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 NO 3 − 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 NO 3 − leaching was indirectly estimated by the periodic measurement of the leaf enrichment in 15 N isotope ( 15 N leaf ). The two water sources showed a different nutrient loading. Thus, NO 3 − and K + , were approximately 5 and 7 times higher, respectively, in the RW. Furthermore, the average contents of NO 3 − 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 NO 3 − leaching observed in RW treatment. In line with this, the mean contents of NO 3 − 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 NO 3 − 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 NO 3 − and K + in the soil samples. The use of RW promoted an accumulation of NO 3 − and K + in the soil and leaves of grapefruit trees, but also enhanced soil NO 3 − leaching, indicating that the proper management of this water source is necessary to avoid soil contamination. The mobilization of NO 3 − and K + from soil to leaf was the highest in the FG stage, to ensure fruit development and vegetative growth.