CIMMYT- Conservation agriculture-
based practices and soil C: Between
myth and farmer reality
Clare Stirling

CA comprises three basic components:
§ Reduced tillage.
§ Retention of crop residues on soil surface.
§ Crop diversification (rotation, intercropping).
What do we mean by Conservation
Agriculture?

• 125 on-farm trials established between 2004 and 2009 across 23 sites in Malawi,
Mozambique, Zambia and Zimbabwe. Paired plot comparison of conventional
agricultural practice and conservation agriculture.
• CA had no consistent effect on soil C stocks.
• ‘Limited potential for CA to significantly increase soil C stocks after up to 7 years of
CA practices’.

Meta-analysis of SOC changes under CA practices in two tropical regions, the Indo-
Gangetic Plains (IGP) and Sub-Saharan Africa (SSA), show modest increases in SOC
stock:
• IGP annual increases in SOC stock compared to conventional practice were between
0.16 - 0.49 Mg C ha-1 yr-1.
• SSA increases between 0.28 - 0.96 Mg C ha-1 yr-1 , but with much greater variation
and a significant number of cases with no measurable increase.

• Potential to sequester SOC due to higher carbon input rather
than from the effect of reduced soil tillage.
• ZT + residues increased SOC by 0.67 t ha-1 yr-1 (i.e. total 4.7 t
ha-1 over 7 years in 0-0.6m soil depth).

Burning crop stubble burning in India:
• India accounts for 1/3 of total crop
residues burned globally each year.
• ~ 90 million tonnes of surplus cereal
residues in NW India alone (~ 85% from
rice and wheat.
• Retention of crop residues in
these systems offers a
genuine opportunity for C
sequestration.
Satellite photo: NASA

Will an increase in SOC from crop residues contribute
to long-term build of soil C stocks and CC mitigation?
• How much of the additional C enters more
recalcitrant SOC pools?
• What are the impacts on other GHGs e.g. N2O
emissions from crop residues?
• How does it compare with alternative use
options e.g. biomass energy, animal fodder,
paper and cardboard industry, anaerobic
digestion etc?

Summary
• Evidence that CA-based practices can increase the SOC concentration
in the surface layers of soil but increases in SOC stock are generally
small and probably insignificant in terms of CC mitigation.
• However, where residue burning is an issue (e.g. IGP) retention of crop
residues offer a genuine, and potentially substantial, means of
sequestering soil C.
Based on C:N ratios implementing the 4p1000 initiative on all agricultural soils
would require 100 Tg N yr-1 = an increase of ~75% of current global N-fertilizer
production or extra symbiotic N2 fixation rates twice the current amount.

Thank you
for your
interest!