23.47 2.3 22.95 + 24.21 5.5 22.95 + 24.45 6.5

5.25 12.58 14.99

'Using figures for the number of cattle (83.95 X lo6) and pigs (120.33 x lo6) in the EUR15 (Eurostat 1995) and figures for manure production (excluding slurry) per year for individual animals (MAFF 1994), the total amount of animal manure available in the EUR15 can be calculated as 821.86 X lo6 t y-'. Area of arable land covered calculated using figures in Table 2.

Table 5 Changes in SOC in long-term experiments concerning sewage sludge application

SOC to Difference (%) No. Rate per y 30 cm from inorganic

Experiment Crop/ Rotation years Treatment (t ha-' y-') (t ha-')' only Reference

Ultana, Sweden

Wobum Market Garden - Sewage Sludge, UK Wobum Market Garden - Sludge/ straw compost, UK2

Arable only 31 13 t ha-' 2y-' Inorganic only

Various market 30 61.4 t ha-' y-' garden crops 30.7 t ha-' y-'

Inorganic only

Various market 30 61.4 t ha-' y-' garden crops 30.7 t ha-' y-'

Inorganic only

6.5 0

61.4 30.7 0

43 21.5 0

105.3 98.5 Witter et al. (1993)

111.9 181.4 Johnston (1975)

53.0

78.0 96.1 39.8

97.1 130.6 Johnston (1975) 74.1 75.9 42.1

'Converted to t ha-' to 30 cm from SOC at given depth or from SOC content ("7) using soil bulk densities given in reference or in GCTE SOMNET database (Smith et al. 1996a). Where unavailable, bulk density of 1.3 g ~ r n - ~ assumed. 2Sludge/straw compost contained about 70% organic matter of pure sewage sludge (Johnston 1975). Rate per y determined from this percentage.

Table 6 Amendment of arable soils with sewage sludge

Increase in SOC in Increase in SOC in

100 years (Pg) years in SOC (Tg y-')

% increase in SOC in Amendment rate % of arable land arable land over 100 total EUR15 soil over total EUR15 over 100 Yearly increase (t ha-' y-') covered' years (Pg)

1 11.17 0.85 22.95 + 23.79 3.7 8.45 2.59 4.31 0.36 22.95 + 23.30 1.6 3.56

'In 1984, 5.56 X 106 t of sewage sludge was produced by a population of 310 X lo6 (Bowden 1987). Allowing for enlargement of the European Union, the population has increased to 341 X lo6 and assuming a 33% increase in sewage sludge production to 1994 (Bowden 1987), current sewage sludge production for the EUR15 is estimated to be 8.13 X lo6 t. Percentage arable land covered is estimated from figure in Table 2.

Table 8 shows the changes that would occur if straw were incorporated into cereal land.

Incorporation of all cereal straw back into the soil in which it was grown would increase total SOC stocks in Europe by 6.1% over 100 years (Table 4).

estimated that by 2010, 20-30% of current arable land in Europe will be surplus to agricultural requirements (Flaig & Mohr 1994). We examine the effect of afforestation of either 20% or 30% of current arable land by natural woodland regeneration.

There are only two long-term field experiments concerning changes in SOC during and after natural woodland regeneration in Europe, both from the same research station in the UK. As such, a robust statistical relationship

Scenario (iv): Aforestation of surplus arable land through natural woodland regeneration

In this scenario, all surplus arable land is allowed to revert to natural woodland over a century. It has been

between afforestation through natural regeneration and yearly change in SOC content cannot be derived. For this



Table 7 Changes in SOC in long-term experiments in which cereal straw was incorporated

SOC to No. Rate per y 30 cm Difference (%)

Experiment Crop/Rotation years Treatment (t ha-' 7') (t ha-')' from no straw Reference

Essai Permanent, Belgium

36 Parcelles, France

Meckenheim 11, Germany Rothamsted, UK

Askov, Denmark

Ranhave. Denmark

Studgaard, Denmark

Ultana, Sweden

2-3 y arable rotation

Bare fallow2

Cereal crops

Continuous winter wheat

Continuous spring barley

Continuous spring barley

Continuous spring barley Arable only

30

30

22

7

10

18

18

35

2.4 t ha-' y-l No straw 7 t ha-' y-' No straw

6.5 t ha-' y-' No straw3

18 t ha-' y-' 9 t ha-' y-I 4.5 t ha-' y-' No straw 12 t ha-' y-' No straw 4 t ha-' y-l No straw4 4 t ha-' y-' No straw4

