Fe and Zn citrate nanoparticles: effect on soil enzyme activities and microbiome.

Iron and zinc citrate nanoparticles (NCs) were developed and evaluated as potential soil-applied plant nutrients. This study investigates the effects of seven synthesized NC formulations-comprising individual and combined Fe and Zn citrate compositions-on soil enzymatic activity and microbial diversity. The impact of NCs was assessed across three concentrations (250, 500, and 1000 mg kg of soil) and three incubation periods (30, 60, and 90 days), and was compared to commercial Fe and Zn sources, including salts, chelates, and nano-oxides. Enzyme activities measured included dehydrogenase, urease, acid phosphatase, and alkaline phosphatase. Culture-based microbiological assays were used to quantify fungi, bacteria, and actinomycetes. The results revealed that NCs generally stimulated enzyme activity and microbial populations, particularly at concentrations ≤500 mg kg. While slight inhibition was observed at 1000 mg kg in some treatments, these effects diminished over time. Strong correlations were found between microbial abundance and enzymatic responses, particularly for dehydrogenase and urease. These findings demonstrate that citrate-stabilized Fe and Zn nanoparticles exhibit low toxicity, support microbial-mediated nutrient cycling, and represent a biosafe alternative to conventional micronutrient sources for soil application.