The aim of this paper is to examine the environmental effect of different chemical characteristics of Zinc (Zn) sources, assessing the transport, leaching and accumulation in the environment in sand, acidic (AS) and calcareous (CS) soils. Comparative short- and long-term studies applying ZnO sources [bulk, laboratory-synthesized and commercial nanoparticles (NP)], ZnSO₄, complexed Zn (Zn-lignosulfonate) and chelated Zn (Zn-diethylenetriamine-pentaacetate, Zn-hydroxy-ethylenediamine-tetraacetate and Zn-ethylenediamine-tetraacetate) were carried out. For all treatments and media, the medium- and long-term dissolution kinetic model was fitted to a logistic function. The applied Zn in the form of ZnO was mainly retained at the upper half (0–7.5 cm) of the columns and in very available form (ranging 40–61% of added Zn in AS and 29–31% in CS). Leached Zn reached values below 6% in AS and 3% in CS. The use of chelated Zn in both soils and complexed Zn, and ZnSO₄ in AS resulted in excessive leaching of Zn (around 70%). The amount of Zn available at the upper half of the column was less than 5%. In the CS, these fertilizers showed a higher aging than ZnO sources, negatively affecting its availability as a nutrient for crops. ZnO NP showed short- and medium-term dissolution kinetics that allow gradual nutrient release and high availability at the medium-term. These products provide an effective solution for plant nutrition while mitigating the environmental problems observed with more traditional sources related to leaching. Graphical Abstract

Zinc (Zn) is a crucial micronutrient essential for the growth and development of crops. Recently, there has been growing interest in harnessing its benefits through the application of zinc oxide (ZnO) nanoparticles (NPs) as an alternative to conventional fertilizers. Different types of ZnO NPs were synthesized in the laboratory by a co-precipitation method using different precursor metal (Zn(NO₃)₂ for ZnO-1 and ZnSO₄ for ZnO-2) and sources of hydroxyl groups (NaOH for ZnO-1 and NH₄OH for ZnO-2) or by a sol–gel method, using ZnC₄H₆O₄ (ZnO-3) or ZnSO₄ (ZnO-4) as precursor metal. This study focused on the effect of these Zn sources on the impact on lettuce (Lactuca sativa L.) cultivation under acidic and Zn-deficient soil conditions. The efficacy of these Zn sources was evaluated by measuring the lettuce fresh weight, the stem diameter, the Zn concentrations in young leaves and mature leaves, the photosynthetic pigment content (chlorophyll and carotenoid), and the overall Zn status in the soil and soil pore water. The ZnO NPs with particle sizes of 76–104 nm positively affected the stem thickness (with an increase of up to 1.4 times that of the control) and crop biofortification with Zn (up to 3.2 and 12.6 times the Zn in young leaf and mature leaf, compared to the control). The smaller ZnO NPs (ZnO-1 and ZnO-3) showed the highest concentrations of bioavailable Zn and Zn in pore water at the end of the cultivation period (with an average increase of 41% compared to larger sources), resulting in high biofortification levels in both mature and young leaves. Peak concentrations of dissolved Zn in soil pore water were observed at 18 days after planting, followed by a decline attributed to the retention of Zn in more insoluble forms in the soil. The difference in Zn concentration between mature leaves and young leaves indicated its limited mobility in the plant, with more Zn accumulating in mature leaves.

The micronutrient Zinc (Zn) is essential for the proper growth and development of crops. Zinc oxide nanoparticles (NPs) and fine particles are presented as an emerging alternative to more traditional fertilizers. In this study, the possible toxic effect of four laboratory-synthesized ZnO NPs and fine particles with different characteristics on tomato seed germination and vigor parameters was examined in comparison to bulk ZnO. Different metal precursors were used for the chemical synthesis of the particles: Zn(NO₃)₂ and ZnSO₄, for particles named NIT-. and SUL-., respectively. In addition, the synthesis process was modified to obtain coated particles (denoted as UW-, NIT-UW and SUL-UW) and washed particles (denoted as W-, NIT-W and SUL-W). These particles were applied at different toxic doses (0, 1.4, 2.8, 5.6 and 11.2 g L⁻¹). The results indicated that although the constant contact time between the ZnO particles did not affect the germination capacity of the seeds, it affected the growth of roots and hypocotyls, with a higher impact on the roots’ development. This toxicity was more evident from the lowest particle dose used, although it did not prevent radicle and hypocotyl elongation during the development period studied (14 days). The synthesized coated particles (NIT-UW, SUL-UW) generated high toxicity on radicle and hypocotyl development, and this effect was observed from the first days of contact with the particles. The observed toxic effects on radicle length were minimized by the application of bulk ZnO particles. In the case of hypocotyl growth, these minor toxic effects were observed by using NIT-W particles and bulk ZnO. The possibility of positive effects on seed germination and development (radicle and hypocotyl length) when in continuous contact with ZnO, whether in fine particles, nanoparticles, or bulk sizes, was excluded. Furthermore, no benefits on germination parameters were observed by suppressing the final washing step in the particle’s synthesis process, suggesting that particle coating did not provide any advantage for seed germination under these continuous contact conditions.

