carbon sequestration and saving potential associated with changes to the management of agricultural...

Carbon sequestration and saving potential associatedwith changes to the management of agricultural soils

in England

J.A. King1,*, R.I. Bradley2, R. Harrison1 & A.D. Carter1

Abstract. The potential for soil organic carbon sequestration, energy savings and the reduction of theemission of greenhouse gases were investigated for a range of changes in the management of tilled land andmanaged grassland. These parameters were modelled on a regional basis, according to local soils and croprotations in England, and avoided the use of soil related indices. The largest carbon sequestration and sav-ing contribution possible comes from an increase in the proportion of permanent woodland, such that a10% change in land use could amount to 9 Mt C yr21 in the initial years (arable and grassland). Changes inarable management could make a significant contribution to an abatement strategy if carried out in concertwith greater use of permanent conservation field margins, increased returns of crop residues and reducedtillage systems, contributing 1.3 Mt C yr21 in the initial years. It should be noted, however, that true soilcarbon sequestration would be only a minor component of this (125 kt C yr21), the main part being savingson CO2 emissions from reduced energy use, and lower N2O emissions from reduced use of inorganic nitro-gen fertilizer.

Keywords: Carbon sequestration, greenhouse gases, land management, energy use, SOC, farm systems

I N T RO D U C T I O N

The estimated global soil organic carbon (SOC) pool is1550 Pg C (1 petagram ¼ 1015 g), which is approxi-

mately twice that of the atmospheric pool (770 Pg C) (Batjes1998) and 2.5 times that of the biotic pool (610 Pg C). Assuch, SOC occupies a position of controlling influence overcarbon flux dynamics. In the UK, concerns have beenexpressed recently about the perceived decline in SOClevels in arable soils. A small-scale study in Norfolk (EastAnglia) indicated that in the last 20 years SOC values havedeclined between 4 and 23% in the topsoils of various soilseries (Bullard et al. 2001). Earlier, Smith et al. (2000)had calculated 562 Tg C (1 teragram ¼ 1012 g) to be presentin the topsoil of the estimated 66 000 km2 of arable land inEngland and Wales. Taken together, these studies indicatethat perhaps 56 Tg C (i.e. 10% of the topsoil pool) is theor-etically recoverable by re-sequestration to soil.

Past modelling exercises have sought to predict the effectof land use and management changes (designed to mitigategreenhouse gas emissions) on soil carbon content, basedupon the measured response of soil carbon from relatively

few long-term experiments (Smith et al. 1997, 2000a,b).The approach described here uses a soil-related coefficientof annual change in SOC derived from the change in man-agement, while other models such as CQESTR (Rickmanet al. 2002) have used an additive model, calculating gainsand losses due to each activity over short rotations of fiveyears or so.

Some of the management changes suggested run the riskof altering the emission rates of non-CO2 greenhouse gasessuch as nitrous oxide (N2O), which have been shown tohave the potential to attenuate considerably any carbonsaving (Smith et al. 2000c; King & Bullard 2001). Theymay also interact unfavourably with other forms of nitrogenpollution, such as ammonia emission and nitrate leaching,and policies for their control (Shepherd & Harrison 2000).

Whatever scale of changes in farming systems are con-sidered – be they whole system changes (e.g. conversion toorganic farming), rotational changes (e.g. including energycrops or increased proportion of grassland), fertilizerchanges (e.g. substituting more inorganic with organicnitrogen) or tillage changes (e.g. zero-cultivation) – analteration in the pattern of energy use will also occur.Detailed energy accounting of agricultural operations isalready available (Leake 2000; Bullard & Metcalfe 2001)and this must also be incorporated into assessments of theimpact of agricultural practices to mitigate the overall emis-sion rates of greenhouse gases.

