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^-1 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^-1. While slight inhibition was observed at 1000 mg kg^-1 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.