Soru

Passage:
For decades, soil scientists and archaeologists maintained that the nutrient-poor oxisols of the Amazon basin precluded the development of dense, sedentary pre-Columbian populations prior to European contact. However, the discovery of Terra Preta—anthropogenic dark earths enriched with biochar, organic waste, and essential nutrients like phosphorus and calcium—demonstrates that ancient populations actively engineered soil fertility. Recent micro-botanical investigations analyzing phytolith assemblages within stratified Terra Preta sites have identified high concentrations of domesticated maize (Zea mays) and palm species alongside charcoal deposits dating to 500 CE. Anthropological archaeologist Dr. Elena Vance argues that the systematic accumulation of biochar in these sites was deliberately intended to stabilize organic matter against rapid tropical leaching, thereby enabling continuous, multi-generational intensive agriculture. Vance concludes that Terra Preta technology directly facilitated the expansion of persistent, high-density urban settlements across the central Amazon basin.

However, skeptical soil chemists note that biochar can form passively through recurring low-intensity domestic hearth burnings over centuries without deliberate agronomic intent. Furthermore, recent hydrological studies show that regional floodplain dynamics periodically deposit nutrient-rich alluvial silt, which enhances crop yields independently of soil charcoal content. Despite these qualifications, Vance maintains that the spatial uniformity of phosphorus enrichment across inland Terra Preta plots—far removed from riverine floodplains—proves that intentional soil management was the primary driver of agricultural intensification.

Which of the following is an unstated assumption on which Vance's argument regarding the role of Terra Preta in enabling high-density settlements depends?