Iron oxides in mangrove soils boost carbon sequestration

These minerals stabilize soil organic carbon, preventing the release of greenhouse gases. However, these ecosystems, known as “blue carbon forests,” have suffered degradation due to changes in land use.

By Luciana Constantino  |  Agência FAPESP – Using a novel approach, researchers have gained insight into mechanisms that may be helping flooded soils in coastal areas, such as mangroves, to retain carbon more efficiently. Understanding this process opens up opportunities to develop tools that address the negative impacts of climate change caused by human land use.

Mangroves are recognized as some of the most effective ecosystems in the world for capturing greenhouse gases, surpassing even tropical forests like the Amazon. This capacity was previously attributed solely to the absence of oxygen in these environments, which slows the decomposition of organic matter and consequently the release of carbon dioxide (CO₂).

A study published in Nature Communications reveals that low-crystallinity iron oxides (including ferrihydrite and lepidocrocite), found in mangroves, act as stabilizers of soil organic carbon. These oxides protect the most unstable fractions, called labile in biogeochemistry, which would otherwise be vulnerable to biological decomposition and the subsequent release of CO₂.

When land use changes, such as for the construction of shrimp ponds or for grazing (situations recorded in the areas analyzed in the study), the geochemical environment changes drastically, leading to soil acidification. This process transforms less crystalline iron oxide minerals into more crystalline forms that are less effective at stabilizing organic carbon.

Crystallinity refers to the way atoms are organized and arranged in a repetitive and orderly manner. This creates a three-dimensional structure that affects the physical and chemical properties of the material.

“Our study brings important innovations. One of them is in the methodology we created. We used established techniques, but in an innovative sequence that allowed us to infer the importance of iron in carbon stabilization. Another highlight was being able to demonstrate the mechanism involved in protecting the most labile fractions of organic matter,” explained Francisco Ruiz to Agência FAPESP. Ruiz is a researcher at the Department of Soil Science at the Luiz de Queiroz College of Agriculture at the University of São Paulo (ESALQ-USP) in Brazil.

The group used infrared spectroscopy to study the interactions between matter and radiation. They also used TG-DSC (thermogravimetric-differential scanning calorimetry) and selective chemical extraction to evaluate samples from the Mocajuba-Curuçá estuary in the Brazilian state of Pará, east of the mouth of the Amazon River.

The first author of the article published in the scientific journal Nature Communications, Ruiz has a scholarship from FAPESP, which also supported the work through the Research Center for Greenhouse Gas Innovation (RCGI) and the Center for Carbon Research in Tropical Agriculture (CCARBON). 

Ruiz’s advisor and corresponding author of the article, agronomist Tiago Osório Ferreira, considers the results to be “a paradigm shift.”

“The study advances our real understanding of how mangrove soils function as carbon sinks in an important climate change scenario and seeks strategies to mitigate its effects. When we understand the processes behind stabilization, it’s possible to discern what type of land use is more or less harmful, in addition to the possibility of enhancing or curbing certain mechanisms to achieve more efficient carbon stabilization and lower greenhouse gas emissions,” says the ESALQ-USP professor.

With over 25 years of experience researching mangrove areas, Ferreira currently leads the RCGI project, “BlueShore: Blue Carbon Forests for Offshore Climate Change Mitigation.” The initiative’s objectives include studying carbon sequestration and stabilization mechanisms in soils, creating a soil health index to classify regions as more or less degraded, and analyzing how mangrove biodiversity responds to higher CO₂ concentrations.

Importance for the planet

Mangroves are called “blue carbon forests” because of their sink characteristic. For example, emissions from the loss of mangrove vegetation in the so-called “Legal Amazon” could be up to three times higher than those recorded in an equivalent forest area. Therefore, halting the deforestation of this ecosystem could prevent around 1,228 tons of CO₂ emissions per hectare (for more information, see the article “The inclusion of Amazon mangroves in Brazil’s REDD+ program”). 

The Legal Amazon, created by the Brazilian government for regional development purposes, is an area covering all nine states where the Amazon biome occurs.

To raise awareness about the importance of these coastal ecosystems and the need to protect them, the United Nations designated July 26 as World Mangrove Protection Day.

Brazil has the second largest mangrove area in the world, spanning approximately 1.4 million hectares along the coastline, and the largest continuous stretch, which is located between the states of Amapá and Maranhão. However, it is estimated that 25% of Brazil’s mangroves have been destroyed since the beginning of the 20th century. This destruction may be accelerated by rising sea levels, climate change, more frequent extreme weather events, deforestation, and urban expansion.

Soil and gas collection in a mangrove area converted to pasture (photo: Angelo Fraga Bernardino/Federal University of Espírito Santo)

Approximately 500,000 Brazilians depend directly on the resources of these ecosystems for survival. This group includes artisanal fishermen, shellfish gatherers, and extractivists. These areas are also important for fishing due to their extensive biodiversity of more than 770 species of fauna and flora, as well as their role in the initial stage of development for various types of fish.

“The problem isn’t crab harvesting or extractivism, but rather the disruption of the biogeochemical balance when vegetation is removed or there’s an inappropriate change in land use. In this sense, the research also sheds light on the importance of conservation and land-use control in mangroves,” Ferreira adds.

The study warns that efforts to restore these ecosystems must go beyond reforestation and incorporate innovative strategies to restore the geochemical balance of the soil. Natural recovery of minerals in mangrove soils is usually slow due to erosion and degradation.

Building knowledge

Ruiz points out that the techniques used in the study of well-drained soils, such as those found in forests, are applied more frequently than those used in the study of waterlogged soils. “For mangroves, we’re in the early stages of assessing this interaction between iron and carbon. I began focusing on analyzing stabilization mechanisms in organomineral interactions while studying technosols,” the researcher says.

During his master’s and doctoral studies, Ruiz worked with types of constructed soils (technosols) that can restore degraded areas. He received the USP Outstanding Thesis Award in Agricultural Sciences and the CAPES Thesis Award – 2024 Edition (Honorable Mention in Agricultural Sciences I), which is promoted by the Coordination for the Improvement of Higher Education Personnel (CAPES), a federal agency linked to the Ministry of Education.

The article “Iron’s role in soil organic carbon (de)stabilization in mangroves under land use change” can be read at www.nature.com/articles/s41467-024-54447-z.