126-4 The Impact of Volcanic Eruptions on the Phosphorous Availability of Raised Fields in the Ecuadorian Highlands

Session: Geoarchaeology of Sites to Landscapes: Current Research on Long-Term Water and Soil Management and Maladaptation, Part II

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The ability of raised fields to cycle nutrients and provide crop-beneficial fertilizers has been posited as one of the major reasons these agricultural systems became widely used in some areas of the prehistoric Andes. Phosphorous deficiency is a well-known issue in volcanic ash soils due to sorption by active amorphous materials such as allophane. In volcanically active highland Ecuador, plant available phosphorous decreases with elevation while concentrations of amorphous materials increase due to enhanced weathering at higher elevations. Ridged fields appear to have been constructed in the marshy flats in valley bottoms where nitrogen and plant available phosphorous is plentifully. At the site of Zuleta, the drainage provided by the ridged fields allowed crops to access this phosphorous, created a greater balance of C, N, and P, and formed concentration gradients with higher C/N ratios on the ridge and lower C/N ratios in the canals creating conditions suitable for a wide range of crops. However, after their construction, enhanced weathering within the canals appears to have contributed to a decrease in available phosphorus. When the VEI-6 eruption of Quilotoa volcano occurred around 1280 AD, the deposition of fresh volcanic ash into the canals led to a dramatic decrease in available forms of phosphorus which would have inhibited the overall productivity of the fields. These results suggest that, while ridged fields may have developed as an adaptation to overcome environmental obstacles in the volcanic landscape, their use and maintenance was not without its own pitfalls.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8510

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