Watershed responses to soybean cropland expansion in the southeastern Amazon

Background/Question/Methods

The surface waters of the Brazilian Amazon now face pressures from human activities that are unprecedented in the region's history. These pressures come primarily from deforestation to expand agriculture and cattle ranching. Since 2000, expansion of soybean croplands has become an important driver of land-use change in the region. Replacement of forests with croplands reduces evapotranspiration and alters the water balance, with the potential to significantly increase stream flows, alter stream habitat, and reshape regional rivers. Intensive cropping of soybeans and corn increases fertilizer use and could increase leaching of inorganic nutrients from farm fields to surface waters, as observed in intensively farmed regions in temperate zones. However, the fertilizer regimes needed for tropical croplands – together with the deep, well-drained soils that occur over much of the Amazon soybean cropping region – may slow land-water nutrient transport and cause different responses than in the temperate zone. Here, we compare stream flows, solute chemistry and water temperature in streams draining forested watersheds and single-cropped soybean watersheds receiving no N fertilizer. Data were collected in ten first and second order watersheds over two years at Tanguro Ranch, a large (80,000 ha) soybean farm in the southeastern Amazon (Mato Grosso, Brazil). 

Results/Conclusions

Stream flows in soybean watersheds were three times higher than in forested watersheds but hydrographs remained similar, with insignificant increases in flashiness during storms. There were few differences in concentrations of streamwater solutes between forest and soybean watersheds, although soybean streams exported more solutes than forested streams because of the increased volume of water exported. Streamwater temperatures in soybean watersheds were significantly higher than those of forested watersheds, with average daily maxima over 3 ^o C higher in soybean watersheds. Warming was attributed to the presence of impoundments and lower riparian forest cover along soybean streams. Although results from these single-cropped systems suggest watershed resilience to changes in stream nutrient concentrations, the region is now rapidly shifting to double-cropping, with soybeans as the main crop and corn as a second late-season crop. The corn portion of this cropping cycle requires inputs of N fertilizer that could change land-water solute transport. Preliminary sampling of groundwater in recently double-cropped watersheds showed small increases in N concentrations, but further research is required to understand the impact of these landscape changes on stream chemistry. Results from this tropical study system show some marked differences from those observed in temperate systems under similar cropping regimes.