Impact of Soil Nutrients on Chemical Composition and Antioxidant Activities of Dysphania ambrosioides Essential Oil in Southern Ecuador

Dysphania ambrosioides is a widely distributed species with a traditional use in folk medicine, but it exhibits marked chemical variability that limits its standardization. This study is the first to characterize the essential oil (EO) of three Ecuadorian populations—Arenillas (ARE), Pasaje (PAS) and Piñas (PIN)—using gas chromatography–mass spectrometry/flame ionization detection (GC-MS/FID), and to correlate its composition with edaphic properties and antioxidant activity. Chemical profiles revealed three distinct chemotypes: ARE (α-terpinene 65.35%, o-cymene 24.83% and ascaridole 3.30%), PAS (α-terpinene 56.31%, o-cymene 10.09% and ascaridole 10.84%) and PIN (α-terpinene 56.89%, o-cymene 17.07% and ascaridole 7.60%). The EO yield was low (0.030–0.064% w/w), coinciding with acidic and nutrient-poor soils. On the other hand, PAS, with its neutral soil and high nitrogen, produced the highest number of oxygenated compounds. Only PAS exhibited strong ABTS radical-scavenging activity (SC50 = 37.99 ± 1.01 µg/mL). In contrast, ARE showed weak activity (SC50 = 424 ± 1.01 µg/mL), while PIN showed moderate activity (SC50 = 112.26 ± 1.01 µg/mL), which was correlated with its high total phenol content (298.48 mg EAG/L). The 2,2-diphenyl-1-picrylhydrazyl (DPPH) activity was low in all samples. Principal component analysis (PCA) confirmed clear separation of the chemotypes, which was linked to edaphic factors such as pH, heavy metals (Cu, Fe and Mn) and organic matter. These findings suggest that edaphic conditions may modulate the chemical composition and antioxidant activity of D. ambrosioides, indicating a potential approach for the sustainable selection of plant material.