8.7 t ha-' y-' No straw

2.4 0

7 0

6.5 0

18 9 4.5 0

12 0

4 0

4 0

8.7 0

41.8 14.3 36.5

52.3 39.6 37.4

59.6 19.7 49.8

78.9 10.2 74.5 4.0 75.6 5.6 71.6

83.3 29.5 64.3

30.2 5.2 28.7

61.9 4.4 59.3

71.2 49.6 47.6

Frankinet et al. (1993)

Houot et al. (1989)

Kick & Poletschny (1980)

M.J. Glendining, N. Bradbury & C. Grace (personal communication)

Thomsen (1993)

Powlson et al. (1987)

Powlson et al. (1987)

Persson & Mattsson (1988); Kirchmann et al. (1994)

'Converted to t ha-' to 30 cm from SOC at given depth or from SOC content (%) using soil bulk densities given in reference or in GCTE SOMNET database (Smith et al. 1996a). Where unavailable, bulk density of 1.3 g cm" assumed. 2Mean of plots 133 and 136 (Houot et al. 1989) 3Value at beginning of the experiment before straw incorporation began (Kick & Poletschny 1980) 4Straw burned (Powlson et al. 1987)

0.5

P I

0 2 4 6 8 10 12 14 16 18

Straw added (t hd"y-')

Fig. 3 yearly percentage change in SOC content (difference between straw incorporation and no straw incorporation) with increasing yearly additions of cereal straw. W = Plotted points (from Table 7), - = Linear regression trend line following y = 0.1115~ + 0.192.

scenario, a simple mean of the two figures is used. Table 9 presents details of these experiments.

The relationship between natural woodland regeneration and yearly change in SOC content (1.66% y-l) was then used to explore scenarios involving afforestation of

surplus arable land. Table 10 shows the changes that would occur if surplus arable land were allowed to revert to woodland.

Afforestation of either 20% or 30% of (surplus) arable land would increase total SOC stocks in Europe by 5.6% or 8.4% over 100 years (Table 10). Afforestation also leads to an additional accumulation of carbon in tree biomass. Jenkinson (1971) estimated the carbon in standing woody biomass to be three times that found in soil on natural woodland regeneration experiments. Standing woody biomass would therefore accumulate 3.85 Pg C over 100 years if 20% of current arable land were afforested or 5.77 Pg C if 30% were afforested, in addition to the carbon sequestered in soil. This is sequestered only temporarily, however, unless converted to durable bio- products (Carter & Hall 1995).

Wood production allows a further carbon mitigation option not so far considered, the substitution of fossil fuels with biofuels for energy production. Although natural woodland regeneration is probably not the most efficient means of woody biofuel production, we have used the estimate of Jenkinson (1971) for carbon in wood after natural woodland regeneration as a basis for


74 P. SMITH et al.

Table 8 Incorporation of straw to all cereal land

Increase in SOC in Increase in SOC in % increase in SOC in Incorporation rate % of cereal land cereal land over 100 total EUR15 soil over total EUR15 over 100 Yearly increase in (t ha-' y-') ' covered years (Pg) 100 years (Pg) years SOC (Tg y-')

5.07 100 1.40 22.95 + 24.35 6.1 14.02

'Total cereal grain yield in the EUR15 in 1993 was 177 X lo6 t (Eurostat 1995). Assuming a harvest index (grain to straw ratio) of 50% this indicates that 177 x lo6 t of straw was produced. Assuming that all straw is incorporated into the cereal land on which it grew (see Table 2), there is enough to provide 5.07 t ha-'. Percentage cereal land covered is by definition 100%.

Table 9 Changes in SOC in long-term experiments concerning afforestation of arable land by natural woodland regeneration

Experiment No. years

Geescroft Wilderness, 102 UK (1883-1985)

Broadbalk Wilderness, 100 UK (1880-1980)

SOC to 30 cm Difference (%) When SOC measured (t ha-')' from beginning Reference

At beginning of period 80.5 117.3 Poulton (1996) At end of period 37.0

At beginning of period 82.2 215.0 Jenkinson (1990) At end of period 26.1

'Converted to t ha-' to 30 cm from SOC at given depth from SOC content (%) using known soil bulk densities

Table 10 Afforestation of surplus arable land through natural woodland regeneration

Increase in SOC in Increase in SOC in % increase in SOC in % current arable area of arable afforested area over total EUR15 soil over total EUR15 over 100 Yearly increase in land afforested land (lo3 ha)' 100 years (Pg) 100 years (Pg) years SOC (Tg y-')

20 30

14560 1.28 21841 1.92

22.95 4 24.23 5.6 22.95 + 24.87 8.4

12.83 19.24

'Area of arable land covered is estimated from figure in Table 2.

