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29 August 2023
By
Hannah Bird
Afforestation carbon sequestration
projects found to be less effective than grasses in tropical savannas
Tree cover in tropical and subtropical
savannas, with soil sample locations identified for analyses of carbon
storage, including Kruger National Park, South Africa. Credit: Zhou et
al. 2023.
Global warming's ever-increasing toll on the
planet has been a focus of mitigation strategies in recent years, with
carbon sequestration projects playing a more prominent role in drawing
carbon dioxide out of the atmosphere to store in solid or liquid form
and therefore decrease the abundance of this greenhouse gas. By doing
so, this reduces the trapping of solar radiation on Earth and thus
aids a decline in the planet's temperature.
One such carbon sequestration project has targeted tropical savannas
and used the planting of trees (known as afforestation) to remove
carbon dioxide from the atmosphere for storage in woody biomass of the
growing tree and the soil as organic carbon. The former above-ground
carbon storage is known to be effective, but that of underground
carbon storage in soils is less well understood.
Measuring the success of this project remains challenging as grasses
also produce soil organic carbon, and new research published in Nature
Geoscience aims to uncouple the two so that the efficiency and
expansion of afforestation carbon sequestration projects can be
determined.
Dr. Yong Zhou, Assistant Professor at Utah State University, U.S., and
colleagues investigated such a project in Kruger National Park, South
Africa, alongside a host of global tropical savannas.
The researchers discovered that grasses contribute over half of the
soil organic carbon down to a depth of 1m, and this was still the case
for soils directly below trees. Soil organic carbon also varied
positively and negatively with increasing tree cover, having a maximum
6% increase, therefore highlighting that afforestation may not be as
effective at carbon sequestration as initially hoped, especially when
compared to grasses.
Despite this, trees may have a somewhat symbiotic effect in increasing
grass productivity in arid and semi-arid savannas (experiencing less
than 700mm of rainfall annually), which affects the overall carbon
storage of the system. Conversely, in mesic savannas (over 700mm
rainfall annually) the effect of tree cover on grass productivity is
negative, thus leading to a decline in overall soil organic carbon.
Analyses of tree and grass-derived soil organic carbon
in the Kruger National Park, South Africa, compared to other global
tropical and subtropical savannas.
Credit: Zhou et al. 2023.
Another factor affecting carbon storage in the
ground is the type of soil; clay-rich soils have a higher capacity for
carbon storage than sandier substrates. Yet, the latter allows for
greater drainage and enables woodier vegetation to grow, which aids
carbon storage in biomass above ground.
The researchers conducted analyses on the type of carbon stored (based
upon isotopes of carbon, the same element with different atomic
masses) which are produced by different photosynthesis pathways: C4
for grasses and C3 for trees. By doing so, they could determine which
was the primary source for the carbon stored within the test soils.
This was then combined with 148 soil profiles from global tropical and
subtropical savannas to generate an overview of the contribution of
grasses versus tress across varying rainfall and tree cover gradients.
Across Kruger National Park, 98 soil samples were taken and analyses
revealed that 76% of the soil organic carbon was derived from grasses,
while over all tropical regions this was 57%, and still at 51%
grass-origin directly underneath trees. For Kruger, the researchers
identified a distinct positive correlation between soil organic carbon
and grass biomass, but a negative response to increasing sand content.
In comparison, grasses are better adapted to obtaining water and
nutrients from clay-rich soils over trees (which accounts for just 24%
of carbon in the park). Looking more globally, South American savannas
had the lowest soil organic carbon compared to those in Africa and
Australia, which the researchers attribute to nutrient deficiency
inhibiting substantial grass growth. Additionally, the higher rainfall
in South America may encourage denser tree canopy growth, with the
subsequent shade at ground level impeding significant grass
development, and therefore carbon storage. Instead, carbon storage in
tree biomass dominates here.
Nevertheless, the research team calculated that afforestation in
grassy ecosystems increased soil organic carbon by 5.74 megagrams of
carbon per hectare. This was most significant in the upper 30cm of the
1m soil profiles analyzed.
Given that much of this organic carbon is concentrated in the surface
layer, there is risk that it may be released back into the atmosphere
during forest fires that scorch the ground, and are becoming an
increasingly frequent occurrence each summer.
Therefore, the suggested approach is to rely upon grasses as the major
driver of carbon sequestration in savannas and reconsider the
incremental gains of afforestation projects. While these do have some
positive effects on increasing grass productivity for storage of more
carbon at greater depth away from potential bush/forest fires as tree
roots extend deeper, it may not be sufficient to outweigh the costs to
water resources management and biodiversity in their growth.
Further work needs to be conducted on determining the effect of
different grass types on soil carbon sequestration, clay content and
minerals, the age of tree biomass, as well as the action of herbivores
grazing in the area.
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