The impact of agriculture on tropical mountain soils in the western Peruvian Andes: a pedo-geoarchaeological study of terrace agricultural systems in the Laramate region (14.5° S)

Abstract. This integrated pedo-geoarchaeological study focuses on three abandoned prehispanic terrace agricultural systems near Laramate in the southern Andes of Peru, aiming to unravel the pedological and land-use history of the region. The investigation involved contextualizing the former agricultural management system within its paleoecological framework and assessing the impact of agricultural practices on soil development and quality. The Laramate terrace complex, with its diverse terrace systems and varied geomorphological settings, provided an ideal setting for the investigation. Comparative analyses between non-irrigated agricultural terrace soils and undisturbed reference sequences were conducted, employing a range of methodologies, including surveys, soil analysis, GIS and remote sensing, palaeobotany, and radiocarbon dating. The study identifies three WRB Reference Soil Groups in the Laramate region: Phaeozems, Andosols, and Anthrosols. Unique characteristics of Phaeozems challenge typical descriptions, influenced by factors such as climatic seasonality, vegetation, fauna, lithology, and aeolian inputs. Despite long-term use, terrace soils (Anthrosols) show no severe degradation, maintaining balanced acidity and nutrient availability for successful crop cultivation. Tillage horizons of all terrace soils exhibit elevated organic matter content, highlighting the sustainability of prehispanic agricultural practices through a consistent application of organic manure. Phytolith concentrations indicate extensive agricultural activities, particularly maize cultivation, with varying patterns suggesting changes in cultivation or fertilization practices over time. Starch grain identification aligns with phytolith analyses, reinforcing maize's significance in the region. Notably, the abandonment of the prehispanic cultivation system was not linked to soil exhaustion or terrace instability. The prehispanic history of terraced agriculture in the Laramate region extends over four development phases, reflecting dynamic interactions between environmental, cultural, and agricultural factors. The initial phase, from the Formative Paracas period to the Early Nasca period (800 BCE–200 CE), witnessed the establishment of agricultural terraces with simple terrace architecture, while the Middle Horizon (600–1000 CE) saw systematic areal expansion influenced by the Wari culture. Adaptations to drier conditions included terraced agriculture on volcanic soils. The Late Intermediate Period (1000–1450 CE) witnessed hydrological variability and further terrace expansion to lower altitudes and less agriculturally suitable locations. The final phase, marked by the onset of the Hispanic colonial period in 1532 CE, saw the gradual abandonment of terraced agricultural systems due to demographic shifts and reorganization of production systems. Despite this, the historical trajectory underscores the adaptability and resilience of prehispanic communities in the Laramate region, showcasing innovative terrace agriculture as a means of coping with changing environmental conditions across diverse landscape units.