The rise and fall of human civilizations throughout history have been linked to both environmental and social factors.  During the 13^th and 14^th centuries AD, the people of the Perry Mesa Tradition lived in semi-arid grasslands north of the Phoenix basin, yet in the early 1400s, prior to European contact, these inhabitants mysteriously abandoned their settlements. Archaeologists in our research group are investigating the importance of sociopolitical drivers of population dynamics. In concert with these studies, we investigated whether prehistoric populations may have been destabilized due to agricultural practices that exhausted soil fertility. We characterized deep soil properties and chemistry beneath terraces and non-terraced areas within and outside of agricultural fields near a 130 person settlement, Pueblo La Plata, within the Agua Fria National Monument. Although nitrogen is thought to limit primary production in agricultural ecosystems, its mobility within ecosystems limits its usefulness as an indicator of soil fertility of the past. Thus, we compared concentrations of plant-available and total soil phosphorus, an essential but relatively immobile nutrient, between prehistoric agricultural and non-agricultural areas.  To quantify rates of P depletion through harvest or weathering, we used a mass balance approach that required the determination of soil parent material.

Results/Conclusions

Soils at Perry Mesa showed net gains in silica (20-80%) throughout the soil profile when compared to the underlying andesitic bedrock, which suggests mass additions from an outside source such as dust.  Moreover, rare earth element ratios from soils resembled those of dust samples rather than bedrock, suggesting that dust is the parent material of these soils.  Total and extractable phosphorus concentrations, extractable to total phosphorus ratios, and phosphorus depletion were not significantly different between agricultural fields and non-agricultural sites, or between terraced and non-terraced patches on the landscape.  These data suggest that ~150 years of crop harvesting on Perry Mesa did not deplete soil phosphorus pools, despite possible phosphorus removed through estimates of crop yields required to sustain the pueblo population.  Furthermore, the dust-derived nature of these soils has implications for both the sustainability of prehistoric agriculture and the resilience of these semi-arid ecosystems in the southwestern United States.