Ashes from deforestation and wildfires amplify the carbon dioxide uptake of a Central African forest

A new study by an international research team from the Le Laboratoire des Sciences du Climat et de l'Environnement (LSCE) demonstrates that ashes produced by fires and transported by wind to lowland tropical forest in Central Africa increase their capacity to remove and sequester carbon dioxide.

Fires are common in subtropical and tropical Africa, and help to enhance and promote soil fertility where they occur by producing a nutrient-rich layer of ash. Ecosystems have evolved which are adapted to and promote fires as a mechanism to sustain highly productive despite growing on old infertile soils. This also has been recognized by humans and forms the basis of the widely-used farming method of 'slash & burn' that involves the cutting and burning of plants in a forest or woodland to create fertile agricultural fields. Not all the ash produced by fires remains at place, but some is transported by winds over long distances and in other biomes. "We have recently noted that the massive extent of biomass burning in tropical Africa produces a huge amount of nutrients, which are in turn deposited on central African forests." says co-author Marijn Bauters from Ghent University. However, it has not been shown that the amount of nutrients from transported ash is substantial enough to actually enhance plant growth where it is deposited.

A first indication of the importance of nutrients concentrated in ash, is that phosphorus has been identified as a major constraint on the capacity of tropical forest to fix carbon dioxide from the atmosphere via photosynthesis. Based on nearly 18,000 measurements of photosynthesis, a team of researchers which include Daniel S. Goll and Philippe Ciais from the LSCE showed that higher the concentration of phosphorus in leaves, the higher their capacity to adsorb carbon dioxide. They further demonstrate how this factor can be accounted for in predictions of global photosynthesis, and show that tropical forests could adsorb much more carbon from the atmosphere if these ecosystems were richer in phosphorus than they actually are.

In the new study, the ecosystem modellers from the LSCE joined forces with scientists working in the field from Ghent University to build and test an observation informed computer model for the African forest biome which resolves interlinkages between phosphorus and carbon cycle processes, including the one mentioned above. They used the model to simulate the extent to which the carbon fixation and sequestration in biomass of a Central African forest is affected by nutrients brought by windblown fire ashes. To do so, they compared simulations in which ash deposition either increased as observed from the 1960s to today or was artificially held at the low level of the 1960s throughout the simulations. This setup allows the researcher to single out the influence of past changes in ash deposition from multiple other environmental changes affecting the carbon balance (like climate change, increasing atmospheric carbon dioxide concentration, etc).

The computer simulations show that past changes in ash deposition due to an increase in deforestation fires from the 1990s to the 2000s could have enhanced the carbon dioxide sequestration at an extent which is comparable to the one of increasing atmospheric carbon dioxide concentration. The latter is believed to drive the removal and sequestration of about ⅓ of carbon dioxide emission by human activities. Thus, the finding of another equally powerful driver of carbon dioxide removal and sequestration could reshape our understanding of the role of the African biome for the global carbon cycle. "Continuous greenhouse gas measurements and forest and soil inventories from this poorly studied yet an important component in the global greenhouse gas balance are critical to elucidate the impact of ash deposition on the global carbon cycle." says co-author Hans Verbeeck from University Ghent which operates the very first eddy covariance flux tower in the tropical forest of the Congo Basin.

"These new insights not only affect our understanding about the response of tropical forests to global change, but also open up possibilities to enhance the productivity of tropical forests." says lead author Daniel Goll from the LSCE. "For example, ground rock material containing a comparable nutrient mix than ash could be given to forests to promote their carbon dioxide fixation, like we had postulated in a previous study".

Goll, D.S, Bauters, M., Zhang, H. et al. Atmospheric phosphorus deposition amplifies carbon sinks in simulations of a tropical forest in Central Africa New Phytologist <DOI>

Bauters, M., Drake, T.W., Wagner, S. et al. Fire-derived phosphorus fertilization of African tropical forests. Nature Communications 12, 5129 (2021). https://doi.org/10.1038/s41467-021-25428-3

Ellsworth, D.S., Crous, K.Y., De Kauwe, M.G. et al. Convergence in phosphorus constraints to photosynthesis in forests around the world. Nature Communications 13, 5005 (2022). https://doi.org/10.1038/s41467-022-32545-0

Goll, D.S., Ciais, P., Amann, T. et al. Potential CO2 removal from enhanced weathering by ecosystem responses to powdered rock. Nature Geoscience 14, 545-549 (2021). https://doi.org/10.1038/s41561-021-00798-x