Chemical Forms of Nitrogen Fertilizers Differentially Influence the Content and Composition of Aroma Volatiles and Phytonutrients in Strawberry Fruits

The amount of nitrogen (N) fertilization influences the content and composition of phytochemicals in plants. However, the influence of different chemical forms of N fertilizers on the phytochemical profiles of fruits remains less known. We investigated the effect of nitrate [NO₃^–; supplied as CaNO₃], ammoniacal [NH₄⁺; supplied as (NH₄)₂SO₄], and organic-N (supplied as meat and bone meal [MBM]) fertilization on the profile of volatile compounds, primary, and secondary metabolites in the fruits of Alpine strawberries (Fragaria vesca “Reine des Vallées”) using global metabolomic approaches. The form of plant-available soil N varied as a function of fertilization, with (NH₄)₂SO₄ and MBM fertilization retaining a higher proportion of N as ammoniacal-N throughout the growing period. Leaf nitrate reductase activity was 5 times higher in CaNO₃ treatments, suggesting NO₃^– as the major N form taken up by plants in this treatment. Although ammoniacal-N fertilization resulted in lower plant biomass, the fruit yield and tissue nutrient content were similar to those of nitrate-N treatments. The plant biomass of organic-N treatment was similar to that of nitrate-N fertilization, but the fruit yield was 30% lower. Compared to nitrate-N treatment, aroma-related volatile compounds increased under organic- and ammoniacal-N fertilization, including up to a 5-fold increase in esters, a 6-fold increase in alcohols, a 3-fold increase in volatile fatty acids, and a 60% increase in organic acids. The content of primary metabolites, especially sugar, sugar alcohol, and amino acids, exhibited an opposite trend and increased in nitrate-N than under ammoniacal- and organic-N fertilization. Compared to nitrate-N, the secondary metabolites generally were higher under ammoniacal- and organic-N fertilization, with an 80% increase in the activity of phenylalanine ammonia lyase, resulting in up to 38% increase in flavones, 28% increase in flavanols, and 33% increase in anthocyanins, except for dihydroflavonols and proanthocyanidins, which decreased by 6 and 13%, respectively. The hydrolyzable tannins, including galloyl glucosides, ellagitannins, and ellagic acids, were 25% abundant under organic-N fertilization. Our results indicate that while nitrate-N generally increased the primary metabolites, organic-N and ammoniacal-N fertilization enhanced the aroma-related volatiles and secondary metabolites in strawberry fruits. Thus, precise management of the chemical form of N fertilization can be a valuable tool to improve the phytonutrient content of strawberries.