Terrace agriculture in a mountainous arid environment – A study of soil quality and regolith provenance: Jabal Akhdar (Oman)

Abstract

In the Sultanate of Oman remnants of deteriorating terrace agricultural systems offer important insights into long-term human adaptation in the arid tropics. Irrigation and terrace agriculture in the mountainous Jabal Akhdar region reveal historic agricultural practices in a rugged, high elevation context. The present study examines soil quality and regolith provenance in abandoned agricultural soil terraces. Three soil profiles in each of the Villages of Hadash and Wijma were excavated and analyzed. Physical, chemical and mineralogical analyses were conducted for all soil horizons. In addition, six other soils, 3 possible soil parent rocks (regolith) and soil’s bedrock were collected. Soil ages were constrained by 14C assays and stable isotope, (13C and 18O) on the bulk carbonates in the calcrete (caliche). The results demonstrate that both sites display poor soil quality with very low average total organic carbon (TOC) (6.2–5.0 g kg−1) and mean weight diameter (MWD; 0.27–0.48 mm), with low water stable aggregate content (<42%). All the geochemical, mineralogical and the thin section analyses show that the soils exhibit unique characteristics that differ from those of other sediments (possible parent regolith) and soils in the vicinity. The finding of ostracod shells in the soil terraces in both areas and 14C dating of calcrete (10.193 ± 30–13.887 ± 40 a BP) indicate that regolith was human-transported to terraces to create soil. The 14C ages of the bulk carbonates match well with a dry period of high calcite precipitation contemporaneous to the Younger Dryas. The Hadash and Wijma soil terraces are located ~45 km away from each other, but still display significant similarities in terms of regolith provenance and soil development and were likely filled with regolith from the same source. These results offer new perspective on agricultural terrace development and oasis agriculture in a rugged, high-elevation, arid environment.