Carbon sequestration projects for afforestation are found to be less effective than for grasses in tropical savannas

Carbon sequestration projects for afforestation are found to be less effective than for grasses in tropical savannas

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Tree cover in tropical and subtropical savannas, with soil sample sites identified for carbon storage analysis, including Kruger National Park, South Africa. Credit: Zhou et al. 2023.

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Tree cover in tropical and subtropical savannas, with soil sample sites identified for carbon storage analysis, including Kruger National Park, South Africa. Credit: Zhou et al. 2023.

Global warming’s increasing toll on the planet has been the focus of mitigation strategies in recent years, with carbon sequestration projects playing a more prominent role in pulling carbon dioxide from the atmosphere to store it in solid or liquid form and thus reducing carbon abundance. This is a greenhouse gas. By doing so, this reduces the trapping of solar radiation on Earth, thus helping the planet’s temperature to cool down.

One carbon sequestration project targeted tropical savannas and used tree planting (known as afforestation) to remove carbon dioxide from the atmosphere to store it in the woody biomass of growing trees and soil as organic carbon. Previous aboveground carbon storage is known to be effective, but belowground carbon storage in soils is not well understood.

Measuring the success of this project remains a challenge because grasses also produce soil organic carbon, and new research has been published in Natural Earth Sciences It aims to separate the two so that the efficiency and expansion of carbon sequestration projects for afforestation can be identified.

Dr Yong Zhu, an assistant professor at Utah State University in the US, and his colleagues investigated such a project in the Kruger National Park in South Africa, alongside a range of global tropical savannas.

The researchers discovered that grasses contribute more than half of the organic carbon in soil up to a depth of one meter, and this was the case for the soil directly under the trees. Soil organic carbon also varied both positively and negatively with increasing tree cover, with a maximum increase of 6%, thus highlighting that afforestation may not be as effective in carbon sequestration as initially hoped, especially when compared to grasses.

However, trees may have a somewhat symbiotic effect in increasing grass productivity in arid and semi-arid savannas (which experience less than 700 mm of rainfall per year), affecting the overall carbon storage of the system. Conversely, in medium savannas (>700 mm of rainfall per year) the effect of tree cover on grass productivity is negative, leading to lower total soil organic carbon.


Analyzes of tree- and grass-derived soil organic carbon in Kruger National Park, South Africa, compared to other global tropical and subtropical savannas. Credit: Zhou et al. 2023.

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Analyzes of tree- and grass-derived soil organic carbon in Kruger National Park, South Africa, compared to other global tropical and subtropical savannas. Credit: Zhou et al. 2023.

Another factor that affects carbon storage in the ground is soil type; Clay-rich soils have a higher capacity to store carbon than sandy substrates. However, the latter allows greater drainage and enables the growth of woody plants, which helps store carbon in aboveground biomass.

The researchers conducted analyzes on the type of stored carbon (based on carbon isotopes, which is the same element with different atomic masses) that is produced by different photosynthesis pathways: C4 For herbs and C3 For trees. In doing so, they were able to identify the main source of carbon stored within the test soil. This was then combined with 148 soil images from global tropical and subtropical savannas to create an overview of the contribution of grasses versus trees across different rainfall and tree cover gradients.

Across the Kruger National Park, 98 soil samples were taken and analyzes revealed that 76% of soil organic carbon was derived from grasses, while in all tropical regions this was 57%, and still 51% was of grass origin directly under the trees. For Kruger, researchers identified a clear positive relationship between soil organic carbon and grass biomass, but a negative response to increasing sand content.

In comparison, grasses are better adapted to getting water and nutrients from the clay-rich soil above trees (which accounts for only 24% of the garden’s carbon). Looking at the global scale, South American savannas have the lowest soil organic carbon compared to those in Africa and Australia, which researchers attribute to nutrient deficiencies that inhibit significant grass growth. In addition, higher rainfall in South America may encourage denser tree canopy growth, with the resulting ground-level shade significantly inhibiting grass growth, thus storing carbon. Instead, carbon storage in tree biomass is dominant here.

However, the research team calculated that afforestation in grassland ecosystems increased soil organic carbon by 5.74 megagrams of carbon per hectare. This was more significant in the upper 30 cm of the 1 m soil sections analysed.

Because much of this organic carbon is concentrated in the surface layer, there is a risk that it could be released back into the atmosphere during land-scorching wildfires, which are becoming an increasingly frequent event each summer.

Therefore, the proposed approach is to rely on grasses as the main driver of carbon sequestration in savannas and reconsider the incremental gains of afforestation projects. While these have some positive effects on increasing grass productivity to store more carbon deeper away from potential bush/forest fires as tree roots extend deeper, they may not be enough to outweigh the costs of managing water resources and biodiversity in their growth. .

More work is needed to determine the effect of different grass species on soil carbon sequestration, clay and mineral content, the age of tree biomass, as well as the functioning of herbivores grazing in the area.

more information:
Yong Zhou et al., soil carbon in tropical savannas is mostly derived from grasses, Natural Earth Sciences (2023). doi: 10.1038/s41561-023-01232-0.

Magazine information:
Natural Earth Sciences

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