Soil iron biogeochemistry reveals a sensibility of mangrove forests in the face of climate change

The death of extensive mangrove forests by extreme weather events affects the soil geochemical environment and iron dynamics. These changes cause the loss of important ecosystem services.
Published in Earth & Environment
Soil iron biogeochemistry reveals a sensibility of mangrove forests in the face of climate change

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Changes in soil iron biogeochemistry in response to mangrove dieback - Biogeochemistry

Fe biogeochemistry is associated with important ecosystem services provided by mangrove forests, including carbon sequestration and the retention of potentially toxic elements. The biogeochemical processes controlling Fe fate in mangroves are naturally affected by the soil geochemical environment, which controls Fe dynamics. However, ongoing climate changes and the associated extreme weather events may drastically affect the biogeochemistry of this important micronutrient for both terrestrial and oceanic environments. Therefore, this study aimed to evaluate how massive mangrove mortality after an extreme weather event altered the Fe dynamics in mangrove soils. The results show a significant decrease in soil carbon stock in the dead mangrove forests (25 kg m−2), as compared with the undisturbed forests (37 kg m−2). In addition, we observed a substantial Fe loss (greater than 50% of soil Fe forms, i.e., 17,000 mg kg−1) in the dead mangrove soils, which was associated with pyrite (9000 mg kg−1) and low crystallinity Fe oxyhydroxides (2400 mg kg−1). These impacts led to a decrease in the pyritization in soils, which resulted in a loss of 170 tons of Fe from 500 ha of dead mangrove forests within one year. Thus, the pyritization process may critically compromise a mangrove forests’ ability to immobilize pollutants (e.g., metals) and sequester carbon in the long term, thereby altering their ability to provide these ecosystem services. Overall, our results revealed that the Fe biogeochemical cycle of mangrove forests is very sensitive to future climate change scenarios and increased extreme weather events.

Mangroves are ecosystems recognized for their role in mitigating climate change mainly because of the high potential for carbon sequestration in their soils. With the global climate change concern, much research has focused on evaluating the effects on mangrove carbon stocks (below and above the ground). However, our research focused on evaluating the effects of a massive mangrove death on the soil biogeochemistry of forests impacted by extreme weather events (long drought period combined with hailstorm) related to climate change.

In mangrove soils, iron biogeochemistry plays a key role in several ecosystem functions. For example, iron is an essential nutrient for marine organisms and plants and acts on phosphorus bioavailability, geochemical pathways of carbon sequestration, and immobilization of contaminants.

Therefore, understanding how iron biogeochemical dynamics in mangrove soils will respond to climate change effects is an important step towards strategies for monitoring, recovering, and preserving these forests.

The research was carried out in the estuary of the Piraquê-açú-mirim river in southeastern Brazil. The studied mangroves belong to a protected reserve with ~ 1,746 ha of pristine forests (Long-Term Ecological Research Program, CNPq-CAPES-PELD, subproject 441243/2016-9). Despite the humid tropical climate with an average annual rainfall of > 1,140 mm, the region experienced a severe drought (after the 2015 El Niño), which, combined with a hailstorm (in June 2016), led to the death of around 500 ha of mangrove forests. One year after the death of the mangrove forests, soil samples were collected in four mangrove forests (two dead mangroves and two live mangroves i.e., non-impacted).

Overview of the dead mangrove forest
Overview of the dead mangrove forest one year after the extreme weather events

Our findings reveal that the massive death of mangroves resulted in significant changes in the soil geochemical environment. Oxidizing conditions led to a significant overall loss of Fe, especially pyrite (FeS2), a well known metal sink. In addition, soil carbon stocks decreased sharply as a result of the oxidizing soil conditions and the absence of organic carbon inputs from vegetation. These changes in iron biogeochemistry impacted the potential for both contaminant immobilization and carbon sequestration.

Comparison between live and dead mangroves
On the left side, one of the dead mangrove forests, on the right side is a live mangrove

These results evidence the sensibility of mangroves in face of climate change and the potential loss of ecosystem services provided by these forests. Furthermore, it highlights the need for future studies on climate change to target the role of Fe in mangrove soils. Based on our findings, mangrove forests may change their role as carbon and metal sinks to become potential sources.

Our Research Group on Soil Geochemistry (GEPGeoq) is addressing efforts in studying the possible effects of climate change (e.g., increase in salinity, temperature, and droughts) on mangrove soils. 

Overview of the dead mangrove forest during the sampling
View of the dead mangrove forest during the soil sampling

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