Table 11 Carbon emission reduction potential of wood produced by afforestation of surplus arable land through natural woodland regeneration

% current arable Carbon in woody Yearly accumulation Assumed Energy substitution Yearly carbon land biomass after of carbon in wood percentage energy factors (lower & emission reduction afforested 100 years (Pg) (Tg 7') utilization' higher)2 (Tg Y-?

20 3.85 38.48 75 0.5-0.75 14.47-20.26 30 5.77 57.72 75 0.5-0.75 21.70-30.38

'From IPCC (1996); 2From Sampson et al. (1993)

calculating fossil fuel substitution. Not all of the carbon sequestered by trees can be converted to energy to reduce carbon emissions. Table 11 details the calculation of the carbon emission reduction potential of the wood produced by afforestation.

The lowest estimate of yearly carbon emission reduction is slightly larger than the amount of carbon sequestered in soil under afforestation. This indicates that the carbon mitigation potential of the afforestation scenario is more than doubled by including biofuel substitution for fossil fuels in the calculations.

Scenario (v): Agricultural extensifcation

Estimates of the proportion of arable land that will become surplus to agricultural requirements assume the current level of intensive agriculture. An alternative to changing the land-use of surplus arable land is to use the total current arable land in a less intensive way. In this scenario, we assume that food requirements in the EUR15 will remain constant over the next century (the human population is predicted to remain constant at least to 2025; Sauerbeck 1993) and that the number of



animals farmed for meat and other products will not increase (total livestock numbers in the EUR15 have remained fairly constant over the past 10 years; Eurostat 1995). Current grassland is left in its present form since this is currently sufficient to support the needs of livestock. Cattle farming is not altered in the present scenario. Potential problems regarding the suitability of various soils for ley-arable farming are ignored. The figure for surplus arable land by 2010 (up to 30%) of Flaig & Mohr (1994) suggests that present arable production could be achieved on 70% of current arable land.

In this scenario, all arable land is put into ley-arable rotation so that at any one time 30% of land is in a ley phase. The ley phase of the rotation (two years in a six year rotation) could then be used to extensify pig and poultry production by moving intensively farmed pigs and poultry to outdoor units.

The total number of pigs in the EUR15 is 120.33 X lo6 (Eurostat 1995) and the total number of poultry placed in hatcheries for meat production or egg laying is 3957.85 X lo6 (Eurostat 1995). Stocking densities for pigs and poultry when farmed outdoors are 13 ha-' for pigs (MAFF 1983) and 1000 ha-' for poultry (Lampkin 1990). Even if all pigs and poultry within the EUR15 are assumed currently to be intensively farmed, the area of land in the ley phase at any one time (21841 x lo3 ha) would allow all pigs and poultry in the EUR15 to be farmed in outdoor units.

Three long-term experiments on land formerly under exclusive arable agriculture concerning the effects on SOC of ley-arable rotation in Europe, and a study referring to a further three, have been identified. Since rotations, lengths of the ley phase and amount and type of ley grazing vary, a robust statistical relationship between ley- arable farming and yearly change in SOC content cannot be derived. For this scenario, a simple mean of the figures available is used. Table 12 presents details of the experiments.

The relationship between ley-arable rotation and yearly change in SOC content (1.02% y-') was then used to explore the agricultural extensification scenario. Table 13 shows the changes that would occur if all current arable land were put into 1 / 3 ley-arable rotation.

Conversion of current arable land to a ley-arable system would increase total SOC stocks in Europe by 17.1% over 100 years (Table 13), the greatest effect on SOC content of any of the scenarios examined in this paper. Based on manure production per animal (MAFF 1994) and the stocking densities above, pigs would add 19.5 t ha-' and poultry would add 31.5 t ha-' animal manure to the leys. Data are not available to quantify whether or not this would further increase the SOC content of the soil.

