Solute and sediment export from Amazon forest and soybean headwater streams

Area of Science:

• Hydrology
• Biogeochemistry
• Tropical Ecology

Background:

• Intensive agriculture often elevates stream solute concentrations and export.
• Tropical regions are experiencing significant cropland expansion, yet their watershed responses are understudied.
• Understanding these impacts is crucial for managing tropical freshwater ecosystems.

Purpose of the Study:

• To investigate the effects of soybean cultivation on stream solute concentrations and export in Mato Grosso, Brazil.
• To compare watershed responses between forest and soybean land uses in a rapidly expanding agricultural frontier.
• To identify factors controlling stream chemistry and export in tropical agricultural watersheds.

Main Methods:

• Bi-monthly to monthly sampling of streamflow and 13 solutes (e.g., NO3-, PO43-, DOC) in forest and soybean watersheds.
• Stormflow and stable isotope (δ18O) analysis to characterize flowpaths.
• Comparison of solute concentrations and export between land uses.

Main Results:

• Soybean watersheds showed significantly higher annual export of NH4+, PO43-, Ca2+, Fe3+, Na+, SO42-, DOC, and TSS (5.6-fold increase), largely due to a 4.3-fold increase in water export.
• No significant differences in solute concentrations were observed between land uses, except for Fe3+.
• Wildfires influenced seasonal streamwater and rainwater solute concentrations through atmospheric deposition.
• Despite increased export, total solute yield per unit area remained low across all watersheds.

Conclusions:

• High soil hydraulic conductivity, groundwater-dominated flowpaths, and lack of nitrogen fertilization buffered tropical streams from major solute concentration increases.
• Tropical agricultural watersheds exhibit unique responses compared to temperate regions.
• Continued monitoring is essential to understand long-term impacts of agricultural expansion on tropical water quality.