Zeolites and Biochar Modulate Olive Fruit and Oil Polyphenolic Profile

Soil degradation processes and climate change threaten the sustainability of Mediterranean rainfed olive orchards, with repercussions on crop yield and quality of olives, olive oil and olive by-products. Using soil amendments can enhance soil fertility for sustained environmental quality and plant performance. For two years, we evaluated, under rainfed conditions, the effects of a fertilizer compound (FC) and its combination with zeolites (ZL) and biochar (BC) amendments on soil moisture, yield, fruit and oil polyphenols and quality indices. The polyphenolic composition was strongly influenced by treatments, although no effects were observed on crop yield. ZL improved soil moisture (average increase of 26.3% compared to FC), fruit fatty acid composition (increase of 12.4% in oleic/linoleic ratio in 2018) and oil quality, BC enhanced the concentrations of polyphenols with high nutritional value (average annual increase of 25.6, 84.8 and 11.6% for 3,4-dihydroxyphenylglycol, oleuropein and rutin, respectively). In contrast, olive oil from FC fruits showed the poorest quality, with oxidation and hydrolytic breakdown signals. The applied soil amendments appear to be a promising sustainable strategy to implement in olive rainfed orchards.