تأثير معدلات الري بمياه الصرف الصحي لمدينة البيضاء بالجبل الأخضر علي خصائص التربة ونمو النبات

Abstract

Fresh water resources are limited in most arid and semiarid regions. Libya is not an exception . Jabal Al-Akhdar north – east of libya, receives the highest annual rainfall in the country, but fluctuation in rainfall coupled with increasing demand for fresh water to meet domestic, agricultural and industrial activities posed serious pressure on groundwater which practically represents the sole source of fresh water in the province . A national strategy to utilize domestic sewage effluent to irrigate some forage crops was implemented early in the 1973's. The strategy was limited to two projects namely, Al-Hadba project near Tripoli and Al-Gawarsha project near Benghazi. Other parts in the country including Jabal Al-Akhdar were not included in this national strategy. The current study was aimed toward evaluating the potential for utilizing the untreated domestic sewage effluent of Al-Bayda city in Jabal Al-Akhdar for irrigation on soil chemical properties and the growth of maize ( Zea mays L.), barley ( Hordeum vulgare L. ) and alfalfa ( Medicago sativa L.) in pot experiment under semi–controlled conditions . The two soils are representing forest soils ( Lithic Rhodoxeralfs ) northern part of Jabal Al-Akhdar and pastoral soil ( Lithic Xeric Haplocalcids) southern part of Jabal Al-Akhdar. Groundwater and the untreated sewage effluent of Al-Bayda city were used for irrigation. Two irrigation levels were evaluated based on plant vegetative and root biomass as well as their nutrient and toxic elements content. Each treatment was imposed in triplicates as in Complete Randomized Block Design (RCBD). The data were analyzed statistically by utilizing the software (Genstat ). The impact of irrigation practices on the chemical properties of the two soils were also evaluated . The results of this study has revealed that sewage water contains good concentrations of essential plant nutrients; including nitrogen, phosphorus, potassium, calcium, magnesium, copper and zinc, In addition some pollutant elements such as nickel and lead. Their concentration of the polluting elements was within the accepted range for agricultural use according to local standards as well as Food and Agriculture Organization ( FAO, 1976) and the United States Environment Protection Agency ( USEPA, 1993 ) guidelines. The wastewater was also chacteristized by its low content of soluble salts . The use of the domestic wastewater affected the two soil’s chemical properties positively even though the time span of this study was limited. A significant increase in available phosphorus, total nitrogen, soil organic matter content and cation exchange capacity was observed. The irrigation with ground water or wastewater caused a marginal change in soil рH and soluble salt contents in the two soils . Results has indicated higher plant root weights were produced by crops included in this study when irrigated with the untreated effluent as compared to groundwater. This seems to enhance plant ability to absorb more water and plant nutrients as well as pollutant ions from the soil, which were further reflected positively on wet and dry weight of vegetative parts of these crops and their contents from nutrients such as phosphorus, potassium and calcium . The use of the untreated effluent for irrigating maize, barley and alfalfa led to an increase in nickel, copper, lead and zinc concentration in the two soils and plants tissues, but their concentrations remained within the local and international permissible limits. The concentrations of pollutant elements were highest in the roots of these plants. This reflects important plant selectivity for absorbing ions, and appreciable control for their translocation to the upper plant parts through accumulation in their root systems. This study indicated that the utilization of untreated domestic effluent of Al-Bayda city even for supplementary irrigation of maize, barley and alfalfa under semi controlled conditions and for one season has resulted in significant increase in weight of the vegetative part production of these crops. Moreover, there is no increase in the concentration of pollutant elements in plant tissues that may pose risks to human or domestic animals were detected. Moreover, the use of this nontraditional water source for irrigation under the prevailed conditions of this study has improved the two soil’s chemical properties. [ It seems imperative to pursue this annovative investigation for further evaluation on long term effects of this water resource under field conditions. The chemical make-up of Al-Bayda domestic effluent appear to be acceptable. This will enhance the potential for its use to improve crop production, combating desertification and rehabilitating rangeland. In addition to improvement of the natural ecosystems distinguishing Jabal Al-Akhdar, the adoption of a well planned strategy in this direction will no doubt result in lifting some of the pressure on the limited fresh groundwater of the region.