On soil carbon sequestration to mitigate climate change: potentials and drawbacks

Keith Goulding, David Powlson

and Andy Whitmore

Department for Sustainable Soils and Grassland Systems, Rothamsted Research

SOIL AS A SINK

• Dictionary definition of sequestration: ‘to hold on to’.

• Using this definition, any increase in Soil Organic Carbon (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 to land ….. not just a movement between land C compartments.

Carbon sequestration

• Finite – SOC moves towards new equilibrium value.

• Reversible – depends on continuing the new land management practice

Also: • Should asses impacts on

other GHGs - N2O and CH4 - need full GHG budget

• Note whether given as C or CO2 equiv. (i.e. all GHGs)

Soil C

Time

Management change

Initial Equilibrium

Transition

Final Equilibrium

Carbon 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 • Biochar

LUC that could sequester C

Biosolids

• Manure increases SOC: c 25% of the C in manure is retained in SOC*

• But most manure applied to land anyway, s no C sequestration, merely a movement of C from one field to another

• Genuine sequestration from organic ‘wastes’ if previously sent to landfill

* Johnston et al., 2009, Adv. Agron., 101: 1-57.

Biosolids

Risk of increased direct and indirect (from emitted and re-deposited NH3 and leached NO3

-) N2O emissions if applied N is not effectively utilised

But 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 of larger uptake of N over a longer period by grass

*Van der Meer, H.G. (2007) Optimising manure management for GHG 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 60

Digested biosolids 8.3 180

Green compost 23 60

Paper crumble 30 60

Cereal straw 7.5 50

Potential increases in SOC following the application of a range of organic materials at 250 kg/ha total N

From Table 12 in Bhogal et al., 2008, Defra science report SP0561.

Crop residues

• Increase SOC: 22% crop residue C is retained by soil*

• But as with manure, if the residue would have been applied to land anyway, even on another farm, there is no C sequestration unless the residue would have been burned

* Bhogal et al., 2009, Europ. J. Soil Sci., 60, 276-286.

Soil

orga

nic

C (%

)

0

1

burntincorporated

Soil

tota

l N (%

)

0.00

0.05

0.10

Bio

mas

s C

(kg

ha-1

)

0

50

100

150

200

250

300

350

400

Bio

mas

s N

(kg

ha-1

)

0

15

30

45

60

75

%C %C %N BC BC BN BN%N

And impacts on total C tend to be small: 18-year old straw incorporation experiment, Denmark

No measurable effect on total C or N

40% increase in microbial biomass C or N

Alternative uses of biosolids and crop residues

• Incinerate straw for generation of electricity and heat.

• Anaerobic digestion of biosolids to produce biogas (methane); residue can add some nutrients to soil.

Both deliver greater CC mitigation than adding the materials to soil, through displacement of fossil fuel, 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 yields and returns of organic C in roots and residues to soil (Ladha et al., 2011, JEQ 40, 1756-1766).

• A genuine transfer of C from atmosphere to land and an increase in food production.

Fertilizers

• SOC on Broadbalk increased by on average 0.4 t CO2 eq ha-1 yr-1 for only 50 years, then at equilibrium.

• But there are large GHG emissions (CO2 + N2O) from manufacturing N fertilizer (4 kg CO2 eq per kg N as urea) and losses of N2O ( and nitrate and ammonia) after application.

N applied at 144 kg N ha-1 yr-1

GHG emissions ~ 0.6 t CO2 eq ha-1 yr -1

& N continues to be applied after SOC stabilised

Plough Min till

Many 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 25 0

5

10

15

20

30

40

Soil depth (cm)

1.0 2.0

Carbon content (mg/g soil)

0 Whole 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-1

sequestered 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 every

few years. • Other benefits of No- / Min-till:

Concentration of organic matter near surface: good for soil structure, seedling emergence water 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 offset sequestration of 0.3 t C ha-1 yr-1 *. (Rothamsted experiments found an extra net emission of 4 kg N2O ha-1 yr-1 from min-tilled land compared to ploughed land)

• No consistent pattern but reviews suggest N2O emissions usually increase under Min-Till

• NB. Most agricultural systems produce a net increase 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 from

crop production at one location on the planet does not cause land clearance (deforestation, ploughing grassland, wetland drainage) elsewhere.

• Expect increase in CH4 oxidation and reduction in N2O emission provided N deposition low.

