Long-Term Perspectives On Anthropogenic Soil Change From Ancient Agriculture.

Among agents of anthropogenic soil change, agriculture’s impact on soil is immense in magnitude, spatial extent, and duration. Agriculture has profoundly altered soil properties, processes, and formation pathways world-wide since its inception about 10 millennia ago. Much knowledge about recent agricultural soil change at scales of years to a century has been gained through monitoring long-term experiments and observational studies. However, far less is known about agricultural soil change at scales of centuries to millennia. Deep time perspectives on soil change can help predict long-term effects of agriculture on land resources and to test for sustainability. Information on soil change in longer time frames can be obtained by studying ancient agricultural soils, even though data are more limited than those from modern agricultural soils. Ancient agricultural sites in archaeological and contemporary traditional contexts in the Americas and other regions are presented to illustrate the wide range of soil change in relation to complex, interacting factors such as kind of agricultural system, time scale, and environmental setting and resilience. Inferring soil 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 and pedogenic settings. Soil changes detected in ancient fields are interpreted as a gradient from degradation to enhancement in the context of agricultural productivity and land resource conservation. Main forms of soil degradation inferred from ancient agricultural sites are accelerated erosion, structural deterioration, compaction, declines in organic matter and macronutrients such as nitrogen and phosphorus, salinization, and acidification. Soil enhancement inferred at some sites is A horizon thickening, structural stabilization, increased available water capacity, and gains of organic matter and nutrients.  Findings from these studies are relevant to current agricultural challenges involving climate change, water resources, soil quality and conservation, and sustainability.