1ADAS Ltd, Boxworth Research Centre, Battlegate Road, Boxworth,Cambs CB3 8NN, UK. 2National Soil Resources Institute, CranfieldUniversity, Silsoe, Beds MK45 4DT, UK.*Corresponding author. Fax: þ 44 (0)1954 268268.E-mail: john.king@ adas.co.uk

Organic carbon dynamics and soil management394

Soil Use and Management (2004) 20, 394–402 DOI: 10.1079/SUM2004270

It is now timely to make a focused appraisal of the scaleof response that can practically be expected for differentregions and soil types. In this article, therefore, we seek togenerate a comprehensive list of management practices inEngland that may result in a change in the retention oforganic matter within the soil, and quantify the effect ofadopting each of these practices. In so doing we use onlyestimates of the absolute change in carbon (or the equival-ent for energy and other gases) and avoid using soil relatedindices (SRIs), which depend upon the existing soil carbonstock, and for which there is no theoretical justification.

M E T H O D S

Many articles published recently have addressed measuresthat may either sequester carbon within the soil or saveCO2 emissions from reduced energy use. Those thought tobe most suitable and likely to be tried in England are listedin Table 1.

The measurement of carbon sequestration and savingwas based on the eight ‘Government Office Regions’(Wales was not included and London was subsumed in theSouth East). These broadly mirror the climate and weatherpatterns across England, chiefly on an east–west axis forrainfall and north–south for temperature, and are similarin area size. The type of farming that is carried out in aregion and the potential of the land to sequester carbon areboth partially governed by the soil types in that region. Toavoid the use of SRIs in estimating soil carbon sequestra-tion (there is no theoretical determination of potential soilcarbon from existing levels), only a broad definition of soiltype is required and a detailed inventory of current soil car-bon stocks unnecessary. Therefore the seven broad classesof soil type defined in the current handbook of FertiliserRecommendations (Sandy, Shallow, Deep Silty, Clay, Othermineral, Organic, Peaty) (MAFF 2000) were adopted.

At any given location, soil carbon stocks are largely aresult of management practices and crop residue inputsresulting from rotations over several years. Therefore theimpact of changes in cultivation and management practiceneeded to be assessed on the same basis. A recent survey

carried out by ADAS revealed the most common range ofrotations adopted within each of the aforementioned soiltextural classes and regions of England and Wales (Carter& Jordan 2003). Using this list and the actual distributionof crops by area within each region during 1999, the typicalrotation on each soil type in each region was estimatedtogether with likely secondary and set-aside variants.

A simple additive model of the total net amount of car-bon sequestered or saved (TSC) in response to a manage-ment change was adopted, based upon the changes in fourcomponents: (1) changes in soil organic carbon (SOC)itself; (2) changes in the direct energy (DE) used on site(energy used to power machinery and operations); (3)changes in indirect energy (IE) used on site (energy usedin the manufacture and supply of fertilizers, agrochemicals,etc.); and (4) changes in the emission from soils of othergreenhouse gases (GGs) such as N2O.

The emissions associated with inorganic N fertilizerwere calculated from energy costs of their production(Bullard & Metcalfe 2001; Mortimer et al. 2002). Thevalue of 0.074 kg CO2 emitted for each megajoule of energyexpended was derived from several fuel sources, and wasused as such for other direct and indirect energy costs inthe farm energy audit studies of Bullard & Metcalfe (2001)and Cormack (2000).

Values for SOC, DE, IE and GGs were derived fromthose reported in the literature and simply summed toobtain a value for TSC of each hectare of land to which thechanges apply (TSC ¼ SOC þ DE þ IE þ GGs). Thesummation of the components on to spreadsheets wascarried out as part of a geographical information system(GIS) mapping process. These spreadsheets were designedto detail how this value would be applied across eachrotation on the seven soil types within each region, suchthat an annual equivalent value was obtained for each. Sav-ing rates (and ranges) per unit area of land to which themanagement changes were applied are given in Table 2,expressed as the equivalent amount of carbon emitted foreach mole of CO2.

