Transformation of Soils and Landscapes by Agriculture: Evidence in the Geoarchaeological Record

In weighing the influence of human land use on soils and the stratigraphic record, agriculture has had a profound and widespread impact since its inception about 10,000 years ago, altering pathways of soil and landscape development. Studies of ancient agricultural sites provide deep time perspectives on anthropogenic soil-geomorphic change. Research from these sites is presented to illustrate the wide range of soil-geomorphic change in relation to complex, interacting factors such as kind and intensity of agricultural system, time scale, geomorphic and environmental setting, and terrain and ecosystem resilience and sensitivity to disturbance. Inferring soil-geomorphic change is primarily based on a space-for-time substitution method in which ancient cultivated soils are compared with nearby uncultivated reference soils with similar geomorphic settings and pedogenic development. Soil change detection is complicated by post-agriculture environmental change and land use. Major agricultural activities that alter soils are vegetation clearing and change, landscape modification (e.g., terracing, leveling), tillage, water management, and nutrient additions and removals. These practices lead to surface cover change, accelerated erosion and sedimentation, and changes in soil morphological, physical, chemical, and biological processes and properties. These changes are interpreted in the context of plant needs and land resource conservation. Main forms of soil degradation inferred from ancient agricultural sites are structural deterioration, compaction, and declines in organic matter and macronutrients such as nitrogen and phosphorus. Soil enhancement inferred at some sites is A horizon thickening, structural stabilization, increased available water capacity, and gains of organic matter and nutrients. Case studies are presented from past or contemporary traditional agriculture in the North American Southwest and Peru, also drawing on evidence from other ancient and modern agricultural systems worldwide. Findings from these studies are relevant to current agricultural challenges involving climate change and aridity, water resources, soil quality, and sustainability.