On soil carbon sequestration to mitigate climate change: potentials and drawbacks
On soil carbon sequestration tomitigate climate change:potentials and drawbacksKeith Goulding, David Powlsonand Andy WhitmoreDepartment for Sustainable Soils and Grassland Systems,Rothamsted ResearchSOIL AS A SINK
• Dictionary definition of sequestration: ‘to hold onto’.• Using this definition, any increase in Soil OrganicCarbon (SOC) could be called ‘sequestration’.• But in the context of Climate Change (CC),‘sequestration’ usually implies some CC mitigation.• Must be a net transfer of C from atmosphere toland ….. not just a movement between land Ccompartments.Carbon sequestration
• Finite – SOC movestowards new equilibriumvalue.• Reversible – depends oncontinuing the new landmanagement practiceAlso:• Should asses impacts onother GHGs - N2O and CH4- need full GHG budget• Note whether given as C orCO2 equiv. (i.e. all GHGs)Soil CTimeManagementchangeInitial EquilibriumTransitionFinalEquilibriumCarbon sequestration in soil is:But extra C good for soil quality
• Biosolids• Crop residues• Fertilizers• Plough to Min-Till (reduced tillage)• Arable to grass or forest• Grassland• Deeper rooting plants• BiocharLUC that could sequester C
Biosolids• Manure increases SOC: c 25% of the C inmanure is retained in SOC*• But most manure applied to land anyway, sno C sequestration, merely a movement of Cfrom one field to another• Genuine sequestration from organic ‘wastes’if previously sent to landfill* Johnston et al., 2009, Adv. Agron., 101: 1-57.
BiosolidsRisk of increased direct and indirect (from emittedand re-deposited NH3 and leached NO3-) N2Oemissions if applied N is not effectively utilisedBut evidence* suggests direct losses small:Average loss, as N2O, of N applied in slurry to:• Arable land 0.8%• Grassland 0.3%Smaller loss from grassland thought to be because oflarger uptake of N over a longer period by grass*Van der Meer, H.G. (2007) Optimising manure management forGHG outcomes. Aust. J. Exp. Ag. 48: 38-45.
Organic material Application rate Potential increase in SOC(t/ha dry solids-ds) (kg/ha/yr/t ds)Farm manures 10.5 60Digested biosolids 8.3 180Green compost 23 60Paper crumble 30 60Cereal straw 7.5 50Potential increases in SOC following the applicationof a range of organic materials at 250 kg/ha total NFrom Table 12 in Bhogal et al., 2008, Defra science report SP0561.
Crop residues• Increase SOC: 22% crop residue Cis retained by soil*• But as with manure, if the residuewould have been applied to landanyway, even on another farm,there is no C sequestration unlessthe residue would have beenburned* Bhogal et al., 2009, Europ. J. Soil Sci., 60, 276-286.
SoilorganicC(%)01burntincorporatedSoiltotalN(%)0.000.050.10BiomassC(kgha-1)050100150200250300350400BiomassN(kgha-1)01530456075%C %C %N BC BC BN BN%NAnd impacts on total C tend to be small: 18-year old strawincorporation experiment, DenmarkNo measurable effecton total C or N40% increase inmicrobial biomass C or N
Alternative uses of biosolids and cropresidues• Incinerate straw for generation of electricity andheat.• Anaerobic digestion of biosolids to produce biogas(methane); residue can add some nutrients to soil.Both deliver greater CC mitigation than adding thematerials to soil, through displacement of fossilfuel, but few benefits for soil quality.Powlson et al., 2008. ‘Carbon sequestration in European soils…’Waste Manag. 28: 741-6.
Fertilizers• Fertilizers (especially N) increase crop yieldsand returns of organic C in roots and residuesto soil (Ladha et al., 2011, JEQ 40, 1756-1766).• A genuine transfer of C from atmosphere toland and an increase in food production.
