Carbon sequestration in soils has potential to mitigate climate change but also drawbacks. While increasing soil organic carbon could be considered sequestration, it must result in a net transfer of carbon from the atmosphere to land. Options to sequester carbon include converting arable land to grassland or forest, but this may displace agriculture elsewhere. Maintaining or increasing soil carbon through reduced tillage, cover crops or organic amendments provides other benefits but may not genuinely sequester new carbon. Overall, too much focus on soil carbon risks neglecting larger climate threats, and priorities should be good land stewardship and integrated solutions.

1. 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

2. • 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

3. • 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

4. • 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

5. 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.

6. 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.

7. 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.

8. 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.

9. SoilorganicC(%)
0
1
burnt
incorporated
SoiltotalN(%)
0.00
0.05
0.10
BiomassC(kgha-1)
0
50
100
150
200
250
300
350
400
BiomassN(kgha-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

10. 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.

11. 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.

12. 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

13. 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)

14. 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

15. 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.

16. 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.

17. 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.

18. Arable Grassland
Arable to Grass
Grass to Arable
Permanent grass
Permanent arable

19. 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 wilderness
Data modelled by RothC-26.3 (Solid lines)
Woodland
Arable
Arable Forest

20. 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.

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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!

30. C sequestration summary:
Maximum CO2-C ‘savings’ from land management options
‘Year 1’
-1000
-500
0
500
1000
1500
2000
N2O change
SOC change
kg/ha/yrCO2-C
N2O + CH4 change

31. 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

32. 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

33. 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.

34. 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!

35. 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).