In this scenario pigs are no longer housed indoors so their excreta cannot be used in farmyard manure

production. This leaves only cattle manure of which there is 533 X 106 t produced in the EUR15 (calculated from the number of cattle in the EUR15 (83.95 X lo6; Eurostat 1995) and the amount of manure produced per individual (6.35 t y-'; calculated from figures in MAFF 1994). On the arable land not in the ley phase at any one time, this is enough animal manure to apply 10.46 t ha-' y-'. Although this level of organic manure application may further increase the SOC content of the soil, data are not available quantify the effect.

Discussion

It is generally recognized that globally, the greatest potential for carbon mitigation is in the tropics (Sauerbeck 1993). However, the potential for carbon sequestration in temperate agricultural soils needs to be considered. It is estimated that between 400 and 800 Tg C y-' could be sequestered in agricultural soils globally (IPCC 1996). The figures of 19.24 Tg y-I for soil sequestration and up to 30.38 Tg y-' for carbon emission reduction in the afforestation scenario show that despite covering only 2.43% of the earths land area, the EUR15 could contribute nearly 1 / 8 toward the lower global estimate assuming afforestation of 30% of current arable land.

To put the figures for carbon sequestration in context, global anthropogenic carbon dioxide production in 1991 was 6188 Tg C y-' (Marland et al. 1994). Not all figures for EUR15 member states are available but the figure for Western Europe is 918 Tg C y-' (Marland et al. 1994). The maximum carbon mitigation potential of the five scenarios examined is shown in Table 14.

The figures in Table 14 show that the afforestation and extensification scenarios have the greatest potential for carbon mitigation, but even these can account for only 4.3- 5.4% of anthropogenic C02-carbon produced in Western Europe, or 0 .648% of that produced globally.

Soil organic matter accumulation, and hence carbon storage, is just one of many environmental considerations when assessing these scenarios. As expected, addition of different sources of organic matter (animal manure, sewage sludge and cereal straw) led to different rates of SOC accumulation (Kolenbrander 1974; Van Dijk 1982). However, the addition of any of these organic materials to arable soils has only a small impact on the amount of anthropogenic C02-carbon produced in Western Europe. A combination of organic amendments [sewage sludge at 1 t ha-'( =11%), and animal manure at 15 t ha-'( =75%), of arable land] could increase the potential slightly. There are, of course, other benefits from the greater and more efficient use of animal manures (Arden-Clarke & Hodges 1988) but it is important to weigh the benefits against potential undesirable side-effects such as increased risk of nitrate leaching (CEC 1989), trace gas fluxes from the


76 P. SMITH et al.

Table 12 Changes in SOC in long-term experiments concerning ley-arable rotation

SOC to Difference (%) No 30 cm from arable

Experiment Ley-arable rotation years Rotation (t ha-')' only Reference

Fosters, UK2 3 year ley (lucerne / clover) - 15 Ley-arable 59.5 13.4 Johnston (1973) three year arable Arable only 52.5

Woburn Ley-Arable, 3 year ley (grazed lucerne) - 28 Ley-arable 44.1 28.4 Johnston (1973) UK3 two year arable Arable only 34.3 Orja, Sweden4 1 year ley (clover / grass) - 12 Ley-arable 61.3 14.9 Nilsson (1986)

three year arable Arable only 53.3 Three SOM farms from Rotatiori unspecified 25 Ley-arable 58.1 23.0 Van Dijk (1982) the Netherlands5 Arable only 47.3

'Converted to t ha-' to 30 cm from SOC at given depth or from SOC content ("7) using soil bulk densities given in reference or in GCTE SOMNET database (Smith et al. 1996a). Where unavailable, bulk density of 1.3 g cm-3 assumed. 'Lucerne/clover ley not grazed (Johnston 1973). SOC values are mean of two halves of the experiment; one beginning with a ley phase, the other beginning with an arable phase. 3Lucerne grazed by sheep (Johnston 1973). 4Clover/grass ley grazed by cattle. 20 t ha-' of animal manure added to ley-arable and arable only rotations every fourth year (Nilsson 1986). 5Three soil organic matter farms using both ley- arable farming and 'mineral fertilizer only' treatments described in Van Dijk (1982). Mean values for three farms given. Ley phase grazed by livestock and animal manure also added to ley-arable system (Van Dijk 1982)

Table 13 Agricultural extensification

% current arable land put in to ley- arable land over 100 total EUR15 soil over total EUR15 over 100 Yearly increase in arable years (Pg) 100 years (Pg) years SOC (Tg y-')

