Little is known about the sustainability of irrigated oasis agriculture in northern Oman. The objective of this study therefore was to examine which factors allowed agricultural productivity to be apparently maintained during the two millenia of a mountain oasis' existence. Soil moisture and physico-chemical properties were measured in a typical flood-irrigated field sown to alfalfa (Medicago sativa L.). Particle size, organic (C org ) and inorganic carbon content, pH and electrical conductivity (EC) of the soil profile were analyzed at 0.15, 0.45 and 1.00 m. Saturated hydraulic conductivity and the soil's apparent bulk density and water potential were determined from undisturbed samples at 0.05, 0.25 and 0.60 m. During irrigation cycles of 6-9 days, volumetric water contents ranged from 30% to 13%. A tracer experiment with potassium bromide revealed that 52-56% of the irrigation water was stored in the upper 0.4 m of the soil. The rest of the water moved further down the profile, thus providing the necessary drainage to avoid the build-up of toxic salt concentrations. Due to differences in pore size, plant-available water in the topsoil amounted to 18.7% compared to 13% and 13.5% at 0.25-and 0.60-m depth, respectively. The aggregate structure in the upper 1.0 m of the profile is likely preserved by concentrations of calcium carbonate (CaCO 3 ) from 379 to 434 mg kg À1 and C org from 157 to 368 mg kg À1 soil. The data indicate that the sustainability of this irrigated landuse system is due to high water quality with low sodium but high CaCO 3 concentration, the elaborate terrace structure and water management which allows adequate drainage. D

Several botanical studies have been conducted in different parts of Oman, but knowledge about agro-biodiversity in the rapidly decaying ancient mountain oases of this country remains scarce. To fill this gap we assessed the genetic resources of three mountain oases in the al-Hajar range using a GIS-based field survey and farmer interviews. While arid conditions prevail throughout the mountain range, the different elevations of Balad Seet (950-1020 m a.s.l.), Maqta (930-1180 m a.s.l.) and Al Jabal al Akhdar (1750-1930 m a.s.l.) provide markedly differing agro-climatic conditions. Overall, 107 different crop species were identified belonging to 39 families. Species number was highest among fruits (33 spp.), followed by vegetables (24 spp.). Intensive irrigation allows cultivation of a broad range of species at all oases. However, the number of species varied significantly between sites. Fruit species diversity and homogeneity of distribution of individual fruit species was highest at Balad Seet and lowest at Maqta as indicated by respective Shannon indices of 1.00 and 0.39 and evenness values of 32% and 16%. Century plant (Agave americana L.), faba bean (Vicia faba L. var. minor Peterm. em. Harz) and lentil (Lens culinaris Medik.) were identified as relict crops, supporting oral reports of past cultivation and providing evidence of genetic erosion. Some species, such as the temperate fruits of Al Jabal al Akhdar, were exclusively found at the coolest site, while others only occurred at the hotter locations. Overall greatest species similarity was found between Balad Seet and Al Jabal al Akhdar as indicated by a Sørensen coefficient of similarity of 67%. At all oases a multilayered vegetation structure dominated with a canopy, an understory and a ground layer. Greatest species richness was recorded in the lowest stratum. Overall the study shows a location-specific but surprisingly diverse mosaic of crops in Omani mountain oases which merits further studies and conservation efforts.Dedicated to the memory of Richard N. Lester , keen researcher of plant genetic resources.

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Little is known about nutrient fluxes as a criterion to assess the sustainability of traditional irrigation agriculture in eastern Arabia. In this study GIS-based field research on terraced cropland and groves of date palm (Phoenix dactylifera L.) was conducted over 2 years in two mountain oases of northern Oman to determine their role as hypothesized sinks for nitrogen (N), phosphorus (P) and potassium (K). At Balad Seet 55% of the 385 fields received annual inputs of 100-500 kg N ha À1 and 26% received 500-1400 kg N ha À1 . No N was applied to 19% of the fields which were under fallow. Phosphorus was applied annually at 1-90 kg ha À1 on 46% of the fields, whereas 27% received 90-210 kg ha À1 . No K was applied to 27% of the fields, 32% received 1-300 kg K ha À1 , and the remaining fields received up to 1400 kg ha À1 . At Maqta N-inputs were 61-277 kg ha À1 in palm groves and 112-225 kg ha À1 in wheat (Triticum spp.) fields, respective P inputs were 9-40 and 14-29 kg ha À1 , and K inputs were 98-421 and 113-227 kg ha À1 . For cropland, partial oasis balances (comprising inputs of manure, mineral fertilizers, N 2 -fixation and irrigation water, and outputs of harvested products) were similar for both oases, with per hectare surpluses of 131 kg N, 37 kg P, and 84 kg K at Balad Seet and of 136 kg N, 16 kg P and 66 kg K at Maqta. This was despite the fact that N 2 -fixation by alfalfa (Medicago sativa L.), estimated at up to 480 kg ha À1 yr À1 with an average total dry matter of 22 t ha À1 , contributed to the cropland N-balance only at the former site. Respective palm grove surpluses, in contrast were with 303 kg N, 38 kg P, and 173 kg K ha À1 much higher at Balad Seet than with 84 kg N, 14 kg P, and 91 kg K ha À1 at Maqta. The data show that both oases presently are large sinks for nutrients. Potential gaseous and leaching losses could at least partly be controlled by a decrease in nutrient input intensity and careful incorporation of manure.