Soils from cultivated areas in tropical regions constitute largely underused carbon sinks. Increasing carbon stocks in soil has become critical to mitigate the effects of climate change but also increase soil fertility. What are the options?
Carbon storage in soil is central to reducing greenhouse gas emissions and improving food safety, as defined in the 4 per 1,000 initiative. Two studies have just highlighted the untapped potential of certain soils: cultivated land in tropical areas (1) (2). “While carbon storage concerns all types of land worldwide, we focused on tropical soils as they have been poorly documented on a global scale in the past, explains soil scientist Tiphaine Chevallier. By analysing 258 studies conducted in 27 countries of sub-Saharan Africa and Latin America, we identified the maximum levels of carbon measured they could store according to their use. Forest and grassland soils have the largest stock, whereas cultivated soils are less carbon-rich. They receive less plant debris and organic matter, and can be degraded by farming practices”.
Storing and increasing fertility
Converting this cultivated land into forests or grasslands could therefore offer great carbon storage opportunities, as there is 30% less carbon in cultivated soils. However, agriculture should be maintained on these lands, not just to mitigate the effects of climate change but also to ensure their fertility. Carbon sinks are the result of a balance between the decomposition and mineralisation of organic matter and its stabilisation in conjunction with clay in the soil. Plant debris, essentially crop residues or organic amendments, feeds these reserves first. As it decomposes thanks to the biological activity in the soil, organic matter subsequently generates nutrients through mineralisation, which are essential to plant growth.
New farming practices
Which farming practices are conducive to these two aspects, i.e. conservation of or even increase in carbon stocks and fertility? Firstly, farmers can introduce grassy weeds through crop rotation, which increases the quantity of plant biomass returned to the soil. Cover crops are also conducive to carbon storage. In addition, they can introduce organic amendments or combine agriculture with breeding. Finally, by reducing tillage, they limit carbon stock loss due to erosion and mineralisation. “Organic matter resources are limited in tropical countries, points out soil specialist Kenji Fujisaki. It is essential to develop such practices in these regions, where food safety is not always guaranteed. Their success depends on local weather conditions, soil properties and the socio-economic resources of the farmers involved. It is therefore essential to study the implementation of these practices at regional level and in the long run to assess their effects”.
Local, long-term studies
Researchers wish to continue inventorying carbon stocks in soil based on farming practices in tropical countries. The objective is to determine storage potential at finer regional levels, and issue recommendations on carbon management on this scale. This overview will help guarantee soil resilience to climatic change.
1. Fujisaki, K., Chapuis-Lardy, L., Albrecht, A., Razafimbelo, T., Chotte, J.-L., Chevallier, T. Data synthesis of carbon distribution in particle size fractions of tropical soils: Implications for soil carbon storage potential in croplands. 2018, Geoderma, 313, 41-512.
2. Fujisaki K., Chevallier T., Chapuis-Lardy L., Albrecht A., Razafimbelo T., Masse D., Ndour Y., Chotte J-L. Soil carbon stocks in tropical croplands are mainly driven by carbon inputs: A synthesis, Agriculture, Ecosystems and Environment.