Brazil's underprotected Cerrado savanna stores a staggering amount of carbon, study finds

The Cerrado savanna occupies about 26% of Brazil and is home to more than 12,000 plant species and diverse animal life. It's also speckled with groundwater-fed wetlands that serve as the headwaters for two-thirds of Brazil's major waterways, including the Amazon River, making it not only a biodiversity hot spot but also a critical ecosystem to preserve water security in the region.

This savanna's wetlands also have another superpower: storing carbon in their waterlogged soils. According to a new paper published today in New Phytologist, the Cerrado's wetlands store carbon at a density about six times greater than the Amazon rainforest's vegetation.

The Cerrado's wetlands "are probably one of the most important ecosystems in the Americas to accumulate carbon," said Larissa Verona, lead author of the new study and an ecologist at the Universidade Estadual de Campinas in Brazil and the Cary Institute of Ecosystem Studies. "But this carbon is vulnerable."

The team's findings underscore the need to protect these critically important ecosystems, especially as land use changes, agriculture, and climate change threaten to degrade the dark, wet soil and release its carbon into the atmosphere.

Digging for carbon

Previous studies in the Cerrado indicated that its soils held high amounts of carbon. But researchers typically did not dig deeper than about a meter [three feet] or expand their sampling beyond a few high-elevation areas in the region. The carbon storage potential of the savanna had been overlooked because its groundwater-fed wetlands aren't easy to spot from aboveground, said Amy Zanne, an ecologist at the Cary Institute of Ecosystem Studies and coauthor of the new study.

Because the ecosystems had been so overlooked, their carbon-storing potential has not been included in Brazil's national carbon accounting, either, said Rafael Oliveira, an ecologist at Universidade Estadual de Campinas in Brazil and coauthor of the new paper. Without detailed scientific information, "we have no clue what the emissions are" when these wetlands are degraded. "What are we losing in terms of carbon?" he asked.

To answer that question, Verona and the research team extracted meters-long soil cores from across seven sites in the Cerrado, then tested the layers of those soil cores to determine how much carbon was stored in each. The study's data richness makes an important contribution, said Julie Loisel, a peatland ecologist at the University of Nevada, Reno, who was not involved in the new study. "It's filling a really big data gap," she said. "In terms of the importance of wetlands in the tropics to understand modern-day carbon cycles, most of our information comes from satellite-derived products. We have very little information from field science."

"It's really nice to see a study that has gone really above and beyond in terms of measurements."

The researchers found that on average, each layer of the soil cores stored carbon at a density of 1,200 metric tons of carbon per hectare. That was a surprisingly high number for the types of soils tested, Loisel said. Though scholarly descriptions differ, one classical definition of peat — the type of carbon-rich soil usually considered in carbon accounting — requires that soils consist of about 30% organic matter; the soils studied by the research team contained about 16%, on average. Still, the amount of carbon stored in the Cerrado soils was much higher than that in some peatlands because the Cerrado soils were so dense, Loisel said.

"These are substantial carbon sinks," she said, adding that research like the new study "opens interesting research questions about understanding carbon dynamics in the continuum between mineral soils, wetland soils, and peat soils."

These dense, carbon-rich soils do not occur throughout the entire Cerrado, though, so Verona and the research team set out to estimate the complete geographic range of the wetlands using remote sensing data on land cover, land use information from landowners, and a machine learning approach. They estimated that these ecosystems cover 16.7 million hectares, about 8% of the total area of the Cerrado.

Next, the team measured greenhouse gas emissions from the Cerrado's soil during the wet, dry, and transitional seasons. They found that about 70% of wetland emissions occurred during the dry season. That may pose a problem as the climate changes and the wetlands dry out — because a steady influx of water maintains the environment that allows the soil to store so much carbon, drought could release a lot of carbon quickly.

Protecting tropical wetlands

Further analysis of the soils using radiocarbon dating determined that on average, the carbon stored in the Cerrado is more than 11,000 years old, with the oldest dated to be 20,000 years old. The age of the carbon stored indicates how critical ecosystem protections are: "If we lose the carbon in the Cerrado that has accumulated for millennia, we can't put it back so easily," Zanne said.

Though Brazilian law provides legal protections for wetland areas, the laws don't necessarily protect the water sources that feed the wetlands and make them a critical carbon-storing system. "We need to maintain the hydraulic dynamic," Verona said. "If you protect only the wetlands per se and don't protect the water in the landscape…we will lose the hydraulic system."

Furthermore, Verona refers to the Cerrado as a "sacrifice biome" because it absorbs some of the water-intensive land use needs that can't occur in the better-protected Amazon rainforest. To Verona, that's counterintuitive: "If you sacrifice the Cerrado for agriculture so that you can protect the Amazon, then you remove part of the water that flows to the Amazon, which [was] protecting the Amazon."

Keeping the Cerrado's wetlands functional could be critical to meeting global climate targets, however. Better protections — such as laws that recognize the connectivity of groundwater to the wetlands and better water usage laws — could help to maintain the Cerrado's carbon-storing capacity.

"We are just losing a lot of these wetlands silently, invisibly," Oliveira said. "They remain invisible in policy in Brazil, and even for the global scientific community. They really deserve urgent, stronger protection and recognition at the global level."

This article was originally published on Eos.org. Read the original article.