The actual rate at which SOC was deemed to accumulateafter a change in management was largely derived from thework of Smith et al. (1997), based on results from the

Table 1. Changes in land use and management practice assessed for carbon sequestration and saving potential within the arable and managed grasslandsector of farming in England.

Changes in land use and management practice to arable land Changes in land use and management practice to managed grassland

Change to permanent woodland Change to permanent woodlandChange to willow energy crop Change to willow energy cropChange to Miscanthus energy crop Change to Miscanthus energy crop

Conventional to zero tillage Change to clover based pasturesConventional to reduced tillageAddition of straw residuesAddition of livestock manure to arable land rather than grasslandSet-aside field margins on arable landExtensification scenario of converting break crops to grass in rotationExtensification scenario with outdoor pig breeding on grass in rotation

Conversion to stockless organic management Conversion of conventional to organic dairy management systemConversion to organic management with livestock

J.A. King et al. 395

GCTE-SOMNET database of long-term experiments.This was the case for changes in arable management towoodland, straw incorporation, sewage sludge and manureadditions, set-aside field margins and extensification,although the results from other studies were noted. How-ever, the results from which Smith et al. (1997) calculatedsoil related indices for SOC increase were re-worked suchthat an absolute amount of SOC increase was calculated foreach unit of management change. For changes from arableto energy crops, sequestration rates calculated by Bullardet al. 2001 were employed, and for changes in tillage prac-tice the results from over thirteen studies and reviews weremeaned across different soil types, while the work of Con-ant et al. (2001) revealed a lack of SOC sequestrationpotential following changes in grassland management.

In this study ‘managed’ grassland refers to leys and low-land permanent pasture, not to upland minimally managedand extensively stocked grassland areas. Changes from ara-ble management and managed grassland to woodland wereapplied to 11% of the agricultural area, this being thedifference between current woodland cover (9% of totalland area) and the average for the rest of Europe (15% oftotal land area); this has been suggested as a goal for futureattainment. Changes from arable and grassland managementto willow (Salix spp.– mainly hybrids of viminalis, cinerea,caprea, aurita) and Miscanthus (Miscanthus spp. – mainlygiganteus, but also sacchariflorus and sinensis) energy cropswere also applied to 11% of agricultural land, as a hypothe-tical ‘maximum’ uptake scenario. They were also applied to125 000 ha of arable land to model the current target forenergy crop coverage by 2010.

Zero tillage was only applied to crops that could realisti-cally be drilled by this method and it was assumed thatweather conditions would allow this. Reduced tillage can beapplied to all drilled crops. The change in straw manage-ment was applied to all cereal crops in rotation and strawwas incorporated at rates commensurate with the national

average grain production for wheat and barley (Nix 2003).The inclusion of permanent set-aside field margins forconservation was applied to an area equivalent to 6-m stripsaround fields of an average 10 ha, which amounted to 7.5%of the cultivated area.

The area required for spreading sewage sludge notalready re-cycled on to tilled land was limited at 75 000 ha,and it was assumed that the land area could be readilyfound if required. This may not be the case, however, forlivestock manures and so they were modelled according tothe quantities and areas available region by region. Theseamounts were calculated as the amount of manure alreadyspread on grassland in each region (from ADAS surveys)switched to tilled land instead, at rates calculated to allowthe maximum nitrogen loading of 250 kg N ha21 yr21.

The extensification of certain forms of currently inten-sive livestock production (pigs and poultry) was modelledat two levels. First, the scenario that the pig breeding herdcould be moved to outdoor housing on an extra 51 000 haof suitable (well drained) tilled land in England, andsecond, the potential to use all tilled land for outdoor hous-ing of other pigs and poultry units, by changing allrotations to include two-year grass leys. The change toorganic farming systems was modelled at the rate by whichall current demand for organic produce may theoreticallybe met by English producers, which was by converting anextra 1% of both tillage and managed grassland (in total).

