Geoökologisches Kolloquium WS 2004/05

Alle Termine

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Prof. Dr. Andres Buerkert
Lehrstuhl für Ökologischen Pflanzenbau und Agrarökosystemforschung, Institut für Nutzpflanzenkunde, Universität Kassel
Mittwoch, 09.02.2005 17:15 H6

Agrarökologie, Nährstoffkreislauf, Bewässerungsmanagement und pflanzengenetische Diversität in traditionellen Gebirgsoasen des N-Oman

Little is known about the causes of the apparently sustainable productivity of ancient irrigation agriculture in the mountains of the Arabian peninsula. To fill this gap of knowledge, interdisciplinary research is conducted since 1999 in two desert oases of northern Oman with their hundreds of tiny multi-cropped terraces. The process-oriented agroecosystems approach chosen hypothesized effective nutrient cycling, water management with adequate leaching, highly buffered springs and a wide intra-specific plant biodiversity to be likely causes for the oases² existence over millennia. Spatially strongly variable, but overall large annual nutrient inputs to crops (up to 1400 kg N, 210 kg P and 1400 kg K ha^-1) seem to cause large surpluses in partial field and oasis balances making the studied oases important sinks for water and nutrients at the landscape level. On the clay-rich man-made terrace soils with mean CO2-evolution rates of 45.5 mg C m⁻² h⁻¹ for landraces of wheat (Triticum sp.) and 97.5 mg C m⁻² h⁻¹ for alfalfa (Medicago sativa L.) the latter showed at an average annual total dry matter production of 22 t ha^-1 an biological N₂-fixation of 480 kg ha^-1 thereby contributing substantially to the positive nutrient balances of the cropland. Partial balances of palm groves (Phoenix dactylifera L.) were with 313 kg N, 43 kg P and 191 kg K ha-1 (on 8.8 ha) and 83 kg N, 14 kg P and 85 kg K ha^-1 (on 3.6 ha) remarkably different in both oases. This was mainly due to the recycling of nutrients in human faeces from the artificially high population in the first oasis living largely on imported food. During irrigation cycles of 6-9 days volumetric water contents in the terrace soils ranged from 30-13%. A tracer experiment 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, thereby 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 (CaCO₃) from 379-434 mg kg^-1 and Corg from 157-368 mg kg^-1 soil. These data indicate that the sustainability of this irrigated landuse system is partly due to the high quality of its irrigation water, the elaborate terrace structure and an irrigation scheme that allows a drainage of 5-10% of the applied water. Morphological and molecular genetic analyses of wheat but also of banana (Musa sp.) in combination with first results of pollen analyses of a 20 m deep soil profile not only confirm the assumed large genetic diversity in some of these oasis systems but also provide evidence for the oases² role as refugia of ancient germplasm likely traded in the region many centuries ago.

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