The results of tRRACES contributed strong evidence of large modification of the landscape by generalized fires that increased soil erosion at Konso. This increased erosion led to the loss of the original topsoil and much of the subsoil in the slopes before the hillside terraces started being built, therefore showing that they were not designed from the outset as an improved soil conservation strategy. This possibility was not recognised by the numerous observational studies of farming in this landscape, and suggest that the farming communities only begun the process of building terraces in order to protect the productive alluvial fields that were the legacy of that first phase of erosion.
At Engaruka, previous researches had suggested that soil exhaustion triggered the failure of the agricultural system. However, this view was premature as it was not supported by reliable information on soils health. From tRRACES results, we now know that soils do not show signals of fertility exhaustion, and upon irrigation and additions of organic matter, these can be readily recovered for agricultural use. Therefore, soil fertility is not considered to be the reason for Engaruka system abandonment and it does not seem to have affected the sustainability of the agricultural use. This is because the technology of field construction was designed for taking advantage of the natural processes occurring at a larger scale (erosion and transportation of the sediments from the mountains to the fields in the lowlands) this way providing new fertile edaphic material each rain season, which tackled the possible problems of fertility exhaustion derived from intensive cultivation, as well as the accumulation of salts.
Pre-hispanic agriculture has produced soil degradation at Topaín in form of calcification of irrigated soils. Carbonated waters are, in turn, known for they capacity for remediating soil salinity and hyper-alkalinity, which are frequent attributes of Atacama Desert soils. Thus, the fertility of Topaín system depended on a fragile equilibrium between the composition of the irrigation water and the rate at which soils were irrigated. It is inferred that a more intensive use of soils would accelerate the formation of carbonate crusts that would hinder productivity in the long term, but whether this degradation processes has been the reason for the expansion and posterior abandonment of the system is still being investigated.
Previous studies on Monte Gaiás demonstrated that continued intensive agricultural use during the last 1500 years have led to an accelerated ageing of soils. This was compensated by the addition of organic materials (shrubs grown on the mountains mixed with cattle faeces) to the soils, which allowed maintaining productivity over time. This illustrates the importance of the broader landscape management when considering agricultural sustainability. Moreover, our results, together with the availability of a wealth of paleoenvironmental and historical information for the site, has allowed to understand that the survival of the agricultural system did not result from its inherent characteristics in terms of soils natural properties and farming techniques, but from the climatic and socio-political history at broader local, regional and global scales.
