Long-term effects of combining reclaimed and freshwater on mandarin tree performance

The current scenario of water scarcity leads to a seek for new irrigation strategies that maintain agricultural productivity while reducing the pressure on freshwater resources, especially in the Mediterranean region, which has a structural deficit of water sources. In this study, the effects of irrigation with water from different origins were assessed over six consecutive years in soil salinity indicators, tree physiology and yield of mandarin trees in south east Spain. The following treatments were considered: i) freshwater from the “Tagus-Segura” water-transfer canal (TW or Control); ii) reclaimed water from a wastewater-treatment plant (RW); iii) irrigation with TW, except in stage II of fruit development when RW was used (TW_c); and iv) irrigation with RW except in stage II when TW was used (RW_c). Soil salinity indicators increased in all treatments with respect to Control, except for TW_c in winter. However, this increase only led to reductions in net photosynthesis and stomatal conductance in the RW treatment, while gas exchange parameters of trees from TW_c and RW_c were not negatively affected. Tree vegetative growth was only affected in the long-term, with trees from the RW_c treatment having greater canopies. Yield was increased in the TW_c and RW_c treatments by 23.1 % and 29.5 %, respectively; while it was reduced by 23.5 % in RW when compared to TW. Likewise, differences among treatments in fruit quality traits were also detected. Our results suggest that combining water from different sources could be a viable alternative for irrigating mandarin trees under semiarid conditions with savings of approximately 50 % of fresh water, although the sustainability of soil health should be ensured. In addition, further research is needed to adapt these strategies to other species and cultivar-rootstock combinations.