Irrigation-induced nitrate losses assessed in a Mediterranean irrigation district
Introduction
Rainfed agriculture dominates the Mediterranean region in terms of crop area acreage, but irrigated agriculture has a disproportionate influence on productivity of the major crops grown in this region. Mediterranean countries, including Turkey, have significant areas of both rainfed and irrigated agriculture which is characterized by two contrasting seasons: mild and relatively wet winters and hot and dry summers (Kassam, 1981) with limited rainfall (300–650 mm). While dryland cropping is practiced in winter and early spring, irrigation is needed in late spring and summer (Ryan et al., 2009).
With increasing land use and intensive crop production, rainfed and irrigated agriculture cannot be sustainable without fertilization. Nitrogen (N) fertilizers are extensively used in the Mediterranean region, and N consumption worldwide and in the region is increasing. Thus, N fertilizer use increased from 50 Mt in 1977 (Bumb, 1995) to 101 Mt in 2006 (FAO, 2008) worldwide. Given the attention to escalated dynamics of N under irrigation, mismanagement of these two major agricultural inputs has led to increasing concerns of environmental N pollution, i.e., NO3 accumulation in water resources and eutrophication in rivers, lakes and coastal areas (Galloway and Cowling, 2002). Therefore, on-site and off-site nitrate pollution is also a common problem in the Mediterranean area, and should be considered accordingly.
The extent to which N pollution can occur depends on many factors, particularly fertilizer characteristics and its subsequent reactions in the soil, application dates, amount of precipitation as well as intensity, irrigation management and disposal of irrigation return flows (IRFs) (Barros et al., 2012a). Ammonium fertilizers and urea can be easily transformed to nitrate especially in dryland soils (Li et al., 2009). A large portion of NO3 is taken up by plants for metabolic processes while much of it is either leached from the topsoil to deeper horizons and ultimately to surface and groundwater (Ma, 1992) or washed off by runoff. High nitrate concentrations in Europe, USA, China (Pratt, 1984, Laegreid et al., 1999, Luo et al., 2008) and northeastern Australia (Keating et al., 1996) have been found in groundwater systems dominated by intensive agriculture with high N fertilizer application rates. Considering the low N fertilizer recovery (generally less than 50% and less than 33% in cereals) (Hardy and Havelka, 1975), the remaining portion of the applied N is a potential source of pollution for both the atmosphere and water resources.
Irrigation return flows have been recognized as the major diffuse or non-point pollution contributor to surface and groundwater bodies (Aragüés and Tanji, 2003). Water quality of IRFs is predominantly affected by salt and NO3 concentrations (Barros et al., 2012b). Nitrate concentrations up to 250 mg L−1 were recorded for shallow aquifers, but the concentration rarely reached 50 mg L−1 for surface waters of irrigated areas, the European limit for waters intended for human consumption (European Union, 1998). High NO3 concentrations have been measured in IRFs, especially during N fertilization to corn (Barros et al., 2012a). Seasonal patterns of N fluxes are influenced by fertilization, irrigation scheduling and rainfall distribution. For example, Isidoro et al. (2006) found that 75% of the total NO3-N load was exported after the irrigation season in an irrigation district in Spain. Consequently, optimizing N fertilization, irrigation scheduling and irrigation efficiency were shown to reduce N exports to drainage water (Cavero et al., 2003, Isidoro et al., 2006, Quemada et al., 2013) and groundwater (Thorburn et al., 2003).
European legislation generally addresses concentration levels of contaminants in waters (European Union, 1998). In addition, most works measure NO3 concentrations in IRFs, few studies have quantified NO3 loads in IRFs and their relationships with soil factors and agricultural management characteristics (Cavero et al., 2003, Isidoro et al., 2006, Barros et al., 2012a) due to the fact that it has been not an easy task to quantify IRFs purely generated by an agricultural catchment. The aim of this work was to analyze irrigation-induced NO3 losses in the Akarsu Irrigation District (AID) of southern Turkey and to assess existing irrigation and fertilization management practices aimed at minimizing such losses.
Section snippets
Location
The study area is located in the Mediterranean coastal region comprising the most intensively cropped area of Turkey. The Akarsu Irrigation District covers an area of 9495 ha (Fig. 1) within the Lower Seyhan Plain, and lies between 36°57′ and 36°51′N latitude and 35°40′ and 35°29′E longitude. The area has been irrigated for over 40 years with conventional irrigation and drainage infrastructures. The drainage waters flow through open ditches along the downstream areas and discharge into the
Precipitation, irrigation, crop evapotranspiration and irrigation return flows
Values of net irrigation diverted to the District, precipitation, crop evapotranspiration and net IRFs solely generated from the District for the 2007–2010 HY, IS and NIS are summarized in Table 2. The annual P values were typical for the study area located in the Mediterranean region, except the high value of 856 mm in 2009. Precipitation was much higher in NIS than in IS, with the rainy season starting in late October and ending in late May (Fig. 2). Therefore, cereals grown in winter and corn
Conclusions
Irrigation and fertilization are poorly managed in Akarsu irrigation District. Wheat, corn, citrus and cotton are the main crops of the region, with a production area almost similar over the years. Based on the actual irrigation and fertilization practices in the District, these mismanagements will continue in the near future. Monitoring the water inputs and outputs as well as the concentrations and loads in IRFs are not only important approaches in terms of evaluating the potential NO3
Acknowledgments
Authors gratefully acknowledge funding for this work by the European Commission reseach project in the context of FP6 with project acronym QUALIWATER (Project No. INCO-CT-2005-015031), IntenC project acronym MedSalin (TUBITAK and German-BMBF, TUBITAK-108O582) and partial support was received from Cukurova University Academic Research Projects Unit (Project No. ZF2006KAP1 and ZF2009KAP3). We are also thankful to Dr. Ramon Aragües at Departamento de Suelos y Riegos, CITA, Zaragoza, Spain for his
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Formerly ICARDA, Aleppo, Syria.