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Resistance and Resilience of Ancient Agricultural Soils

Periodic Reporting for period 1 - tRRACES (Resistance and Resilience of Ancient Agricultural Soils)

Reporting period: 2015-08-17 to 2017-08-16

Before the invention of modern, large-scale engineering projects, terrace systems were rarely built in single phases of construction, but instead developed gradually, and could even be said to have evolved. Understanding this process of landscape change is therefore important in order to fully appreciate how terrace systems were built and functioned, and is also pivotal to understanding how the communities that farmed these systems responded to changes; whether these are changes to the landscape brought about by the farming practices themselves, or changes to social, economic or climatic conditions. Combining the study of soils with archaeological and paleoenvironmental knowledge, tRRACES studied four terraced landscapes at Konso (Ethiopia), Engaruka (Tanzania), Topaín (Atacama Desert, Chile), and Monte Gaiás (Spain). The theoretical background of the project relies on soils capacity to be a sensitive indicator of environment modifications driven by anthropic forces. Proxies for these changes based on soil properties enable the evolution of the environment to be deciphered and a historical narrative to be constructed.
The aim of the project was to provide a long-term, evolutionary perspective on the development of agricultural soils, and to assess the relationships between soil properties and traditional land use and management, with a focus on the assessment of sustainability and the potential contribution to policy development.
Specifically, tRRACES project was designed to meet the following objectives:
(1) To study the changes on edaphic properties induced by ancient agricultural uses under a range of environmental conditions
(2) To contribute to a sound scientific basis for the indigenous knowledge so that information on resilience of past agricultural systems can be used reliably
(3) To provide information on the sustainability of agroecosystems to the work-scales required by planners and policy makers.
This approach provided information useful within land use and adaptation planning within the broader context of environmental and social sustainability. Of special importance has been the incorporation of the knowledge from traditional societies with long histories of surviving the challenges of everyday living and working.
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.
Taking into consideration the results of the four sites together, it outstands as a result of tRRACES that Research that takes into account how terrace systems change through time can thus provide important details of whether the function of the system has changed, and can help assess how the legacies of former practices impact current or future cultivation. The identification of the modifications of the soil formation pathways due to agricultural use can be used as an informative approach on the sustainability of agricultural systems in the past. As these processes can be extrapolated to modern settings, the information yielded by tRRACES can inform the possible patterns of change under current and future environments.
Moreover, the four cases have demonstrated that the assessments of sustainability have to consider a geographical area larger it is usual in traditional archaeological studies, as the processes underpinning agricultural productivity over history were largely dependent in the management of the wider landscape and climatic variations.
Konso riverside sediment traps
Sequence of terraced landscape construction at Konso (from Ferro-Vázquez et al 2017)
Terraces system at Topaín
Konso hillside terraces
Sampling a soil in a sediment trap at Engaruka
Terraces excavation at Monte Gaiás