Seasonal Dynamic of NO₃⁻ and K⁺ in a Citrus Crop Irrigated by Different Water Qualities

This study evaluated the effect of (i) irrigation water source: transfer (TW) and reclaimed water (RW), and (ii) crop phenological stage: winter rest (WR), flowering-sprouting (FS), and fruit growth (FG), on NO₃⁻ and K⁺ dynamics in soil and leaf of a citrus crop. The experiment was carried out during the 2018 and 2019 growing seasons on adult ‘Star Ruby’ grapefruit trees (Citrus paradisi Macf.). The concentration of both nutrients was periodically measured in soil and leaf samples and continuously monitored in the soil soluble fraction with nutrient sensors. Moreover, soil NO₃⁻ leaching was indirectly estimated by the periodic measurement of the leaf enrichment in ¹⁵N isotope (¹⁵N_leaf). The two water sources showed a different nutrient loading. Thus, NO₃⁻ and K⁺, were approximately 5 and 7 times higher, respectively, in the RW. Furthermore, the average contents of NO₃⁻ and K⁺ in the soil samples from the RW treatment were 10.1 and 19.7%, respectively, higher than in TW, with the highest soil NO₃⁻ leaching observed in RW treatment. In line with this, the mean contents of NO₃⁻ and K⁺ in the leaves from the RW treatment were 106.9 and 30.4% higher than the TW ones. As for the different phenological stages, in the FG stage, the lowest concentrations of NO₃⁻ in the soil samples and the highest in the leaf tissue were observed after a high soil leaching event. In this study the nutrient sensors measurements varied according to the dynamic of NO₃⁻ and K⁺ in the soil samples. The use of RW promoted an accumulation of NO₃⁻ and K⁺ in the soil and leaves of grapefruit trees, but also enhanced soil NO₃⁻ leaching, indicating that the proper management of this water source is necessary to avoid soil contamination. The mobilization of NO₃⁻ and K⁺ from soil to leaf was the highest in the FG stage, to ensure fruit development and vegetative growth.