Terrace soils in the Yemen Highlands: Using physical, chemical and radiometric data to assess their suitability for agriculture and their vulnerability to degradation

The Yemen Highlands are well known for their ingenious terrace systems which were constructed at the beginning of Bronze Age in the 3rd millennium BC. Although ancient, these systems are still in use, but modern erosion, deforestation, changes in climate and unsustainable qat cultivation lead to drastically increasing land degradation. A small field terrace system located in Wadi Al-Jidar/Eryan was investigated to ascertain its agricultural suitability and vulnerability to degradation of such systems while focussing on the most important base of agriculture: colluvial deposits and its associated soils. Physical and chemical analyses demonstrate that not only ancient and modern terraces both are still suitable for agricultural purposes, but that these systems are very fragile and highly erodible when abandoned. Whereas ancient and mediaeval colluvial deposits of 4 m thickness were deposited over a period of 2000 years, 2 m of modern deposits have been accumulated in 200 years only. Other noticeable differences between old and young deposits are their distinct coarse sand/fine silt contents, as well as the higher thickness of ApbBw horizons in older deposits or lower salt contents in younger soils. The present study highlights an innovative multi-radiometric (14C, naturally occurring radioisotopes, 137Cs) and soil data approach to evaluate the agricultural suitability of terraces in Yemen on the one hand, and emphasises several potential advantages of integrative research on land degradation on the other, particularly when investigating both ancient and modern soil erosion processes in semi-arid tropical highlands.

► First radiometric data (NOR and 137Cs) of terrace soils in Yemen are presented.
► Ancient and modern terraces differ in the number of soil layers and thickness.
► Despite degradation, terrace soils are still suitable for agriculture.