Elsevier

Ecological Engineering

Volume 79, June 2015, Pages 60-68
Ecological Engineering

Evaluating the sustainability of subsurface drip irrigation using recycled wastewater for a bioenergy crop on abandoned arid agricultural land

https://doi.org/10.1016/j.ecoleng.2015.03.008Get rights and content

Abstract

Producing biofuel from plant crops is considered a potentially viable alternative to the use of fossil fuels, as well as a means of reducing CO2 emissions. In this regard, the use of marginal soils such as abandoned farmland and non-conventional water resources, including recycled urban wastewater (RWW), is posited as a viable and sustainable system for growing such crops. The present study evaluates the sustainability of Jatropha curcas L. (JCL) irrigated with RWW in abandoned farming soils on the island of Fuerteventura (Canary Islands, Spain), which is one of the most arid parts of the European Union. The evaluation of the crop was carried out during its first 48 months and compared the effects of two irrigation systems – surface drip irrigation (SDI) and subsurface drip irrigation (SSDI) – and two irrigation doses – 75% of ETp (dose A) and 100% de ETp (dose B) – on the growth, productivity and nutritional status of JCL, as well as on the soil quality. The results indicate that SSDI does not offer significant advantages compared to SDI in terms of nutritional status, growth or productivity. However, the amount of water used for irrigation does significantly influence JCL growth (199 vs. 220 cm plant height for doses A and B, respectively) but not productivity (815 vs. 1000 kg  seed ha−1 for doses A and B, respectively). Irrespective of the irrigation system and dose applied, RWW irrigation led to a significant increase in soil salinity. For example, in topsoil (0–20 cm), average values of 42.0 dS m−1 and 6.8 mg L−1 of electrical conductivity and soluble boron were reached, respectively. The sustainability of the system under experimental conditions is discussed.

Introduction

The dependence on fossil fuels for energy has triggered two types of concern: on the one hand, concern at the environmental impact and, secondly, concern at the exhaustion of limited reserves. Hence the need to explore alternative energy sources (Mazumdar et al., 2012). In this regard, planting energy crops to produce biofuel could play an important part in the provision of clean energy (Goldemberg, 2007). The EU has set a target of producing 10% of all energy consumption in the transport sector from biofuels by 2020 (EU, 2006). To meet this target, it is reckoned that approximately 18.5–21.1 million ha of arable land will be needed (Özdemir et al., 2009). One consequence of the foregoing is that the ethical debate on “food security vs. energy security” has intensified. Competition between biofuels and food crops for land is considered one of the causes of the increase in the price of basic foodstuffs since 2007 (Ribeiro, 2013, Shortall, 2013). In an attempt to get round the debate, the use of marginal farmland for growing energy crops is posited as a viable and alternative system to produce biofuel (Reijnders, 2009, International Energy Agency, 2010).

Leaving aside the ambiguity and conflict of interests that arises in its conceptual definition (Shortall, 2013), the term “marginal land” covers areas of crop and pastureland that have been abandoned due to degradation processes arising out of intensive use and/or relocation of farming activity due to social and/or environmental factors (Campbell et al., 2008). The estimated global area of abandoned agriculture is 385–472 million ha (Campbell et al., 2008). Much lies in arid and semi-arid parts and the land in question can be of potential use for creating zones suited to sustainable biofuel production (Chávez-Guerrero and Hinojosa, 2010).

Although many energy crops can survive in arid and semi-arid regions under dryland farming conditions, irrigation is required to achieve acceptable productivity (Zema et al., 2012, Drumond et al., 2008). The use of non-conventional water resources, including recycled urban wastewater, is considered a sustainable option for such crops (Rajaona et al., 2012). The main advantage of RWW irrigation is that it ensures continuous availability of water, as well as supplying nutrients and organic matter to the soil (Toze, 2006, WHO, 2006, Duan and Fedler, 2007, Gregory, 2000, Heidarpour et al., 2007). However, it also raises numerous issues concerning possible physico-chemical degradation of the soils (e.g. salinisation, sodification in arid zones), the accumulation of pollutants in the soils and plants, and, above all, the potential health risk associated with the irrigation (Heidarpour et al., 2007). Subsurface drip irrigation has been proposed as a method to reduce this last problem (Pescod, 1992). SSDI, which enables flows of water equivalent to surface irrigation (SDI) to be applied, consists of placing the drip lines below the surface (15–20 cm) (ASAE, 1996). Among the advantages of the system the following deserve mention: (i) less contamination of the above-ground parts of the plants and/or of workers given the lack of direct contact with the wastewater and less dispersal of pollutants by wind (Oron et al., 1999); (ii) a reduction in water loss due to evaporation and elimination of surface runoff (Camp, 1998); (iii) the possible application of fertilisers using fertigation nearer the root zone (Bidondo et al., 2012); (iv) improved weed control (Patel and Rajput, 2007); (v) easier tillage operations (Salvador and Aragües, 2013); (vi) higher crop yield compared to SDI (Singh et al., 2006, Lamm and Trooien, 2003, Patel and Rajput, 2007). Among the main drawbacks, however, is the inability to detect clogged emitters before the effects become very apparent (Bidondo et al., 2012) and the need for a support irrigation system to leach salts forming on the drip lines when low-quality water is used (EC > 0.5 dS m−1) and rainfall is insufficient (<450 mm) (Salvador and Aragües, 2013).

