Moderate Nitrogen Rates Applied to a Rainfed Olive Grove Seem to Provide an Interesting Balance between Variables Associated with Olive and Oil Quality

Nitrogen (N) fertilization has been extensively studied all over the world, not only due to its impact on plant growth and yield, but also on product quality. Olive oil is a natural product highly valued by the markets, due to its role in the Mediterranean diet, but only when its quality is above legally established standards. Thus, it is important to continue to study how cropping practices can impact the quality of olive oil in order to fine-tune the guidelines issued to producers. The objective of the three-year study was to assess the effect of the application of 0, (N0), 40 (N40) and 120 (N120) kg ha−1 of N in a rainfed olive grove on variables associated with fruit biometry and the maturation index, polyphenols in olives and olive oil, the evolution of the quality of the oil over time and sensory analysis. The application of N prolonged the period of coloration, delaying the maturation of the fruits, particularly in the higher doses of N applied. N application changed the phenolic composition and the antioxidant capacity of the fruits and olive oil, but without a clear trend among years. In general, the high N rate negatively affected the quality of the oils, reducing the oxidative stability and bitterness of the oil when compared to N40 treatment; this is due to the lower content of phenolic compounds that have a radical scavenging capacity. Excessive N application (N120) increased the degradation of the oil quality, as it led to an increase in the K232 extinction coefficient. Moderate N rates (N40) seem to be a good compromise to guarantee the quality and stability of the oil, although there was some interference from other uncontrolled environmental variables; these were probably accentuated because the olives came from a rainfed olive grove. The sensory analysis panel detected a defect (winey-vinegary flavour) in the oil from N40 treatment that is difficult to justify and will need more attention in future studies.