Fermented Nettles: Bioactive Profile and Seasonal Variability

In traditional horticulture, fermented nettles (FN) enhance plant growth and resilience. However, their precise mode of action remains unclear. This study aims to characterize the bioactive profile of FN and to evaluate their potential as biostimulants beyond organic fertilizers. For this purpose, FN samples were prepared from Urtica dioica L. harvested in different seasons and analyzed by mass spectrometry (ICP-MS, LC-MS/MS, and GC×GC-MS), electrophoresis, and spectrophotometry to quantify macro- and micronutrients, nitrogen compounds, phytohormones, antioxidant capacity, enzyme activities, and microbial viability. The results show that FN are rich in essential nutrients (N, K, Ca, Fe, and Zn), hydrolytic enzymes (proteases, glycosidases and phosphatases), and phytohormones (auxins, cytokinins, gibberellins, abscisic acid, and salicylic acid). FN contain volatile compounds with potential antimicrobial effects, in addition to strong antioxidant properties. The monitored parameters support the dual role of FN as fertilizers and biostimulants, suggesting that FN act synergistically through nutrient enrichment, enzymatic degradation of macromolecules, hormonal signaling, and microbial priming. Based on our data, particularly because of the highest microbial viability and enzyme activities, the summer FN seem like the most suitable option. Moreover, the seasonal variability in composition highlights the importance of timing the harvest to optimize FN efficacy in sustainable agriculture.