Comparing the impacts of drip irrigation by freshwater and reclaimed wastewater on the soil microbial community of two citrus species

The search for new water resources for irrigation is a mandatory requirement in Mediterranean agroecosystems. The impacts of irrigation with water from different origins were evaluated in the soil microbial community and plant physiology of grapefruit and mandarin trees in the south-east of Spain. The following treatments were considered: i) freshwater with an electrical conductivity (EC) of 1.0 dS m−1 from the “Tagus-Segura” water-transfer canal and well (TW); ii) reclaimed water (EC = 3.21 dS m−1) from a wastewater-treatment-plant (RW); iii) irrigation with TW, except in the second stage of fruit development, when RW was utilized (TWc); and iv) irrigation with RW except in the second stage, when TW was used (RWc). Phospholipid fatty acids (PLFAs) revealed that microbial biomass was higher in the grapefruit soil than in the mandarin soil. In grapefruit soil, TW treatment showed a lower bacterial PLFA content than RW, RWc, and TWc, while RW showed the lowest values in the mandarin soil. In grapefruit soil, β-glucosidase and cellobiohydrolase activities were greater in RW and TWc than in TW and RWc. Under mandarin, the greatest activity of these enzymes was recorded in the TWc treatment. The saline stress caused lower net photosynthesis (A) and stomatal conductance (gs) in plants of RW, RWc and TWc than in plants of TW treatment. The annual use of reclaimed water or the combined irrigation with TWc benefited microbial biomass and enzyme activities of grapefruit soil. In contrast, the microbial community of mandarin soil seemed more affected by the annual irrigation with reclaimed water.