The assumption was made that all managed grassland inEngland would be open to a change in fertilizing regimesuch that clover could be incorporated to provide the equiv-alent of 200 kg ha21 fertilizer N, although much of thisgrassland will already incorporate some clover. The othermajor assumption made for grasslands was that they arecurrently managed for the most energy expensive use poss-ible, that is, dairy production. Therefore, all carbon seques-tration and energy savings made for grassland are at theexpense of dairy farming. Lesser amounts of sequestration

Table 2. Final carbon sequestration and saving rates (kg ha21 yr21 CO2-C) applied to each unit area of land undergoing change according to thescenarios shown.

Change in land use or management SOC DE þ IE GGs TSC

Arable to permanent woodland 552–828 425 327–609 1304–1862Arable to willow energy crop 552–828 304 327–609 1183–1741Arable to Miscanthus energy crop 490–734 275 327–609 1092–1618Conventional to zero tillage 145–235 22 2181 to 284 214–173Conventional to reduced tillage 40 16 0 56Addition of straw residues 532–717 0 261 to 217 471–700Application of additional sewage sludge 610 44 23 651Addition of livestock manure to arable land rather than grassland 50–208 13–25 8–25 71–258Set-aside field margins on arable land 490–734 440 25–46 955–1220Extensification of converting break crops to grass in rotation 479 136 0–172 615–787Extensification with outdoor pig breeding on grass in rotation 479 136 0–2 615–617Conversion to stockless organic management 479 238 7–13 724–730Conversion to organic management with livestock 479 296 7–10 782–785Grassland to permanent woodland 0 1963 2354 4317Grassland to willow energy crop 0 1842 2354 4196Grassland to Miscanthus energy crop 0 1813 2354 4167Change to clover based pastures 0 196 233 163Conversion of conventional to organic dairy management system 0 1749 533 2282

TSC ¼ total sequestered and saved carbon, SOC ¼ soil organic carbon, DE ¼ direct energy CO2-C emissions, IE ¼ indirect energy CO2-C emis-sions, GGs ¼ other greenhouse gases expressed as CO2-C equivalents.


and saving would be made if significant areas of managedgrassland were changed from lowland beef or sheep fatten-ing management.

R E S U LT S

The anticipated net annual additional carbon sequestra-tion and CO2 emission savings in each of the eight Gov-ernment Office Regions in England is given in Table 3.The largest contribution to carbon sequestration/savingwould come from turning over a greater proportion ofland to set-aside as permanent conservation orientatedmargins around fields (about 0.8 Mt C yr21 for England).In addition, ensuring that an amount of cereal strawequivalent to current production were returned to theland would sequester about 0.3 Mt C yr21, as would areturn to grass leys in rotations. Other useful contri-butions to carbon sequestration/saving could be made byapplying livestock manure to arable land rather thangrassland (0.2 Mt C) and changing tillage practice awayfrom plough based systems to shallow tine systems(0.2 Mt C) (Table 3). Only minor sequestration and sav-ing would ensue from increased organic farming andmaybe, surprisingly (if N2O emissions increase), from thewholesale use of zero tillage.

The source of carbon sequestration/saving can be verydifferent for each management change considered. Themain component for increased straw returns, for instance, is

the 800 kg C ha21 that may be accumulated annually as SOCon clay soils (assuming a 7.75 t ha21 wheat crop), with anestimated 50 kg ha21 CO2-C equivalent subtracted for theextra N2O emissions caused by the nitrogen in the straw,but with no change in energy levels used on the land. Set-aside margins, on the other hand, will accumulate SOCat a similar rate, 730 kg C ha21, but will save an extra404 kg C ha21 in energy no longer expended on the land(19 998 MJ ha21) and about 400 kg ha21 CO2-C equivalentfrom expected N2O emissions from the nitrogen fertilizersthat would otherwise have been used on the set-aside strip.For a change from arable production to woodland, similarrates of 800 kg C ha21 as SOC, 425 kg ha21 CO2-C equival-ent in energy (21 068 MJ ha21) savings and 400 kg ha21

CO2-C equivalent for N2O emissions from fertilizer nolonger applied to the land, still apply for a clay soil. How-ever, to these can be added sequestration at the rate of about2800 kg C ha21 as woody biomass accumulating annually.

