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).

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.

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.