Influence of humic acids on the interaction between soil bacterial communities and nutrient-based nanoagrochemicals

Nanoagrochemicals (NAGs) are emerging as an efficient alternative to conventional fertilizers, but their ecotoxicity on soil microbial communities is a concern and remains insufficiently characterized, particularly under realistic soil chemical conditions. This toxicity is strongly modulated by humic acids (HAs). The objective of this study was to evaluate the effect of six NAGs, nano-hydroxyapatite (nH), magnesium phosphate (nMg), potassium sulfate (nK), hematite (nFe), copper oxide (nCu), and titanium oxide (nTi) on bacterial growth using a bacterial suspension extracted from an agricultural soil, and to determine how the addition of HA (0, 0.05, 0.10, and 0.20 mg L -1 ) modifies this toxicity. The [ 3 H]-leucine incorporation method was used to measure bacterial growth inhibition, estimating the toxicity index (log IC 50 ). In the absence of HA, all NAGs showed dose-dependent toxicity, with the following sequence: nH > nMg > nCu >> nK > nTi > nFe. The addition of HAs significantly reduced the toxicity of all NAGs, shifting the dose-response curves to higher concentrations. This mitigating effect varied depending on the NAG: for nK, nFe, and nTi, toxicity was practically eliminated even at the lowest HA concentration (0.05 mg L -1 ); for nH, nMg, and nCu, toxicity was attenuated, although a dose-dependent response remained. Humic acids mitigated the toxicity of NAGs, likely through the formation of an eco-corona that reduces direct nanoparticle–bacteria interactions. These findings highlight the need for ecotoxicological assessments of NAGs in soils to explicitly consider interactions with HAs to avoid overestimating their environmental impact. • Nanoagrochemicals exert type- and dose-dependent toxicity on soil bacteria. • Humic acids strongly reduce nanoagrochemical toxicity to soil bacteria. • Humic acid eco-coronas attenuate nanoagrochemical-bacteria interactions. • Eco-corona formation alters nanoagrochemical interactions at the cell surface. • Nanoagrochemical–humic acid interaction reduce effective exposure to bacteria.