Although ZnO nanoparticles (ZnO-NPs) are promising controlled-release Zn sources, the influence of synthesis route and precursor chemistry on Zn dissolution, leaching, and bioavailability in different soils remains unclear. This study compared five laboratory-synthesised ZnO formulations to determine how formulation design affects Zn mobility and availability in sand and soils. ZnO-NPs (C-NIT-1, C-SUL, C-NIT-2, S-ACE, and S-SUL) were synthesised via co-precipitation or sol–gel routes using different precursors and hydroxide sources, yielding distinct physicochemical properties. Dissolution was assessed in aqueous media, and leaching was evaluated through column experiments with sand, acidic soil (pH 5.8), and calcareous soil (pH 8.3). Zinc in leachates was periodically quantified, and bioavailable Zn was determined using low-molecular-weight organic acids. The formulations showed marked differences in structural and surface properties, with hydrodynamic diameters of 183–603 nm and zeta potentials from − 7.4 to + 16.5 mV. SEM revealed variable aggregation, with C-NIT-2 forming aggregates up to 10 μm. Elemental analysis confirmed precursor-dependent variation in S and Zn contents. These differences resulted in contrasting Zn release patterns: in sand, C-SUL and S-ACE released Zn rapidly, whereas C-NIT-1 and S-SUL showed slower kinetics. In acidic soil, C-SUL leached 1.4 mg Zn (> 80% of available Zn), while in calcareous soil total leaching remained below 0.2 mg. Overall, the results demonstrated that Zn release and mobility are not determined solely by particle size, but by the combined influence of other key parameters, such as porosity, surface charge, solubility, synthesis conditions, as well as soil properties.

This study investigates the current application and ageing effects of various Zn sources on acidic Mediterranean soil. Two successive lettuce crops were grown in soil fertilised with 0, 15, 30, 60, and 140 mg Zn kg⁻¹ using commercial ZnO nanoparticles, Zn complex, and Zn sulphate. Plant growth, Zn biofortification, dietary implications, human health, and the soil Zn status were evaluated. Zinc bioavailability was influenced by the source, application rate, and chemical ageing. The bioavailability of Zn in the soil increased from 4.60 to 66.7 compared to the control treatment. Zinc applied in the form of ZnSO₄ was the most bioavailable form in the first year of cultivation. Advanced specialty fertilisers such as ZnO nanoparticles and Zn-lignosulfonate, along with the conventional fertiliser ZnSO₄, demonstrated a residual effect allowing effective Zn uptake by plants in the second crop. Zn concentrations in lettuce leaves were 3.33–34.6 times higher than the control treatment. Application of 30 mg Zn kg⁻¹ and higher of commercial ZnO nanoparticles, Zn complex, and Zn sulphate heptahydrate resulted in some toxicity. Higher application rates of these sources may pose a potential risk to the population, as indicated by the health risk index. These Zn sources represent a promising alternative for enhancing plant growth and providing a sustained release of Zn in several successive crops, making them a potential alternative to conventional fertilisers. Their unique properties can optimise nutrient management strategies and promote sustainable crop production.

Zinc (Zn) is an essential micronutrient for plant growth, serving as a co-factor in enzymatic processes and pigment biosynthesis. In horticultural crops such as lettuce, Zn fertilization is increasingly relevant for optimizing yield and nutritional quality. In this study, a greenhouse pot experiment was conducted using Lactuca sativa L. cv. Romana Verano (Ramiro Arnedo) to evaluate the effects of four Zn sources with contrasting physio-chemical properties—ZnSO₄, a synthetic chelate containing DTPA, EDTA, and HEDTA, a Zn–lignosulphonate complex, and ZnO nanoparticles—applied to soil at rates of 15, 30, 60, and 120 mg Zn·kg⁻¹. Morphometric traits, photosynthetic pigmentation, and photosystem performance were assessed to determine differences in plant response. Results showed that low to moderate Zn supply (15–60 mg Zn·kg⁻¹) maintained growth, leaf number, stem diameter, and biomass without significant changes compared to the control. In contrast, the highest dose (120 mg Zn·kg⁻¹), particularly in chelated forms, led to reductions in growth and yield exceeding 80%, reflecting supra-optimal effects. Although lignosulphonate and nanoparticles sources lowered soil Zn availability, they did not affect lettuce growth or yield, indicating their potential as safer agricultural alternatives to conventional Zn fertilizers. Photosynthetic efficiency, measured through chlorophyll fluorescence and electron transport activity, was positively modulated by adequate Zn levels but declined at excessive concentrations. These findings highlight that Zn efficiency strongly depends on its chemical form and applied dose, providing practical insights for optimizing Zn fertilization strategies in lettuce and other horticultural crops.