This document summarizes research on the effects of conservation agriculture practices on soil carbon levels. It finds that while conservation agriculture can increase soil carbon concentration in surface layers, the increases in total soil carbon stocks are generally small and unlikely to significantly contribute to climate change mitigation. However, retaining crop residues in areas where residue burning occurs, such as the Indo-Gangetic Plains, offers a meaningful way to sequester soil carbon. For large-scale implementation of initiatives to sequester carbon in soils, major increases in nitrogen fertilizer production or nitrogen fixation would be required to support the additional biomass growth needed.

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

2. 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?

3. • 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’.

4. 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.

5. • 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).

6. 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

7. 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?

8. 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.

9. Thank you
for your
interest!