Increase in SOC in Increase in SOC in YO increase in SOC in

100 3.92 22.95 -+ 26.87 17.1 39.21

Table 14 Carbon mitigation potential of the five scenarios examined

Scenario

Yearly carbon % of anthropogenic carbon % of anthropogenic carbon mitigation potential dioxide carbon produced dioxide carbon produced (Tg y-')' annually in Western Europe' annually globally2

10 t ha-' y-' animal manure to all arable soils 1 t ha-' y-' sewage sludge to arable soils (11.17% land) 5.07 t ha-' y-' straw incorporated into all cereal land Afforestation of 30% surplus arable land by natural regeneration Conversion of all current arable land to 1 / 3 ley-arable farming with extensification of pig and poultry farming

12.58 1.37

8.45 0.92

14.02 1.53

49.62 5.41

39.21 4.27

0.20

0.14

0.23

0.80

0.63

'From Tables 4, 6, 8, 10, 11 and 13. 2From Marland et al. (1994). 3Derived by adding SOC sequestration potential (Table 10) and carbon emission reduction potential assuming higher energy substitution factor of Sampson et al. (1983) from Table 11.

soil (Willison et al. 1995), and increased heavy metal and organic pollutant concentrations in the environment (Sauerbeck 1987). The rates of organic amendment sug- gested here are low, however, and as such are unlikely

to lead to serious environmental side-effects but we have not attempted to quantify this.

The latter two scenarios have a larger uncertainty associated with them since they use figures derived from



fewer long-term experiments, from which it was not possible to establish robust statistical relationships. Based upon these more limited data, both show some potential for sequestering useful amounts of carbon (Table 14). The afforestation scenario uses only surplus arable land and has a number of other potential environmental advant- ages such as enhancement of landscape appearance and the provision of wildlife habitats. A disadvantage is that agricultural production on the remaining land would need to remain at its current intensive level.

The extensification scenario would also yield a number of other potential benefits such as improved animal welfare and decreased requirement for inorganic fertilizers but could increase the risk of nitrate leaching when ley pastures are ploughed under. Of the five scenarios investigated, the extensification scenario uses the largest number of assumptions. It is also the most reliant upon suitability of soil types and climate (outdoor pig farming is better suited to light soils and mild climate; MAFF 1983), aspects not considered here.

The estimates of the potential for carbon sequestration in soils in the EUR15 are based upon the best long-term data available in Europe. Account was not taken in these estimates of the suitability of soils or climate in a given region or of the local availability (or surplus) of resources used (such as animal manure), nor of the potential problems of moving resources from one locality to another. To account for such factors, a more sophisticated approach is required, ideally involving the use of spatial databases of soil, climate, ground cover, etc. in the EUR15 coupled with a suitable dynamic simulation model (similar to the approach used by Donigan et al . 1994 and Lee et al. 1993). A model evaluation and comparison exercise (Smith et al. 1996c) involving nine SOM models and 12 long-term datasets has recently been undertaken to determine the suitability of models for such purposes (Smith et al . 1996d).

The estimates provided here serve to define only the potential for carbon sequestration in the EUR15. Given that they may tend to overestimate the level of carbon sequestration attainable in practice, and given that the scenario with the greatest potential for carbon mitigation can sequester only 0.8% of the annual anthropogenic C02-carbon produced globally, the figures presented in this paper do not contradict the view that, although efforts in temperate agriculture can make small contributions to global carbon mitigation, more significant impacts can be made by halting tropical and sub-tropical deforestation (Sauerbeck 1993) and by reducing anthropogenic C02 emissions from fossil fuel burning (Houghton et al . 1992).

Acknowledgements We are grateful to Liz Grover, Nicky Bradbury and Paul Poulton for helpful discussion during the preparation of this manuscript.

P.S. was supported by a grant from the Terrestrial Initiative in Global Environmental Research (TIGER) programme of the U.K. Natural Environment Research Council (NERC). IACR receives grant-aided support from the Biotechnology and Biological Sciences Research Council of the United Kingdom.

References

Arden-Clarke C, Hodges RD (1988) The environmental effects of conventional and organic/ biological farming systems. 11. Soil ecology, soil fertility and nutrient cycles. Biological Agriculture and Horticulture, 5, 223-287.

