The Amazon rainforest is extremely productive despite growing on some of the world’s most nutrient-depleted soils. To achieve this, the forest relies heavily on the recycling of nutrients. Organic compounds as the product of organic matter transformation contain essential nutrients like nitrogen, phosphorus, and sulfur. As they dissolve in soil water, they create what’s called dissolved organic matter (DOM), which indirectly or directly is an essential nutrient pool for plants and microorganisms. But different forest types in the Amazon may face different nutrient demands and limitations.
Frederik Lange and his colleagues investigated dissolved organic matter in soils of two contrasting Amazon rainforest types: terra firme forests on clay-rich soils and white-sand forests on sandy soils. Using high-resolution mass spectrometry, they could identify thousands of different compounds and track how organic nutrients cycle through the ecosystem. They used this molecular perspective to evidence nutrient limitations and the intensity of nutrient cycling in the mineral soil.
Nutrient limitations and soil nutrient cycles
Their analysis revealed three key findings:
- Terra firme forests showed clear signs of phosphorus limitation
- White-sand forests unexpectedly showed evidence for high sulfur demand rather than the expected nitrogen limitation
- The two rainforest types process nutrients very differently – terra firme forests rely heavily also on soil processes while white-sand forests predominantly cycle nutrients in their surface organic layer
The findings help us understand how different Amazon rainforest types maintain their productivity despite nutrient limitations. This knowledge is crucial for predicting how these forests might respond to climate change, particularly how nutrient limitations will restrict their ability to absorb increasing atmospheric CO2. The evidence for phosphorus limitation in terra firme forests agreed with the hypotheses. In contrast, this is the first study to show that Amazonian white-sand forests might experience elevated sulfur demand and cycling. Previous research had focused mainly on nitrogen and phosphorus. This unexpected finding also reveals how much we still have to learn about nutrient cycling in Amazon rainforest ecosystems.
Lange et al. published the study “Cycling of dissolved organic nutrients and indications for nutrient limitations in contrasting Amazon rainforest ecosystems” Open Access in the journal Biogeochemistry.
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