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

Hightlights

• The anthropogenic infill of mountainous soil terraces is of lacustrine provenance.

• Recurring wet/arid climate alternations enhanced calcrete formation.

• Younger Dryas event is responsible for the sedimentation of the soil regolith in Oman.

• The soil quality of abandoned mountainous soil terraces in arid environments considerably deteriorated.

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.

Introduction

Soil terraces are a mode of agriculture found in many regions worldwide (Varotto et al., 2019). Constructing soil terraces to create arable land in rugged terrain is one of the most labor-intensive forms of agriculture practice in arid regions. In semi-arid circum-Mediterranean areas, soil terraces are still a common form of agriculture (Moraetis et al., 2015, Tarolli et al., 2014). However, large areas of terraced landscapes in the Mediterranean area and elsewhere (Bolivia, Peru etc.) have been abandoned, often for social-economic reasons (Koulouri and Giourga, 2007, Tarolli et al., 2014). Cultivation abandonment can have positive or negative influences on soil fertility and erosion (Kosmas et al., 2000b, Novák et al., 2014). There are several studies on soil abandonment resulting in improved soil quality under Mediterranean climatic conditions and with specific site conditions of slope and soil lithology (Dunjó et al., 2003, Kosmas et al., 2000a, Kosmas et al., 2000b). On the contrary, abandonment of soil terraces in more arid environments can be detrimental to soil quality given limited precipitation and the risk of mineral precipitate accumulation. In Yemen, soil terraces, which appear as early as 5000 BP (Wilkinson, 1999), are threatened in many areas due to deterioration and abandonment (Pietsch and Mabit, 2012, Varisco, 1991, Vogel, 1988). Overall, in most other cases adverse environmental impacts have been shown to follow terrace abandonment, including examples in southern Peru (Londoño, 2008), on the Greek islands (Koulouri and Giourga, 2007, Moraetis et al., 2015) and in Spain (Lasanta et al., 2001).

In Oman and on the southern flanks of the Jabal Akhdar, agriculture and eco-tourism in mountainous high elevation areas (up to 1980 m) offer significant economic opportunities. However, some land abandonment is still taking place, which threatens agricultural sustainability and cultural landscapes (Al-Rawahi et al., 2014, Buerkert et al., 2010). In 2008, Luedeling and Buerkert identified the water shortages in the mountain oases of Oman as a contributing factor to soil terrace abandonment. The same study also indicates that shifts toward irrigation demanding crops (roses and fruits) and precipitation pattern changes have contributed to land abandonment. In an extremely arid environment like Oman, terrace abandonment leads to soil desertification, erosion and terrace wall collapse (Luedeling, 2007).

With time, such techniques of soil agriculture in extreme environments are in danger of falling into oblivion especially when these areas are getting abandoned by young generation (Faccini et al., 2018). Investigations of ancient soil terrace landscapes contribute to knowledge of food production among ancient civilizations (Beach et al., 2002, Bevan and Conolly, 2011, Bevan et al., 2013: Charbonnier, 2017, Harrower and Nathan, 2018). Understanding ancient agricultural terracing requires an understanding of how soils form, including their regolith provenance. Natural soil development in countries like Oman is often poor due to aridity, resulting in a lack of vegetation and low TOC values, while extreme precipitation events often result in soil erosion (Ministry of Regional Municipalities, Environment, and Water Resources, 2005). Nagieb et al. (2004) suggested that in Oman mountainous areas, the sediment/regolith was brought in to soil terraces from nearby wadis after heavy rainfalls. However, no studies have demonstrated the regolith provenance in mountainous soil terraces in Oman. Details of such agricultural measures for terrace construction in harsh environments are significant for re-evaluation of ancient agricultural practices. The regolith provenance in such anthropogenically influenced landscapes is not only of local interest, but also of global importance. Such features have a special significance to settlements in extreme environments.

Since the lack of pedogenesis due to high aridity, mountainous terraced soils in Oman constitute a significant sedimentary record under an imminent risk of loss. Moreover, terrace abandonment leads to adverse impacts on the soil quality especially on those marginal areas. Herein, we demonstrate the unique natural value of those areas by studying the soil provenance and the complex pedogenesis in two mountain oases of Oman. We also add new findings on the impact of terrace abandonment on soil quality. To achieve the previous, we present geochemical, physical, mineralogical, microscope and ¹⁴C dating evidence of the regolith provenance and soil conditions.