Saturday, 15 July 2006

Soluble Phosphorus Availability in Subsurface Drip Irrigated Agriculture: Transformations of Soluble Inorganic and Organic Forms from Reclaimed Water in Soil and the Effect of the Rizosphere.

M. Pino Palacios-Díaz1, Vanessa Mendoza-Grimón1, Esteban Del-Nero1, Juan Ramón Fernández-Vera2, Francisco Rodriguez-Rodriguez2, and Jose Manuel Hernández-Moreno3. (1) Univ de Las Palmas de Gran Canaria, Agronomia. Fac. Veterinaria. Autovía Las Palmas-Arucas km 6.5, 35416 Arucas, Spain, (2) Granja Agrícola Experimental del Cabildo de Gran Canaria, Autovía Las Palmas-Arucas km 6.5, Arucas, Spain, (3) Univ de La Laguna, Dpto. Edafología y Geología, Facultad de Biología, Avda. Astrofísico Fco.Sánchez s/n, La Laguna, 38204, Spain

Reclaimed Water (RW) reuse is necessary to provide a sustainable agriculture in arid and semiarid regions. The use of Subsurface Drip Irrigation (SDI) with RW has many potential advantages. In spite of this, few studies have been conducted in order to establish the effect of the SDI water management on Phosphorus (P) availability to plants and how the rhizosphere is able to modify the soluble P (Ps) equilibrium in soils. The aim of this study is to increase the knowledge about the Inorganic and Organic P (Pi and Po) forms carried out by RW, its transformation in subirrigated soil and its availability to plants. After a preliminary laboratory study to characterize RW, soil and soil samplers, a field experiment was conducted during 2 years of irrigation with RW and conventional Fresh Water (FW). Different equipments were used to irrigate both RW and FW clay-loam soil. The subsurface irrigation system had 0.2 m buried lines and 0.5 m apart, with pressure compensated drips. Soil moisture samples were buried at 0.1 m depth in both soils, sampling on different periods during two years, 4, 24 and 48 hours after irrigation at 3 different sites: no plant, Medicago sativa and Sorghum bicolor ssp sudanense rhizospheres. Although RW carried out respectively 1.62 and 2.66 mgL-1 of inorganic and total P (Pt), and no fertilization was used in FW soil, less inorganic soluble P (Psi) was found in RW irrigated soils than in FW ones. The same results were obtained both with conventional soil analysis and when using soil water samplers. Lower proportion of Psi from total soluble P (Pst) were found in RW than in FW soil in all the rhizospheres considered. Future studies must be done to better explain these unexpected experimental results. After 48-hour irrigation, higher Psi was measured in RW soil. Moreover, plant rhizosphere affected the obtained results because lower Psi/Pst was found in Sudan grass than in Lucerne in RW.

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