It is no surprise that these measures would be mosteffective in the eastern regions where arable cultivation ismore widely practised (Eastern, East Midlands, Yorks &Humber) (Figure 1). The distribution for the potential sav-ings and sequestration from exchanging livestock manurefrom grassland to arable land is a little different (Figure 2):highest potential is found in Lancashire, West Midlandsand the central part of the South West region. The totalsequestration/saving for England estimated for applyingall remaining sewage sludge to land was estimated at48.9 kt C.

Table 3. Anticipated total net annual carbon equivalent sequestration and saving (kt C) for land use and management changes to arable land and managedgrassland in England.

Change in land useor management

Government Office Region

North East North West Yorks & Humber Eastern South East West Midland East Midland South West Total

Arable to permanent woodland 190 277 361 578 553 463 515 705 3644Arable to willow energy crop 16 17 36 69 60 42 55 54 349Arable to Miscanthus energy crop 15 16 34 66 56 40 52 50 329

Grassland to permanent woodland 284 405 553 791 680 552 671 843 4780Grassland to willow energy crop 52 39 141 277 186 128 211 147 1181Grassland to Miscanthus energy crop 50 37 138 275 182 126 208 143 1158

Conventional to zero tillage 8 2 20.85 17 13 5 7 12 63Conventional to reduced tillage 7 2 21 51 28 20 38 15 182Addition of straw residues 12 4 34 83 50 33 61 28 306Addition of livestock manure toarable land rather than grassland

9 8 22 32 28 25 29 27 181

Set-aside field marginson arable land

50 13 149 184 106 134 138 73 846

Extensification of converting breakcrops to grass in rotation

0 0 18 40 177 10 46 0 292

Extensification with outdoor pigbreeding on grass in rotation

0 0 0.3 0.6 3 0.1 0.7 0 4

Change to clover based pastures 24 13 75 158 98 67 118 72 626

Conversion of conventional to organicdairy management system

12 13 32 59 42 32 47 41 279

Conversion to stocklessorganic management

1 0.5 3 7 4 3 5 3 28

Conversion to organic managementwith livestock

20.1 0.03 20.7 21 20.6 20.3 21 20.2 24


Figure 3 shows the distribution of carbon saving andsequestration potential due to reduced tillage operations,which mirrors the distribution of arable tillage across thecountry. The threefold less carbon sequestration/savingpotential from zero tillage compared to reduced tillage(Table 3) is in contrast to the three-fold increase which isseen between these values when other GGs are not takeninto account (Table 4). This is due to the potential increasein emissions of N2O which have been attributed to zero-tilled land, but which are not expected to occur from mini-mal tillage, where the surface 15 cm is cultivated by discsor tines. The difference this makes to the distribution ofpotential sequestration and saving is also seen in Figure 4,where the values for zero tillage include potential N2Oemissions. It should be noted that in many areas (e.g. thefenland areas of north Cambridgeshire and south Lincoln-shire) negative sequestration/saving is predicted, due toincreased N2O emissions (Figure 4).

The impact that accounting for other GGs has on thewhole range of management changes to tilled land can beseen by comparing the values in Table 3, where N2O isincluded, with those in Table 4, where it has been omitted.In some cases, such as straw addition and organic farming,GGs lead to a decrease in the sequestration potential,though not as much as that for zero tillage, whereas the

exchange of livestock manure and inclusion of permanentset-aside margins actually increases the potential.