Arable Grassland

Arable to Grass

Grass to Arable

Permanent grass

Permanent arable

0

20

40

60

80

100

1860 1880 1900 1920 1940 1960 1980 2000 2020

Organic C in soil

(t C ha-1)

Year

Broadbalk wildernessData modelled by RothC-26.3 (Solid lines)

Woodland

Arable

Arable Forest

Grassland systems

NCS = Net Carbon Storage (kg C/ha/yr)

Grazed = 1290 Grazed and cut = 500 Cut = 710

Including GHG fluxes, the net balance of on- and off-site C sequestration was 380 kg CO2eq/ha/yr.

9 European sites

Soussana et al., 2007, ‘Mitigating the GHG balance of ruminant production systems…’, Integrated Crop Management, 11, 119-151.

Data from the National Soil Inventory of England and Wales obtained between 1978 and 2003 (Bellamy et al., 2005) showed that rotational grasslands gained C at a rate of around 100 kg C/ha/yr. In Belgium, C fluxes on grasslands were from +440 kg 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 grasslands were a highly uncertain component of the European-wide C balance in comparison with forests 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 land contributions to C sequestration worldwide using 19 regions. A positive relationship was found, on average, between the C sequestration rate and the animal stocking density, which is an indicator of the pasture primary productivity. Based on this relationship they estimate a 200 Mt SOC sequestration/year on 3.5 billion ha of permanent pasture worldwide ~ 60 kg C/ha/yr

Worldwide

Grassland summary (kg CO2eq/ha/yr)

9 EU sites, grazed, grazed and & cut (inc GHGs) 380 England and Wales 400 Belgium 1760 to -3600 Europe 2400 ± 3600 Worldwide 240

Deep(er) rooting crops

• Roots are a means of delivering carbon and natural plant-produced chemicals into soil with potentially beneficial impacts: carbon sequestration (at depth) biocontrol of soil-borne pests and diseases inhibition of the nitrification process in soil (conversion of ammonium to nitrate) with possible benefits for improved nitrogen use efficiency 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 Miscanthus Carbon turnover under Miscanthus (14 yr) (Richter et al., unpublished)

• 2 non-tuft (M. giganteus, M sacchariflorus) and 3 tuft-growing (M sinensis) genotypes

• SOC and roots analysed for C3 and C4 contributions based on δ13C

• Considerable C4-based enrichment in 0-30 cm soil

• Some evidence of subsoil sequestration in two genotypes

www.carbo-biocrop.ac.uk

SOC in arable

reference soil

0–30 cm

30–100 cm

Biochar: the solution?

Sources and attributes • Organic material burned slowly under

limited oxygen Bi-product of bioenergy (pyrolysis of biofuel

crops, 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 gases

Almost everything!

C sequestration summary: Maximum CO2-C ‘savings’ from land management options

‘Year 1’

-1000

-500

0

500

1000

1500

2000N2O change

SOC change

kg/h

a/yr

CO

2-C

N2O + CH4 change

Maintaining SOC in cropping systems

1. Ley-arable farming – i.e. intermittent pasture 2. Add crop residues 3. Add manures or other organic “wastes” 4. Min-Till / No-Till mainly redistribution in early years, but

useful to concentrate SOC near surface C sequestration long-term?

5. Grow plants with larger/longer roots 6. Fertilisers

1. Avoid tillage and the conversion of grasslands

to arable 2. Moderately intensify nutrient-poor permanent

grasslands 3. Light grazing instead of heavy grazing (what

about ‘mob grazing?) 4. Increasing the duration of grass leys 5. Converting grass leys to grass-legume

mixtures or to permanent grasslands

Maintaining SOC in grassland

Conclusions for C sequestration

• Not all increases in SOC genuinely sequester C. • Incorporation of organic ‘wastes’ or crop residues does

not usually sequester C: but benefits for soil quality and functioning; greater CC mitigation from using biosolids and residues

for bioenergy production. • Large GHG emissions from N fertiliser manufacture

outweigh any climate change benefit from increased SOC from increased crop residue returns.

• Long-term min-till probably sequesters C and delivers other benefits for soil.

• Conversion of arable land to forest or grass is genuine sequestration, but limited opportunities for this.

General conclusions

Too much emphasis on soil C sequestration risks less attention to major climate change threats: • Land clearance for food or biofuels • Other deforestation • Wetland drainage Priorities: • good land stewardship including increased efficiency of N use, reduced tillage, maintaining ‘green’ cover • integrated solutions

Deforestation in Brazil down 23% - only 2040 km2 in last 12 months!

Acknowledgements

Some of this research was funded by the UK Biotechnology and Biological Sciences Research Council (BBSRC) and some by the UK Department for Environment, Food and Rural Affairs (Defra).