Fertilizers• SOC on Broadbalk increased by on average 0.4 t CO2 eqha-1 yr-1 for only 50 years, then at equilibrium.• But there are large GHG emissions (CO2 + N2O) frommanufacturing N fertilizer (4 kg CO2 eq per kg N asurea) and losses of N2O ( and nitrate and ammonia)after application.N applied at 144 kg N ha-1 yr-1GHG emissions ~ 0.6 t CO2 eq ha-1 yr -1& N continues to be applied after SOC stabilised
Plough Min tillMany claims of C sequestration cf. conventional cultivation, but:• Mainly redistribution of C nearer to soil surface Baker et al, Agriculture, Ecosystems & Environment 118, 1-5 (2007) Blanco-Canqui & Lal, SSSAJ 72, 693-701 (2008)• Some small net SOC accumulation under zero-till in long-term: Angers &Eriksen-Hamel, SSSAJ 72, 1370-1374 (2008)• Periodic cultivation – loss of accumulated SOC Powlson et al, Agriculture, Ecosystems & Environment 146, 23-33 (2012) Conant et al, Soil & Tillage Research 95, 1-10 (2007)• Increased N2O emissions in some situations Depends on soil wetness: Rochette Soil & Tillage Research 101, 97-100 (2008)• No-till sometimes causes yield decrease, so decreased C into soil Ogle et al Agriculture, Ecosystems & Environment1 49, 37-49 (2012)
Impact of 26 years reduced tillage on soil C (Brazil)0 5 15 20 25051015203040Soildepth(cm)1.0 2.0Carbon content (mg/g soil)0Whole soil Free light fraction(Machado et al (2003) Soil Use Manag. 19: 250-256)+ 50 % +100 %10- - - - - Dashed lines = Conventional tillage; Solid lines = no-tillage
Overall benefits of No- / Min-till• Possibly small SOC accumulation: Stern Report estimates 0.14 t C ha-1 yr-1sequestered under No-till. Recent estimate from UK experiments 0.31 (+/-0.18) t C ha-1 yr-1 sequestered under No-till;perhaps half this for Min-till. But in UK Min-tilled land often ploughed everyfew years.• Other benefits of No- / Min-till: Concentration of organic matter near surface:good for soil structure, seedling emergencewater infiltration and retention.Powlson & Jenkinson (1981). J. Agric. Sci. 97: 713-721.Baker et al (2007). Agric. Ecosys. Env. 118: 1-5.
Net GWP effects of change to Min-Till• Extra 3 kg N2O ha-1 yr-1 could offsetsequestration of 0.3 t C ha-1 yr-1 *. (Rothamstedexperiments found an extra net emission of 4 kgN2O ha-1 yr-1 from min-tilled land compared toploughed land)• No consistent pattern but reviews suggest N2Oemissions usually increase under Min-Till• NB. Most agricultural systems produce a netincrease in GWP*Johnson et al. (2007) Env. Poll. 150: 107-124.
Arable Grassland or Forest• Genuine C sequestration.• But must be certain that removal of land fromcrop production at one location on the planetdoes not cause land clearance (deforestation,ploughing grassland, wetland drainage)elsewhere.• Expect increase in CH4 oxidation and reductionin N2O emission provided N deposition low.
Arable GrasslandArable to GrassGrass to ArablePermanent grassPermanent arable
0204060801001860 1880 1900 1920 1940 1960 1980 2000 2020Organic C insoil(t C ha-1)YearBroadbalk wildernessData modelled by RothC-26.3 (Solid lines)WoodlandArableArable Forest
Grassland systemsNCS = Net Carbon Storage(kg C/ha/yr)Grazed = 1290Grazed and cut = 500Cut = 710Including GHG fluxes, the net balanceof on- and off-site C sequestration was380 kg CO2eq/ha/yr.9 European sitesSoussana et al., 2007, ‘Mitigating the GHG balance of ruminant productionsystems…’, Integrated Crop Management, 11, 119-151.