Bosch M, Amberger A (1983) EinfluP langjhringer Diingung mit verschiedenen N-Formen auf pH-Wert, Humusfraktionen, biologische Aktivitat und Stickstoffdynamik einer Acker- Braunerde. Zeitschrift f u r Pflanzenernahrung und Bodenkunde, 146, 714-724.

Bowden AV (1987) Survey of European Sludge Treatment and Disposal Practices. Water Research Centre, Medmenham, UK,

Carter MR, Hall DO (1995) Working Group Report -

Management. In: Terrestrial Ecosystems. In: Carbon Sequestration in the Biosphere. Processes and Prospects (ed. Beran M), pp. 227-238. NATO AS1 133, Springer-Verlag, Heidelberg.

CEC (1989) Intensive Farming and the Zmpact on the Environment and the Rural Economy of Restrictions on the Use of Chemical and Animal Fertilizers. Commission of the European Communities, Luxembourg, 212, pp.

Christensen BT (1996) The Askov long-term experiments on animal manure and mineral fertilizers. In: Evaluation of Soil Organic Matter Models Using Existing, Long-Term Datasets (eds Powlson DS, Smith P, Smith JU), pp. 301-312. NATO AS1 138, Springer-Verlag, Heidelberg.

Dirkzwager AH, L, 'Hermite P (eds) (1989) Sewage Sludge Treatment and Use. New Developments, Technological Aspects and Environmental Effects. Elsevier, London, 536, pp.

Donigan ASJr, Barnwell TOJr, Jackson RBIV, Patwardhan AS, Weinrich KB, Rowel1 AL, Chinnaswamy RV, Cole CV (1994) Assessment of Alternative Management Practices and Policies Afecting Soil Carbon in Agroecosystems of the Central United States. US. EPA Report EPA/600/R-94/067, Athens, 194, pp.

Eurostat (1995) Agriculture Statistical Yearbook, 1995. Theme 5, Series A. Commission of the European Communities, Luxembourg, 250 pp.

Flaig H, Mohr H (eds) (1994) Energie aus Biomasse - eine Chance fuel die Landwirtschaft. Springer, Berlin, 386 pp.

Frankinet M, Raimond Y, Destain JP, Roisin C, Grevy L (1993) Organic matter management and calcic amendments in order to maintain or improve soil fertility. In: Soil Biota, Nutrient Cycling and Farming Systems (eds Paoletti MG, Foissner W, Coleman D), pp. 27-39. Lewis, Boca Raton.

Fraters D, Bouwmann AF, Thewessen TJM (1993) Soil organic matter map of Europe. Estimates ofsoil organic matter content of the topsoil of FAO-Unesco soil units. RIVM Report no. 481505004, Bilthoven, Netherlands, 60 pp.

Garz J, Hagedom E (1990) Der Versuch 'Ewiger Roggenbau' nach 110 Jahren. In: 110 ]ahre Ewiger Roggenbau (ed Liste H-J), pp. 9-30. Martin-Luther-Universitat Halle-Wittenberg

182, pp.


78 P. S M I T H et al.

Wissenschaftliche Beitrage 1990 / 31 (S 72), Halle (Saale), Germany.

Gupta RK, Rao DLN (1994) Potential of wastelands for sequestering carbon by reforestation. Current Science, 66, 378-380.

Hall JE, Sauerbeck DR, L, 'Hermite P (1992) Effects of Organic Contaminants in Sewage Sludge on Soil Fertility, Plants and Animals. Commission of the European Communities, Brussels, 224 pp.

Houghton JT, Callander BA, Varney SK (eds) (1992) Climatic change 1992 - the Supplementary Report to the IPCC Scientific Assessment, Intergovernmental Panel on Climate Change, Report prepared for IPCC by Working Group 1. Cambridge University Press, New York.

Houot S, Chaussod R (1995) Impact of agricultural practices on the size and activity of the microbial biomass in a long-term field experiment. Biology and Fertility of Soils, 19, 309-316.

Houot S, Molina JAE Chaussod R, Clapp CE (1989) Simulation by NCSOIL of net mineralization in soils from the Deherain and 36 Parcelles fields at Grignon. Soil Science Society of America Journal, 53, 451455.

Howard PJA Loveland PJ, Bradley RI, Dry FT, Howard DM, Howard DC (1995) The carbon content of soil and its geographical distribution in Great Britain. Soil Use and Management, 11, 9-14.

IPCC (1996) Climate Change 1995. Impacts, Adaptations and Mitigation of Climate Change. Scientific-Technical Analyses, Cambridge University Press, Cambridge, UK, 878 pp.

