“Using marginal quality water for an energy crop in arid regions: Effect of salinity and boron distribution patterns”

Abstract

Marginal-quality water, such as recycled wastewater (RWW), is now commonly used for irrigation in regions with limited freshwater resources. However, the practice can pose a threat to soil conservation due to the high soluble salts content present in such water resources. Long-term sustainability of use therefore requires a sound understanding of the soil salinity distribution and appropriate selection of both crop and irrigation system. Jatropha curcas L. (JCL) has been proposed as an ideal candidate for biofuel production in arid and semi-arid regions under RWW irrigation, particularly on account of its high salt tolerance. The present study evaluates the evolution and distribution of soil salt and boron (B) as a potential constraint on the sustainability of a “JCL-RWW” farming system on the island of Fuerteventura (Canary Islands-Spain) during five years of cultivation under surface drip irrigation (SDI) and sub-surface drip irrigation (SSDI). The results indicate that, under high evaporative demand conditions, SSDI does not appear to offer any advantage over SDI in terms of soil salt distribution. The results indicate that irrigation with RWW has increased boron concentrations by a factor of 7 (soil Typic Torrifluvents; TT) and 5 (soil Typic Haplocambids; TH), respectively, in relation to the control soils. Irrigation with RWW has also led to a rise in soil salinity reached values of 59 and 81 dS m ⁻¹ for soil TT and TH, respectively. For soil TT and TH SAR, reached values of 56 and 80 (meq L⁻¹)^0.5, respectively as a result of irrigation with RWW. Although salinity does not seem to be a factor limiting crop production, irrigation with RWW can accelerate natural soil salinization processes in regions where desertification is of particular concern.

Introduction

Irrigation plays a fundamental role in crop production and agricultural development in many arid and semi-arid regions, despite their limited freshwater sources (Bezborodov et al., 2010). Competition for water from agriculture, households, industry and the environment gradually intensifies as the population increases and the effects of climate change becomes more pronounced (Ayars et al., 2015). Among the different water conservation practices proposed, the use of non-conventional water resources-particularly recycled wastewater (RWW)-for agricultural irrigation is becoming common practice in such regions (Acosta-Motos et al., 2014).

The benefits of RWW use include conservation of freshwater reserves, less discharge of pollutants into water bodies, and supply of nutrients to crops due to the organic matter and nutrients present in this water (Belaid et al., 2009, Muyen et al., 2011, Khajanchi-Lal and Minhas, 2015). However, the quality of RWW is often considerably worse than that of conventional water resources (e.g. groundwater) and its use can pose a threat both to soil conservation and to the sustainability of the agricultural systems (Bedbabis et al., 2014).

A frequent negative aspect of RWW is the content of dissolved solids (chlorides, sodium, boron (B) and heavy metals), which tend to be present in large concentrations in urban and industrial wastewater and are not eliminated by conventional treatments (secondary) (Belaid et al., 2009). In arid and semi-arid regions, this impact can be exacerbated by the high evaporative rate, which concentrates the mineral components supplied by the irrigation water (Leal et al., 2009). Numerous studies have linked physical and chemical degradation in soils with the short and medium-term application (2–20 years) of RWW irrigation in arid regions (Lado and Ben-Hur, 2009). The negative effects cited include salinization (Xu et al., 2010, Morugán-Coronado et al., 2011), build-up of trace elements (B) (Lucho-Constantino et al., 2005), sodification/alkalization (Morugán-Coronado et al., 2011, Levy et al., 2014) and concomitant changes in soil structure (Sou/Dakouré et al., 2013). In view of the foregoing, appropriate selection of crops, which are tolerant or semi-tolerant to saline conditions, and of irrigation systems that guarantee the long-term sustainability of the crop cultivation in terms of soil salinity are necessary (Malash et al., 2008, Chen et al., 2010).

Drip irrigation (DI), both surface (SDI) and sub-surface (SSDI), is considered one of the most adequate techniques for crop irrigation in arid and semi-arid regions (Selim et al., 2013b). The principal advantages attributed to the DI are its ability to distribute water uniformly, control the amount of water applied and reduce evaporation and percolation (Wang et al., 2011). However, the continuous application of marginal-quality water using drip irrigation systems causes a build-up of salts in or around the root zone (Ayers and Wetscot, 1985, Oron et al., 1995). Soil salt distribution patterns play a key role in the amount of plant available water and depend on diverse factors such as the irrigation system design (SDI vs SSDI), the soil properties (texture, infiltration rate, water retention capacity), water and fertilization management (volume, frequency and quality of the water applied) and crop type (Belaid et al., 2009, Souza et al., 2009, Chen et al., 2010, Wang et al., 2011). Adequate DI management and design require a comprehensive understanding of salt accumulation and distribution patterns (Chen et al., 2014). However, as yet these have received insufficient attention, especially under SSDI. The study and knowledge of the aforementioned salinity patterns could be important when marginal-quality waters are used for planning irrigation with support irrigation system (e.g. sprinkle irrigation) in order to prevent the build-up of salts (Salvador and Aragües, 2013).

Jatropha curcas L. (physic nut or purging nut; Euphorbiaceae family) is a non-edible oleaginous plant that produces seeds for 50 years with a high oil content (∼25–35%). It is a drought-resistant tree easy to establish in different types of soils even on gravelly and sandy soils (Silva et al., 2010). JCL is considered one of the best candidates for the biofuel production in arid and semi-arid regions under RWW irrigation, particularly in view of its moderate-high salinity tolerance potential (Gimeno et al., 2012), up to 12 dS m⁻¹ in irrigation water and 10 dS m⁻¹ in soil salinity (Dagar et al., 2006, Sharma et al., 2008) although yield under these conditions is not documented. However other authors classifies JCL as a crop sensitive to salinity (Niu et al., 2012, Rajaona et al., 2012). In addition to biofuel production, JCL has been used to generate environmental benefits such as improving soil quality, preventing soil erosion, and promoting marginal land reclamation and soil remediation (Wani et al., 2012).

The island of Fuerteventura, which suffers intense desertification on account of its extreme aridity and land use and water resources overexploitation, has hosted since 2010 a research project aimed at evaluating the potential of JCL for harnessing abandoned farming soils and the high volume of RWW from the local tourist industry (Dorta-Santos et al., 2015). The present research evaluates the evolution and distribution of soil salinity and B as a potential constraint to the sustainability of the “JCL-RWW” agricultural system on Fuerteventura during five years of cultivation. The specific objectives of the study were i) to compare the distribution patterns of salts and B under SDI and SSDI in soils of different textures and ii) to evaluate the influence of salt and B accumulation and distribution on leaf mineral composition and JCL yield in terms of seed production. The results of the study will be directly applicable to RWW management RWW management, including desertification control strategies in extremely arid territories. They will also provide useful information concerning the potential benefits and limitations of JCL as a bioenergy crop in such territories.