Data from the National Soil Inventory of Englandand Wales obtained between 1978 and 2003(Bellamy et al., 2005) showed that rotationalgrasslands gained C at a rate of around 100 kgC/ha/yr.In Belgium, C fluxes on grasslands were from +440kg C/ha/year to -900 kg C/ha/yr.England and Wales
In their assessment of the European C balance,Janssens et al. (2003) concluded that grasslandswere a highly uncertain component of theEuropean-wide C balance in comparison withforests and croplands.They estimated a net grassland C sink of 600 ±900 kg C/ha/year.European C balance
Follett and Schuman (2005) reviewed grazing landcontributions to C sequestration worldwide using19 regions. A positive relationship was found, onaverage, between the C sequestration rate and theanimal stocking density, which is an indicator of thepasture primary productivity. Based on thisrelationship they estimate a 200 Mt SOCsequestration/year on 3.5 billion ha of permanentpasture worldwide~ 60 kg C/ha/yrWorldwide
Grassland summary (kg CO2eq/ha/yr)9 EU sites, grazed, grazed and & cut(inc GHGs) 380England and Wales 400Belgium 1760 to -3600Europe 2400 ± 3600Worldwide 240
Deep(er) rooting crops• Roots are a means of delivering carbon and naturalplant-produced chemicals into soil with potentiallybeneficial impacts: carbon sequestration (atdepth) biocontrol of soil-bornepests and diseases inhibition of the nitrificationprocess in soil (conversion ofammonium to nitrate) with possiblebenefits for improved nitrogen useefficiency and decreased N2O emissions.Kell, D. (2011) Annals of Botany 108, 407-418.http://aob.oxfordjournals.org/content/108/3/407.full?sid=24aa69b0-b2ec-4c26-b6b4-0b7bdfee2401
Subsoil sequestration by MiscanthusCarbon turnover under Miscanthus (14 yr) (Richter et al., unpublished)• 2 non-tuft (M. giganteus,M sacchariflorus) and 3tuft-growing (M sinensis)genotypes• SOC and roots analysedfor C3 and C4contributions based onδ13C• Considerable C4-basedenrichment in 0-30 cmsoil• Some evidence of subsoilsequestration in twogenotypeswww.carbo-biocrop.ac.ukSOC inarablereference soil0–30 cm30–100 cm
Biochar: the solution?Sources and attributes• Organic material burned slowly underlimited oxygen Bi-product of bioenergy (pyrolysis of biofuelcrops, straw, or wastes) In natural ecosystems from fire• Highly stable, porous, active surfaces
Biochar: proposed effects on soil• Near-permanent increase in soil C• Greater stabilisation of other soil C• Suppression of greenhouse gas emission• Enhanced fertiliser-use efficiency• Improvement in soil physical properties• Enhanced crop performance• Increased soil biodiversity
Biochar: gaps in process knowledge• Presence of contaminants• Decomposition• Nutrient and water retaining properties (CEC,surface area)• Microbial habitat or microbial substrate• Trace element content and mobility• Impact on greenhouse gasesAlmost everything!
C sequestration summary:Maximum CO2-C ‘savings’ from land management options‘Year 1’-1000-5000500100015002000N2O changeSOC changekg/ha/yrCO2-CN2O + CH4 change
Maintaining SOC in cropping systems1. Ley-arable farming – i.e. intermittent pasture2. Add crop residues3. Add manures or other organic “wastes”4. Min-Till / No-Till mainly redistribution in early years, butuseful to concentrate SOC near surface C sequestration long-term?5. Grow plants with larger/longer roots6. Fertilisers
1. Avoid tillage and the conversion of grasslandsto arable2. Moderately intensify nutrient-poor permanentgrasslands3. Light grazing instead of heavy grazing (whatabout ‘mob grazing?)4. Increasing the duration of grass leys5. Converting grass leys to grass-legumemixtures or to permanent grasslandsMaintaining SOC in grassland
Conclusions for C sequestration• Not all increases in SOC genuinely sequester C.• Incorporation of organic ‘wastes’ or crop residues doesnot usually sequester C: but benefits for soil quality and functioning; greater CC mitigation from using biosolids and residuesfor bioenergy production.• Large GHG emissions from N fertiliser manufactureoutweigh any climate change benefit from increased SOCfrom increased crop residue returns.• Long-term min-till probably sequesters C and deliversother benefits for soil.• Conversion of arable land to forest or grass is genuinesequestration, but limited opportunities for this.
General conclusionsToo much emphasis on soil C sequestration risks lessattention to major climate change threats:• Land clearance for food orbiofuels• Other deforestation• Wetland drainagePriorities:• good land stewardshipincluding increased efficiencyof N use, reduced tillage,maintaining ‘green’ cover• integrated solutionsDeforestation in Brazil down 23% - only2040 km2 in last 12 months!
AcknowledgementsSome of this research was funded bythe UK Biotechnology and BiologicalSciences Research Council (BBSRC)and some by the UK Department forEnvironment, Food and Rural Affairs(Defra).