El Nino-induced drought alters atmosphere and soil chemistry of the Amazon rainforest
The 2023-2024 El Niño caused the most severe drought ever recorded in the Amazon rainforest, along with heat waves and fires. But it also significantly affected the emissions of biogenic volatile organic compounds through the Amazon rainforest’s vegetation and their uptake by soils.
This text is based on two press releases provided by the Max Planck Institute for Chemistry.
Scientists are currently predicting a high chance that the world will experience a strong El Niño beginning at the end of this year 2026. This would be only three years after the last El Niño, the effects of which scientists are just beginning to understand.
The previous El Niño event in 2023 and 2024 brought unprecedented drought to the Amazon rainforest, resulting in historically low water levels in the Rio Negro in Manaus and widespread wildfires, even in typically wet and fire-resistant regions. This extreme weather event also coincided with multiple intense heat waves, which further exacerbated the drought's impact on the rainforest ecosystem. New research from the Amazon Tall Tower Observatory (ATTO) sheds light on the ecosystem's response to these extreme conditions, providing valuable insights into the resilience and vulnerability of the Amazon’s biodiversity.
Emission of stress-defense molecules
On the one hand, the vegetation significantly changed its chemical emissions to cope with environmental stress. Researchers, including scientists from the Max Planck Institutes for Chemistry (MPIC) and Biogeochemistry (MPI-BGC), measured the forest’s release of biogenic volatile organic compounds, or BVOCs, which are carbon-based molecules naturally emitted by vegetation. The results were striking. Emissions of one BVOC group, the so-called sesquiterpenes, increased by 122% over the course of the event. Sesquiterpenes are reactive airborne molecules that trees produce as stress signals and protective substances. A well-known example is caryophyllene, a peppery-smelling compound found in cloves and black pepper.
Even more surprisingly, the study detected unexpected emissions of less volatile sesquiterpene alcohols during the wet season after the drought peak. These findings suggest an adaptive response to oxidative stress, revealing how vegetation metabolically adjusts to adverse conditions. Interestingly, the change persisted long after the immediate stressor had passed. "Our results show that severe drought shifts the atmosphere toward lower-volatility and more reactive compounds”, explains Joseph Byron, the study’s first author and a researcher at the Max Planck Institute for Chemistry. "This reflects underlying metabolic changes as the rainforest attempts to mitigate damage from abiotic stress."
Furthermore, increasing emissions of BVOCs lead to greater loss of carbon from the biosphere to the atmosphere, as these compounds are built on carbon backbones. Sesquiterpenes, in particular, are carbon-rich, containing 15 carbon atoms per molecule.
Suppression of soil uptake
On the other hand, the soil uptake of biogenic volatile organic compounds also changed significantly during the El Niño drought. The research team could show that soils typically take up large amounts of isoprene, one of the most common types of BVOCs. During the drought caused by the 2023-2024 El Niño, isoprene uptake capacity dropped by a factor of over four compared to normal conditions.
“Our results show that during the extreme climatic conditions imposed by the 2023 El Niño, soil isoprene uptake abruptly became unresponsive to the increased ambient isoprene concentrations,” remarked Giovanni Pugliese, the study’s first author and a researcher at the Max Planck Institute for Chemistry. “The results are consistent with a physiological constraint of isoprene-degrading soil microbes when the soil moisture falls below 20%”.
The role of (increasing) BVOCs in the atmosphere
Plants and soils naturally emit biogenic volatile organic compounds (BVOCs). They play a crucial role in Earth’s atmospheric processes and ecological functions. In the atmosphere, BVOCs significantly influence the formation of secondary organic aerosols (SOAs), affect cloud formation, and alter the oxidative capacity of the air by reacting with atmospheric oxidants. The Amazon Rainforest is the world’s largest tropical forest and is thus a major global source of BVOCs.
Jonathan Williams, lead scientist of both these studies, summarizes: “It is interesting to see that when the Amazon ecosystem is faced with short-term drought and heat extremes, it acts to boost atmospheric BVOC levels by increasing canopy emissions while simultaneously weakening the soil sink”. These results align with the consensus that BVOC emission serves as a plant defense mechanism against thermal stress and oxidative damage.
Understanding these chemical responses is crucial, as climate change is projected to make El Niño events more intense and persistent. BVOCs shape local and regional air quality, weather patterns, and even the global climate system. The shift toward more reactive compounds could have significant implications for atmospheric chemistry and the overall resilience of the Amazon rainforest. Understanding the dynamics of BVOC emissions in the Amazon is essential for accurately modeling climate feedbacks and assessing the forest's role in Earth’s changing atmosphere.
