Synthesis of Nano-Fertilizer Based on Hydrochar Derived from Plant Waste and Its Effect on Wheat

Introduction: Climate change, the spread of pollutants, and declining agricultural productivity, coupled with the excessive use of chemical fertilizers, pose serious challenges to the environment and human health. Therefore, the production of efficient, low-risk fertilizers with lower environmental impact has become a necessity. In this regard, nanotechnology, through the provision of nano-fertilizers, holds high potential for enhancing plant growth and yield. Hydrochar, as a non-toxic carbonaceous material compatible with the environment,  can serve as a suitable platform for the controlled release of nutrients. Furthermore, the use of plant waste, such as fruit peels, for hydrochar synthesis represents an approach based on the circular economy and sustainable development. This research aimed to synthesize a nano-fertilizer based on hydrochar derived from plant waste and modified with zinc oxide nanoparticles (ZnO), and to evaluate its effects on the growth and physiological responses of wheat plants.
Materials and methods: The ZnO-modified hydrochar nano-fertilizer was synthesized using tangerine peel waste and zinc oxide nanoparticles via the hydrothermal method. The nanostructural characteristics of the nano-fertilizer were investigated using FESEM, FTIR, XRD, and UV-Vis techniques. The synthesis of nanoparticles and investigation of their effect on the initial responses of wheat were conducted in a completely randomized design with three replications in 2023 at the Plant Physiology Laboratory of Razi University, Kermanshah. The effects of different concentrations of the nano-fertilizer (0, 20, and 40 mg L⁻¹) on wheat plants (Pishgam cultivar) were studied under hydroponic conditions for 21 days. Growth parameters (length and fresh weight of roots and shoots), photosynthetic pigment content (chlorophyll a, b, total chlorophyll, and carotenoids), hydrogen peroxide content, and the activity of antioxidant enzymes (catalase and peroxidase) were measured.
Results: FESEM, FTIR, XRD, and UV-Vis analyses confirmed the successful synthesis of a hydrochar-based nanofertilizer modified with ZnO nanoparticles. FESEM images showed the uniform distribution of spherical ZnO nanoparticles, approximately 20 nm in size, on the nanofibrous hydrochar substrate. FTIR spectra confirmed the presence of various functional groups, such as hydroxyl and carboxyl, as well as the Zn-O bond. The XRD pattern indicated the presence of the crystalline phase of ZnO nanoparticles and the amorphous structure of hydrochar. The 40 mg L⁻¹ Zn-based nanofertilizer concentration had the greatest effect on plant growth, while the 20 mg L⁻¹ treatment produced the greatest increase in photosynthetic pigments. This dual and concentration-dependent response indicates a dynamic physiological adjustment, where the plant shifts its strategy from optimizing photosynthesis at lower concentrations to accelerating growth and activating defense at higher concentrations. The application of the zinc-based nano-fertilizer significantly activated the antioxidant defense system of wheat. At 40 mg L⁻¹, hydrogen peroxide content increased in the roots and shoots, alongside a significant increase was found in the activity of catalase and peroxidase enzymes in both organs. The nano-fertilizer, by inducing mild oxidative stress, elevated hydrogen peroxide levels as a signaling molecule. In response, the plant, through a simultaneous and proportional increase in antioxidant enzyme activity, established a hormetic adaptation that created a dynamic balance between growth stimulation and stress neutralization.
Conclusion: The nano-fertilizer activated the wheat antioxidant system by inducing mild oxidative stress and increasing hydrogen peroxide signaling. In response, the plant established a hormetic adaptation by increasing the activity of catalase and peroxidase enzymes, creating a dynamic balance between growth stimulation and stress management. Accordingly, the hydrochar-based nano-fertilizer modified with ZnO nanoparticles, by improving physiological, and antioxidant indicators, can be introduced as an effective and environmentally friendly strategy for enhancing wheat growth. This research represents a practical step towards valorizing agricultural waste and realizing the principles of the circular economy.