The annual figures for carbon sequestration/savingpotential for changes in arable management (Table 3) allappear to be minor in comparison with more permanentchanges in land use, especially the planting of perennialwoody crops which accumulate biomass carbon year onyear. The values for the maximum planting scenario of upto 11% of agricultural land, bringing the British woodlandcover up to 15%, are given in Table 3 for planting on culti-vated land and managed grassland. Also given is the annualsequestration and saving potential following the planting ofwillow energy crops on 11% of agricultural land. However,at present the targeted planting area for energy crops is125 000 ha, which amounts to carbon equivalents seques-tered/saved of 169 kt C for willow planted on arable land,or 182 kt C if Miscanthus were planted, but 521 kt and525 kt C, respectively, for willow and Miscanthus planted ondairy grassland.

All the changes to grassland entail a general reduction inthe amount of dairy farming in the country, but it is in thedairy industry where a change to organic methods couldmake significant savings with only a limited uptake (Table 3).It is instructive, however, that merely changing two-thirds

of the inorganic nitrogen fertilizer applied to managed

Figure 1. Spatial distribution of the potential annual carbon sequestrationto land and carbon equivalent savings (expressed as kg km22 C) due to theincorporation of a 6 m field margin set-aside strip around all arable fields.

Figure 2. Spatial distribution of the potential annual carbon sequestra-tion to land and carbon equivalent savings (expressed as kg km22 C) dueto the incorporation of livestock manure to tilled land rather thangrassland.


grassland to natural fixation by an active clover componentin the sward, has the potential to sequester, or rather save,more carbon than any arable management change except theuniversal incorporation of set-aside field margins (Table 3).

D I S C U S S I O N

Changes in arable management on tillage landIn the proposed changes to cultivation practice listed inTable 1 the predominant means of sequestration is a

reduction in SOC oxidation. The soil sequestration poten-tial of a move to zero tillage is at first sight impressive(Table 4), but almost entirely negated when the possibleincreased emissions of N2O are accounted for (Table 3). Anincrease in N2O efflux is to be expected as there is generallyan increase in bulk density of the topsoil with zero tillage(Schjønning & Rasmussen 2000) and thereby an increase inpartially anaerobic conditions. We have factored in emis-sions at the level suggested by MacKenzie et al. (1998) andadopted by Smith (2002) but there is a dearth of evidencefor this and it remains a point of controversy. If this is trulythe case, then a wholesale move to zero tillage may actuallybe detrimental to national efforts at using soils as carbonsinks in a greenhouse gas abatement strategy.

Reduced oxidation also lies behind the sequestrationpotential of increased use of set-aside strips, which if takenup nationally in arable areas would sequester/save aboutfour times as much as a move to reduced tillage, mainlybecause higher energy savings would be entailed (Table 2).Although less cost-effective than reduced tillage (due toloss of yield), such management change may be consideredworthwhile, as there would be wildlife, biodiversity andconservation benefits. A greater inclusion of grass leys inarable rotations also has the potential to sequester and savelarge amounts of carbon if widely practised, but less so ifonly sufficient land is changed to accommodate breedingherds of pigs. In both cases sequestration is predominantlyvia reduced SOC oxidation, but also energy savings andreduced N2O emissions as well.

A large contribution could be made, however, fromsimply ensuring that all cereal straw residues are returnedto the land either as straw or farmyard manure (Tables 3 &4). This could be cost-effective, though requiring an extrafinancial incentive to promote it. Farmyard manure couldbe used more efficiently simply by making greater use of itas a fertilizer in arable rotations, rather than just returningit to grassland (Table 2). Carbon sequestration and savingby doing this would be of the same order as a change toreduced tillage (Table 3), and could be carried out with aminimum of transport within regions.

Changes in land management to tillage land and grasslandThe land use with the highest potential for carbon seques-tration inevitably proves to be afforestation, if only to the

Table 4. Anticipated total net annual carbon equivalent sequestration (kt C) and saving without other greenhouse gases (N2O & CH4) being taken intoaccount, for arable management changes shown.