Among energy crops, Jatropha curcas L. (JCL), a tropical oleaginous plant whose seeds are high in non-edible oil content, has adapted to growth in adverse conditions such as drought, low nutritional reserves and salinity (Silva et al., 2010) and is considered a prime candidate for the production of biofuel using RWW irrigation in arid regions (Rajaona et al., 2012). This species also offers ecological benefits such as mitigation of soil degradation and regeneration of marginal or abandoned soils (Contran et al., 2013). JCL plantations are expanding rapidly, with between 1 and 2 million ha currently under cultivation (Contran et al., 2013). However, major knowledge gaps exist as regards optimal cultivation practices, particularly the species' water needs and growth and production response (Maes et al., 2009).

The extremely arid island of Fuerteventura (Canary Islands, Spain) presents a series of factors that make it a potential location for JCL cultivation: (i) the availability of extensive areas of neglected farmland (approximately 8700 ha, 92% of the island’s arable land, has been abandoned in recent decades due primarily to the non-viability of production as a result of the lack of water); (ii) the availability of treated wastewater from the island's tourist industry (around 15 × 106 m3 of urban wastewater are treated every year, of which a mere 9% are used in farming) (FCCA, 2013). These waters present a high salinity. Due to the constant winds (average = 3.2 m s−1), which can lead to dispersal of the pollutants present in RWW applied on the soil surface, SSDI is proposed as a method to control the phenomenon. As noted above, without a support system to remove the salt from the drip lines, this type of irrigation would not be sustainable due to the extreme aridity and to the poor water quality (salts, pollutants). A compromise solution is to apply deficit irrigation with RWW, allowing moderate stress with minimum impact on yields (FAO, 2002), and to complement this with support irrigation.

The present study uses data obtained from the first 48 months of cultivation in field conditions to evaluate the potential for growing JCL using SSDI and urban RWW in abandoned farming soils on the island of Fuerteventura. Specifically, the objectives are to (i) evaluate the effects of the subsurface irrigation system on nutritional status, growth and seed and oil production, under deficit irrigation conditions, (ii) evaluate the impact of RWW irrigation on soil quality. The evolution of salts over time and depth will help decide which type and amount of support irrigation should be applied for salt leaching.

Section snippets

Study site and experiment design

The study was conducted between January 2010 and January 2014 on the island of Fuerteventura (28°45′N, 13°49′W), specifically on the experimental farm owned by the local government (Cabildo Insular). The climatic conditions during the period were as follows: average annual rainfall of 81 mm, evapotranspiration 1706 mm yr−1, average annual temperature 21 °C, average wind speed 3.2 m s−1, average radiation 19.2 M J m−2 day−1 and average sunshine of 11 h day−1 (MAGRAMA, 2013).

An abandoned farming plot of 1000

Effect of irrigation system and dose on morphometric characteristics

Fig. 1 shows the morphometric variables (plant height and stem diameter) under the different treatments of 46-months old plants. The irrigation system did have a significant effect on plant height (p = 0.000), but not on stem diameter (p = 0.132), with greater development achieved under SDI. The irrigation dose had a significant influence on both variables, with greater growth seen under dose B (100% of ETp). The interaction between the two factors (irrigation system and dose) did not prove

Conclusions

The results obtained from the experimental cultivation of JCL on the island of Fuerteventura allow the following conclusions to be reached: (i) SSDI does not offer any advantages compared to SDI in terms of seed production. However, the advantages of SSDI under the study conditions (frequent strong winds and high temperatures) may guarantee greater control of the dispersal of pollutants present in RWW; (ii) increasing the irrigation dose leads to a significant increase in growth, but not in

Acknowledgements

The present work was funded by DISA. The authors are particularly grateful to the Heterogeneous Catalysis Group of the Department of Chemical Engineering and Pharmaceutical Technology at the University of La Laguna for performing the oil extraction and supplying the oil production data. We are also grateful to the staff at the Fuerteventura government's Pozo Negro Experimental Farm, where the study was carried out.

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