Jenkinson DS (1971) The accumulation of organic matter in soil left uncultivated. Rothamsted Report for 1970, Part, 2, 113-137.

Jenkinson DS (1990) The turnover of organic carbon and nitrogen in soil. Philosophical Transactions of the Royal Society, London

Jenkinson DS, Johnston AE (1977) Soil organic matter in the Hoosfield Continuous Barley Experiment. Rothamsted Report for 1976, Part, 2, 87-101.

Jenkinson DS, Hart PBS Rayner JH, Parry LC (1987) Modelling the turnover of organic matter in long-term experiments at Rothamsted. INTECOL Bulletin, 15, 1-8.

Johnston AE (1973) The effects of ley and arable cropping systems on the amounts of soil organic matter in the Rothamsted and Woburn Ley-Arable Experiments. Rothamsted Report for 1972, Part, 2, 131-159.

Johnston AE (1975) The Woburn Market Garden Experiment, 1942-1969. 11. The effects of the treatments on soil pH, soil carbon, phosphorus and potassium. Rothamsted Report for 1974, Part 2, 102-132

Kick H, Poletschny (1980) Auswirkung langjahringer Strohdiingung auf den Nahrstoffhaushalt des Bodens. Landwirtschaftliche Forschung SH, 36, 383-388.

Kirchmann H, Persson J, Carlgren K (1994) The Ultana Long-term Soil Organic Matter Experiment, 1956-1991. Swedish University of Agricultural Sciences, Department of Soil Sciences, Reports and Dissertations 17, Uppsala. 55, pp.

Klir J (1996) Long-term field experiment Praha-RuzynP, Czech Republic. In: Evaluation of Soil Organic Matter Models Using Existing, Long-Term Datasets (eds Powlson DS, Smith P, Smith JU), pp. 363-368. NATO AS1 138, Springer-Verlag, Heidelberg.

B, 329, 361-368.

Kolenbrander GJ (1974) Efficiency of organic matter in increasing soil organic matter content. Transactions ofthe 10th International Congress of Soil Science, 2, 129-136.

Korschens M, Miiller A (1996) The static experiment Bad Lauchstadt, Germany. In: Evaluation of Soil Organic Matter Models Using Existing, Long-Term Datasets (eds Powlson DS, Smith P, Smith JU), pp. 369-376. NATO AS1 138, Springer, Heidelberg.

Lampkin N (1990) Organic Farming. Farming Press, Ipswich,

Lee JJ, Phillips DL, Liu R (1993) The effect of trends in tillage practices on erosion and carbon content of soils in the US Corn Belt. Water, Air, and Soil Pollution, 70, 389401.

MAFF (1983) Pigs: the outdoor breeding herd. Booklet 2431. HMSO, London, 24 pp.

MAFF (1994) Fertilizer recommendations for Agricultural and Horticultural Crops. RB209, 6th edn. HMSO, London, 112 pp.

Marland G, Andres RJ, Boden TA (1994) Global, regional, and national C02 emissions. In: Trends '93: A Compendium of Data on Global Change (eds Boden TA, Kaiser DP, Sepanski RJ, Stoss FW), pp. 505-584. ORNL/CDIAC45. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA.

Mattingly GEG Chater M, Johnston AE (1975) Experiments made on Stackyard Field, Woburn, 1876-1974) 111. Effects of NPK fertilizers and farmyard manure on soil carbon, nitrogen and organic phosphorus. Rothamsted Reportfor 1974, Part 261-78.

Mercik S (1993) Seventy years of fertilizing experiments in Skierniewice. In: Proceedings of the International Symposium 'Long-Term Static Fertilizer Experiments', June 1993, Warszawa- Krakow, Poland (eds Mazur K, Filipek J, Mazur B), pp. 31-54. Agricultural University of Cracow, Poland.

Nilsson LG (1986) Data of yield and soil analysis in the long- term soil fertility experiments. Journal of the Royal Swedish Academy of Agriculture and Forestry, Suppl. 18, 32-70.

Persson J, Mattsson L (1988) Soil C changes and size estimates of different organic C fractions in a Swedish long-term small plot experiment. Swedish Journal of Agricultural Research, 18,

Poulton PR (1996) Geescroft Wilderness, 1883-1995). In: Evaluation of soil organic matter models using existing, long-term datasets (eds Powlson DS, Smith P, Smith JU), pp. 385-390. NATO AS1 138, Springer-Verlag, Heidelberg.