Change in management Government Office Region

North East North West Yorks & Humber Eastern South East West Midland East Midland South West Total England

Conventional to zero tillage 35 13 86 211 117 83 148 65 757Conventional to reduced tillage 8 4 27 61 30 24 43 15 212Addition of straw residues 67 28 218 253 129 193 181 91 1 159Addition of livestockmanure to arable

6 9 13 9 16 18 13 20 103

Set-aside field margins 7 3 25 61 31 22 43 12 203Conversion to stocklessorganic management

1 0.6 3 7 4 3 5 3 28

Conversion to organicmanagement with livestock

1 0.6 4 8 5 3 6 3 30

Figure 3. Spatial distribution of the potential annual carbon sequestrationto land and carbon equivalent savings (expressed as kg km22 C) due to achange in soil cultivation to reduced cultivation techniques.


extent of bringing the woodland cover in Britain up to thesame proportion as in the rest of Europe. This would entaila scale of change in land use much larger than we are usedto, needing about half as much woodland again as is cur-rently on the ground in England. For this reason alone it isunlikely to be taken up in the near future to the extent pro-jected here. Similarly, the current target for energy crops isabout half that projected here, and progress to planting125 000 ha has been slow under the current financialregime. Although sequestering/saving only about a third asmuch carbon as woodland and lacking the same level ofcontribution due to increased biomass, energy crops arenevertheless more cost-effective than woodland as a seques-tration mechanism by generating a regular income. Theyalso have the added bonus of substituting for fossil fuel, soreducing emissions from the power industry, and in thecase of willow and Miscanthus provide 25 and 36 times,respectively, as much energy as they consume inproduction.

For changes to managed grassland, the major differencefrom changes to tilled land are that no true soil sequestra-tion is envisaged. However, for all the comparable scenariosin Table 2, more carbon is considered saved from managedgrassland than for tilled land (Table 3), and there are large

potential savings on changing fertilizer strategy. This isbecause of the assumption made that this grassland wouldbe managed to support dairy enterprises, which have veryhigh energy requirements (Cormack 2000), and that savingswould be made at their expense. This scenario is not unli-kely because the current economics of dairy production areso marginal that reduced production is already occurring.

Scenarios for a national strategyThe amounts of carbon sequestration and saving detailedso far have been quoted as the net annual change. The lit-erature is quite clear that actual sequestration in soil cannotbe maintained at the initial rate indefinitely; a new equili-brium between oxidation of soil organic carbon (SOC) andincreased organic matter returns will eventually be attained.There is no consensus as to when this may be reached,with estimates ranging from only about 6 years (Paustianet al. 1997) to 15–20 years (West & Post 2002) after achange in cultivation regime. On the other hand, data usedin the derivation of the value of livestock manure to arableland continue to record annual increase in SOC after dec-ades of treatment (Smith et al. 1997). It was consideredprudent to adopt a mid-way position when estimating thepotential for carbon sequestration and saving over longerterms, in that for the duration of most rotations (3–6years) an annual increase can be assumed, but not over thewhole economic rotation for energy crops, or a minumumrotation for woodland (25 years). We have calculated thetotal amount of carbon sequestration and saving for periodsof 1, 5 and 25 years in Table 5, on the basis of theserotational periods and assumed that, where significant SOCsequestration occurs, it does so for 10 years, but is dis-counted thereafter.

The conclusion from Table 5 is that the potential forgenuine carbon sequestration to soil by agricultural man-agement changes is very limited under English conditions.In some cases sequestration and savings can be negatedover time by changes in the emission of other greenhousegases and energy use (see conversion to organic systems inarable and the extensification of pigs after 25 years in Table5). It is also clear that large savings in the national inven-tory of greenhouse gas emissions will only come fromwholesale land use change as single measures (e.g. to wood-land, energy crops and a return to temporary grass leys inarable rotations). However, some of the arable managementchanges are not mutually exclusive and could be runtogether. A scenario whereby tillage was universally byminimum methods, all straw returned and 6-m permanent‘set-aside’ margins employed, would give a combinedsequestration/saving potential of 31 Mt C (115 Mt CO2

emissions) over 25 years. This is comparable to increasingthe woodland component of the landscape by about 3%,and more than the contribution envisaged from soil seques-tration and saving due to energy crops (not including thatdue to their energy production). It would, however, involveabout a 7% drop in agricultural production and require aconsiderable financial incentive to overcome the loss sus-tained by farmers.

