New research examines the role of grasses in controlling climate change
A recent initiative has set its sights on carbon sequestration in tropical savannas, an ecosystem characterized by a shared area of trees and grasses. The project initiated tree planting (afforestation) efforts to capture carbon dioxide from the air, storing carbon in two main places: the woody biomass of growing trees, and in the soil. While the effectiveness of carbon storage in trees has been well-established in research, how carbon is stored in soil was less well defined, and Zhu and his colleagues set out to determine what role grasses played in this effort.
The team, which included scientists from Yale University, Lawrence Berkeley National Laboratory, the University of Cape Town, Texas A&M, Kruger National Park, Harvard University, and the University of Oregon, conducted a comprehensive study looking at the contribution of grasses to the carbon content of savannah soils. They evaluated the potential impact of increased tree cover in tropical savannas on soil carbon storage. The study was published in the journal Nature Geoscience.
Using a case study conducted in Kruger National Park, South Africa, and data collected from tropical savannas around the world, the research team showed that savanna soils enriched with carbon from grasses showed relatively higher carbon concentrations. Their findings showed that grasses account for more than half of the carbon content in soils across tropical savannas, including the soil directly under trees. This underscores the important role that grasses play in carbon accumulation within tropical savannas.
Their findings showed carbon gains and losses, with increased tree cover across tropical savannas. The most significant variation was observed in savannas receiving higher rainfall, where tree planting is more likely to flourish, as well as in areas with clay soils and savanna sites that had significant contributions to carbon storage from grasses.
“This highlights the precise nature of increased tree cover on carbon dynamics in savanna soils,” Zhou said. “On average, the increase in soil carbon storage resulting from the expansion of tree cover across tropical savannas is negligible.”
This finding is consistent with the team’s previous research published in Nature, which showed that increased tree cover due to fire suppression increased carbon storage in woody biomass, but did not affect soil carbon storage.
“Our findings challenge the common assumption that afforestation uniformly enhances soil carbon storage,” Zhou said. “However, we have yet to determine the precise factors responsible for the large variation observed in the response of soil carbon storage to increasing tree cover across tropical savannas.”
In general, forests primarily store carbon in above-ground woody stems and leaves. In contrast, a large portion of carbon in grassland ecosystems, such as savannas and grasslands, is stored in the soil, primarily within the extensive root systems of grasses as well as decomposing organic matter. In the context of long-term carbon storage, carbon locked in soil has proven more reliable, he said, especially for a vulnerable future characterized by rising temperatures and an increased likelihood of drought and wildfires.
“It further demonstrates that savannas play critical roles in the global carbon cycle in their own unique ways, underscoring the importance of conserving and protecting these ecosystems in an equitable way,” he said.
(Tags for translation)Utah State University