Powlson DS, Brookes PC, Christensen BT (1987) Measurement of soil microbial biomass provides an early indication of changes in total soil organic matter due to straw incorporation. Soil Biology and Biochemistry, 19, 159-164.

Powlson DS, Smith P, Coleman K, Smith JU, Glendining MJ, Korschens M, Franko U (1997) A European Network of Long- Term Sites for Studies on Soil Organic Matter. Soil and Tillage Research, in press.

Sampson RN, Wright LN, Winjum JK, Kinsman JD, Benneman J, Kiirsten E, Scurlock JMO (1993) Biomass management and energy. Water, Air, and Soil Pollution, 70, 139-159.

Sauerbeck DR (1987) Effects of agricultural practices on the physical, chemical and biological properties of soils: Part I1 - Use of sewage sludge and agricultural wastes. In: Scientific Basis for Soil Protection in the European Community (eds Barth H, L'Hermite P), pp. 181-210. Elsevier, London.

p.374.

9-12.



Sauerbeck DR (1993) C02 emissions from agriculture: sources and mitigation potentials. Water, Air, and Soil Pollution, 70,

Schnieder E (1976) Entwicklung der Ernteertrage und des Humusgehalts im Thyrower Nahrstoffmangel-Dauerversuch. Wissenschaftliche Zeitschrift der Humboldt-Universitut zu Berlin, Mathematisch-Naturwissenschaftliche Reihe, X X V , 4, 447-456.

Smith P, Powlson DS (1996) Trends in soil organic matter content in long-term experiments: Datasets used for soil organic matter model evaluation and comparison. In: Evaluation and Comparison of Soil Organic Matter Models Using Datasets from Seven Long-Term Experiments (eds Smith P, Powlson DS, Smith JU, Elliott ET). Geoderma Special Issue, in press.

Smith P, Smith JU, Powlson DS (eds) (1996a) Soil Organic Matter Network (SOMNET): 1996 Model and Experimental Metadata. GCTE Report 7, GCTE Focus 3 Office, Wallingford, UK, 259 pp.

Smith P, Powlson DS, Glendining MJ (1996b) Establishing a European soil organic matter network (SOMNET). In: Evaluation of Soil Organic Matter Models using Existing, Long- term Datasets (eds Powlson DS, Smith P, Smith JU), pp. 81- 98. NATO AS1 138, Springer-Verlag, Heidelberg.

Smith P, Powlson DS, Smith JU, Elliott ET (eds) (1996~) Evaluation and Comparison of Soil Organic Matter Models Using Datasets from Seven Long-Term Experiments. Geoderma Special Issue, in press.

381-388.

Smith P, Smith JU, Powlson DS, et al. (1996d) A comparison of the performance of nine soil organic matter models using datasets from seven long-term experiments. In: Evaluation and Comparison of Soil Organic Matter Models Using Datasets from Seven Long-Term Experiments (eds Smith P, Powlson DS, Smith JU, Elliott ET). Geoderma Special Issue, in press.

Stiftung RB, ed (1994) Fur eine umweltfreundliche Bodenntzung in der Landwirtschaft. Bliecher Verlag, Gerlingen, 104pp.

Thomsen IK (1993) Turnover of I5N-straw and NH4N03 in a sandy loam soil: effects of straw disposal and N fertilization. Soil Biology and Biochemistry, 25, 1561-1566.

Van Dijk H (1982) Survey of Dutch soil organic matter research with regard to humification and degradation rates in arable land. In: Land Use Seminar on Soil Degradation, Wageningen, October 1980 (eds Boels D, Davies DB, Johnston AE), pp. 133- 143. Balkema, Rotterdam.

Williams JH (1988) Guidelines, Recommendation, Rules and Regulationsfor Spreading Manures, Slurries and Sludges on Arable and Grassland. Commission of the European Communities, Luxembourg, 65 pp.

Willison TW, Goulding KWT Powlson DS (1995) Effect of land- use change and methane mixing ratio on methane uptake from United Kingdom soil. Global Change Biology, 1, 209-212.

Witter E, Mirtensson AM, Garcia FV (1993) Size of the microbial biomass in a long-term field experiment as affected by different N-fertilizers and organic manures. Soil Biology and Biochemistry, 25, 659-669.


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