When considering the above mechanisms in thecontext of meeting Britain’s commitments under the Kyoto

Figure 4. Spatial distribution of the potential annual carbon sequestrationto land and carbon equivalent savings (expressed as kg km22 C) due to achange in soil cultivation to zero tillage techniques.


Protocol, two points are worth noting. The first is that theinitial commitment period is only for five years (2008–2012) for which emissions are compared to 1990 levels(Sleutel et al. 2003). This means that the five-year valuesfrom Table 5 can be used, which include a higher SOCcomponent than the long term 25-year scenario –128 kt SOC-C yr21 compared with 51 kt SOC-C yr21 forthe above arable scenario. The second point is that changesin the SOC component after a land management changeare actually a minor contribution to virtually all changes21.27 Mt C within a total of 31.44 Mt C over 25 years forthe arable scenario above. Much effort is currentlyexpended in trying to achieve adequate verification pro-cedures to monitor these minor changes in soil carbon(Sleutel et al. 2003), whereas ensuring a sound record ofoverall reductions in energy and agrochemical usage on theland would account for far more of the total reduction inemissions. In addition, a much better understanding of theimpact of the changes on N2O flux from soil will be neededto genuinely verify the change in greenhouse gas emissionsfrom soil and agricultural activities.

AC K N OW L E D G E M E N T S

The authors would like to acknowledge the financial assist-ance of the Department for Environment Food and RuralAffairs (Defra) of the UK Government, in commissioningthis study. We would also like to thank Ms H Lyons ofADAS, and Dr V. Jordan and Dr A. Leake of the SoilManagement Initiative (SMI) for assistance and comments.

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Table 5. The final carbon sequestration and savings (Mt CO2-C). possible for England according to the scenarios for changes in agricultural managementand over the timescales shown. Sequestration to SOC is given by the values in parentheses.

Change in management After 1 year After 5 years After 25 years

Arable to woodland 3.65 (0.07) 18.24 (0.34) 90.16 (0.69)Arable to willow energy crop 0.36 (0.07) 1.75 (0.35) 7.71 (0.69)Arable to Miscanthus energy crop 0.33 (0.06) 1.64 (0.31) 7.30 (0.61)Conventional to zero tillage 0.06 (0.02) 0.31 (0.09) 1.29 (0.19)Conventional to reduced tillage 0.18 (,0.01) 0.91 (0.02) 4.49 (0.04)Addition of straw residues 0.31 (0.06) 1.53 (0.31) 6.71 (0.62)Addition of livestock manure to arable 0.18 (0.01) 0.90 (0.06) 4.33 (0.13)Set-aside field margins 0.85 (0.06) 4.23 (0.31) 20.24 (0.61)Extensification scenario of break crops to grass 3.22 (0.05) 16.12 (0.24) 79.91 (0.48)Extensification scenario with outdoor pig breeding 0.05 (0.05) 0.24 (0.24) 0.45 (0.48)Conversion to stockless organic management 0.03 (0.05) 0.14 (0.24) 20.03 (0.48)Conversion to organic management with livestock ,0.01 (0.05) 20.02 (0.24) 20.83 (0.48)Grassland to woodland 4.78 (0) 23.88 (0) 119.42 (0)Grassland to willow energy crop 1.18 (0) 5.91 (0) 29.53 (0)Grassland to Miscanthus energy crop 1.16 (0) 1.64 (0.31) 7.30 (0.61)Change to clover based pastures 0.63 (0) 3.13 (0) 15.64 (0)Conversion to organic dairy management 0.28 (0) 1.40 (0) 6.97 (0)


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Received May 2004, accepted after revision July 2004.

q British Society of Soil Science 2004


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