Aminochelates as Dual Micronutrient Carriers and Biostimulants: Modulating Rhizosphere Enzyme Hotspots and Root–Microbe Interactions in Sunflower

The mechanisms by which organic chelates influence rhizosphere enzyme dynamics and microbial function are poorly understood due to a lack of spatial visualization. To address this gap, we evaluated the effects of iron (Fe) and zinc (Zn) aminochelates on the spatial distribution of β-glucosidase (βG) and leucine aminopeptidase (LAP) activities in the rhizosphere of sunflower (Helianthus annuus L.) using a novel integration of in situ zymography and biochemical assays. Glycine- (Gly) and methionine-based (Met) Fe and Zn aminochelates were synthesized and applied in rhizobox experiments, with untreated soils serving as controls.[Fe(Gly)₂] and [Zn(Met)₂] significantly enhanced βG (7–21%) and LAP (72–120%) activities, while expanding enzymatic hotspot zones by 270–450% and 78–251%, respectively. Kinetic analyses showed that [Zn(Met)₂] achieved the highest catalytic efficiency (Ka) and maximum velocity (Vmax, p < 0.01), while also increasing basal respiration (+ 42.3%) and microbial biomass C (3-fold) relative to the control. Root length and surface area were strongly correlated with hotspot intensity (Pearson’s r = 0.75–0.94), reflecting tight root–microbe feedbacks. Network analysis further revealed that [Fe(Met)₂] and [Zn(Met)₂] promoted the highest system-wide coordination, linking microbial enzyme activity with root architecture (Mantel’s r = 0.56–0.68, p < 0.05). By enhancing microbial activity, expanding biologically active zones, and improving root foraging traits, aminochelates demonstrated a dual functionality as both micronutrient carriers and physiological stimulants. These results establish methionine-based aminochelates, in particular, as promising next-generation biostimulants that can improve soil fertility, optimize nutrient cycling, and support resilient crop production systems.