Climate change is intensifying droughts in the Amazon. We urgently need to understand how forest ecosystems respond. This means not only tree survival but also the invisible chemical processes that sustain soil functions.
To address this, Frederik Lange and his colleagues looked at so-called white-sand forests. White, sandy soils with lower, bushy vegetation characterize this forest type. White-sand forests only make 5% of the Amazon rainforest ecosystems. But they play an outsized role in feeding carbon into the region’s important blackwater rivers. Those rivers are called blackwater because they carry large amounts of organic material, coloring them dark. They transport carbon and nutrients towards the coast and ultimately help fertilize the Atlantic Ocean. Scientists expect that droughts will alter the production and export of organic matter from white-sand forest soils, with consequences for the river and ocean ecosystems that depend on this material as a food and energy source.
Dissolved organic matter in white-sand soils has multiple key sources:
- the decomposition of surface plant litter (for example, dead leaves)
- the breakdown of organic matter deeper in the soil
- and the release of compounds by plant roots.
These sources are accompanied by a wide variety of compounds that plants and microorganisms produce in response to stress, such as during drought. The molecular composition of dissolved organic matter reflects these different sources and stress responses. Therefore, it can reveal which processes are disrupted by drought and how severely organisms are affected. For white-sand forests in the Amazon, this has never been investigated.
© Frederik Lange/MPI-BGC
To fill this gap, the team collected soil water from two white-sand forest sites in the central Amazon over two years. They then analyzed thousands of individual organic compounds – covering one moderate and one extreme drought year under El Niño. They found that drought fundamentally reorganizes where soil carbon comes from. Inputs from decomposing leaf litter collapsed, while contributions from deeper soil organic matter, plant roots, and drought stress compounds increased substantially. White-sand soils are already extremely nutrient-poor today. This shift from aboveground to belowground sources further may deplete resources under repeated extreme drought events.
Furthermore, under extreme drought, stress-related compounds increased dramatically. This is a signal that plants and microorganisms were struggling to cope. It points to severe ecosystem stress under extreme drought, threatening the healthy functioning of white-sand forests. Importantly, these shifts followed deterministic patterns, suggesting that future droughts will trigger similar – and potentially stronger – responses.
This carries two practical implications. First, scientists could develop an early warning system for ecosystem stress in white-sand forests from the molecular composition of soil water. Second, under future climate change, repeated drought-induced disruptions to carbon and nutrient cycling could progressively undermine their functioning and their ability to sustain the blackwater rivers and regional carbon flows that depend on them.
Lange et al. published the study “Drought shifts dissolved organic matter sources from above- to belowground and stress-induced processes in Amazon white-sand forests” Open Access in the journal Biogeochemistry.
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