Crop development based assessment framework for guiding the conjunctive use of fresh water and sewage water for cropping practice—A case study
Graphical abstract
Introduction
Sewage water, including raw, primary and secondary-treated effluents, have already been widely reused for agriculture and horticulture (Emongor and Ramolemana, 2004, Singh et al., 2012, Al-Khamisi et al., 2013). The reused sewage water generally exerts both positive and negative effects on crops in crop–soil–sewage system; this is especially true for the cases of nutrient supplement and environment stress. Sewage irrigation can enhance the concentrations of nitrogen and phosphorus and other nutrient in soil (Thapliyal et al., 2013), while some water pollutants such as salts, nitrates, heavy metal and pathogens, etc., often accumulate in soil and lead to phytotoxicity and potential secondary environment pollution (Jordan et al., 2001, Friedman et al., 2007, Qadir et al., 2010, Xu et al., 2010). The principal mechanism for overcoming any difficulties relating to sewage irrigation is the pre-treatment of sewage water. Reverse osmosis membrane filtration is just one of the effective techniques to remove salt molecules and ions, but it is far too expensive to be economically viable for crop irrigation (Toze, 2006). Therefore, a cost-effective strategy to relief the sewage-deriving stress is recommended with conjunctive use of reclaimed water and groundwater (Al-Khamisi et al., 2013). In doing so, refined management is crucial to improve crop yields. For example, those management approaches, such as transplanting older seedlings, applying balanced nutrients, etc., have great potential for enhancing yield in stress-prone rainfed coastal areas and will help sustain rice yield while ensuring sustainability of the cropping system (Sarangi et al., 2015). However, sewage water is often applied directly to crops, meanwhile fertilizers are also applied empirically for high crop yield in sewage irrigation practice, and it can thus easily lead to unbalanced or excessive nutrient supplement and increasing possibility of leaching (Blum et al., 2013). In order to refine the engineering management of conjunctive use of sewage and fresh water for cropping irrigation, it is important for us to establish an in-situ assessment method for fixing some key engineering parameters, especially sewage dilution ratios (SDRs) and fertilization formula based on crop development stages.
On one hand, SDRs can be easily set through sewage-deriving stress test. Many methods have been developed to test the environment stress. Bioassay methods are popularly used because of its ease of use in getting situ and direct results. Regarding phytotoxicity bioassay, seed germination, early seedling growth and root elongation tests have been well developed into phytotoxicity bioassay methods and adopted by regulatory agencies (Gong et al., 2001, Plaza et al., 2005). Recently crop seed germination test has been reported as a bioassay to assess the phytotoxicity caused by wastewater (Rivera et al., 2013). On the other hand, fertilization formula can be prescribed through crop nutrition diagnosis. There have been three main methods developed for assessing the crop nutrient deficiency, and the Diagnosis and Recommendation Integrated System (DRIS) method is especially claimed to have certain advantages over other tools (Srivastava and Singh, 2008). The DRIS technique is based on a comparison of calculated elemental ratio indices with established norms, which are defined as the average values of foliar nutrient ratio pairs for high yield stands (Beaufils, 1973). DRIS identifies nutrient constraint early in crop growth and allows sufficient time for remediation of identified problem right in the same growing season (Walworth and Sumner, 1987). Apparently, assessment and management of toxicological risks is the key step when establishing sewage irrigation based production (Salgot et al., 2006). Currently, most of guidelines and risk management on agricultural sewage irrigation focus mainly on food and environmental safety (Levy et al., 2010). The above-mentioned biological and chemical testing methods can only assess the effects separately and thus can’t provide cropping-specified data for analysis. This study intends to establish a crop development based assessment framework for conjunctive use of sewage and fresh water for agriculture irrigation.
Most of the species of the Cruciferae family are well known as very important agriculture and/or horticulture crops, they are therefore of particular interests for agriculture environment risk evaluation. Kale (Brassica oleracea L. var. acephala D.C.) is one of them which is commonly used as an important potted flower and as a favorable vegetables in “baby” green salad due to its high levels of lutein and β-carotene, which is good for human health (Lefsrud et al., 2007). Its seeds are easy to obtain and germinate. The crop kale will be employed as material to carry out the experiments illustrating the assessment framework. The overall objectives of this study are: (1) To develop a crop development based framework to diagnosis crop stress and balance fertilization, and further to give some in-situ risk management recommendation for conjunctive use of sewage and fresh water in agricultural irrigation practice; (2) To verify the feasibility of the bioassay through a case study with crop kale.
Section snippets
Assessment framework
Cropping management strategy generally changes with crop development stages, such as germination, seedling and maturity. Sewage irrigation may bring about phototoxicity to crop plants in all stages, while potential crop nutrient deficiency often occurred in maturity stage. Three bioassays, germination test, seedling test and cropping test, are designed for assessing crop phytotoxicity in the above three crop development stages, and a nutrient diagnosis method is established based on DRIS for
Indices for stress diagnosis
As shown in Fig. 2, raw sewage water (100% SDR) treatment significantly enhances the crop NCSI values than that of the diluted sewage water (30% and 60% SDR) and tap water (0% SDR) in all the three tests. This result suggests that sewage water have brought about phytotoxicity to the crop in three development stages, and the sewage-deriving phytotoxicity can be relieved effectively by diluting the sewage water with fresh water. Under the same SDR level, the NCSI values of three tests change in
Assessing phytotoxicity and selecting cost-effective SDR
Linear CNSI curves during germination and seedling stages suggested that the sewage-deriving stress presents SDR-response phytotoxicity, and the phytotoxicity was sensitively determined by germination test and further confirmed by seedling test. Although the crop kale had more strong tolerance ability against sewage-deriving phytotoxicity when growing more mature, it is easy to get damaged under the raw sewage water during germination and seedling stages. In cropping test, CNSI values appeared
Conclusions
This study established an assessment framework for guiding the conjunctive use of sewage and fresh water for cropping irrigation, in which were designed three bioassays to assess the sewage-deriving phytotoxicity and one diagnosis of crop nutrition to optimize the fertilization formula based on crop development stages in agricultural sewage irrigation practice. The results of the case study with crop kale (B. oleracea var. acephala) demonstrated the sewage-deriving phytotoxicity was sensitively
Acknowledgements
The authors would like to thank Mr. Liu Jun-wei, Senior Engineer of Nanhu Park, Xiamen, China, for providing the experiment facilities and his guidance, and the Xiamen Secondary Sewage Treatment Plant (XSSTP) for the water quality data of sewage and tap water of the experiment. The financial support of China Scholarship Council([2013] 3050) is gratefully acknowledged.
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