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The olive (Olea europaea) oil industry is experiencing a transition from traditional rain-fed to intensively managed irrigated orchards. Moreover, since fresh water resources in typical olive cultivation regions are often scarce, alternative water sources, often marginal in quality, are increasingly used for the irrigation of olives. Utilization of reclaimed wastewater (RWW) increases the susceptibility of olive trees to osmotic stress and augments the potential of groundwater contamination by nutrients and salts. The objective of this study was to evaluate tree growth and productivity and to quantify nitrate and chloride (Cl) losses in an olive orchard irrigated with RWW. A four year field study compared two olive tree varieties, ‘Barnea’ and ‘Leccino’, and three treatments: (i) fresh water application with commercial fertilizer at recommended rates (Fr), (ii) RWW application with commercial fertilizer at recommended rates (Re) and (iii) RWW application with commercial fertilizer reduced according to the amounts of the nutritional constituents in the wastewater itself (Re−). No significant difference was found in nutrient and mineral accumulation in diagnostic leaves and no differences in trunk growth, fruit production or oil yields were observed between treatments. In spite of this, lower measured Cl concentration in diagnostic leaves of ‘Barnea’ and higher Cl concentrations in its root zone relative to ‘Leccino’ suggested that ‘Barnea’ trees better controlled Cl uptake. While similar amounts of water were applied, the Re and Re− treatments loaded the soil profile with 1.75 times more Cl then the Fr treatment. Additionally, significantly more nitrates were transported out of the root zone in the Re treatment compared to Fr and Re− for both cultivars. We conclude that RWW used for irrigating olive oil orchards had no effect on vegetative growth and productivity but increased salt loads into and beyond the root zone. The nutritional constituents in the RWW used to irrigate olives should be accounted for in order to increase fertilizer application efficiency and minimize the transport of nutrients into groundwater.

A nutrient management plan (NMP) field experiment was conducted to investigate the fate of nitrogen (N), phosphorus (P), potassium (K) and salts in a semi-arid environment (San Jacinto, CA). Our mechanistic approach to study NMP performance was based on comprehensive measurements of water and N mass balance in the root zone. A cereal crop rotation (wheat-rye hybrid to sorghum, Triticum aestivum L.–Secale cereale L. to Sorghum bicolor L. Moench) that does not fix atmospheric N was employed during 2007, whereas a legume crop (alfalfa, Medicago sativa L.) that forms nodules to fix N was used in 2008. Blending (2007 and 2008) and cyclic (2007) dairy wastewater (DWW) application strategies (no statistical difference in 2007) were implemented to meet crop water and N uptake. The high content of salts in DWW and accurate application of water to meet evapotranspiration (ET) yielded salt accumulation in the root zone. Leaching these salts after the fallow period resulted in the flushing of nitrate that had accumulated in the root zone due to continuous mineralization of soil organic N. This observation suggested that a conservative NMP should account for mineralization of organic N by (i) leaching salts following harvests rather than prior to planting and (ii) maintaining soils with low values of organic N. For the wheat-rye hybrid–sorghum rotation, losses of nitrate below the root zone were minimal and the soil organic N reservoir and P were depleted over time by applying only a fraction of the plant N uptake with DWW (28–48%) and using DWW that was treated to reduce the fraction of organic N (3–10%), whereas K accumulated similar to other salts. Conversely, with alfalfa approximately 15% of the applied N was leached below the root zone and the soil organic N increased during the growing season. These observations were attributed to fixation of atmospheric N, increased root density, and applying a higher fraction of plant N uptake with DWW (76%). Collectively, our results indicate that NMPs should accurately account for water and nutrient mass balances, and salt accumulation to be protective of the environment.