Eco-Friendly Iron-Humic Nanofertilizers Synthesis for the Prevention of Iron Chlorosis in Soybean (Glycine max) Grown in Calcareous Soil
Eco-Friendly Iron-Humic Nanofertilizers Synthesis for the Prevention of Iron Chlorosis in Soybean (Glycine max) Grown in Calcareous Soil. Cieschi M.T., Polyakov A.Y., Lebedev V.A., Volkov D.S., Pankratov D.A., Veligzhanin A.A., Perminova I.V., Lucena J.J. //Frontiers in Plant Science. 2019. V.10. P.413.
Iron deficiency is a frequent problem for many crops, particularly in calcareous soils and iron humates are commonly applied in the Mediterranean basin in spite of their lesser efficiency than iron synthetic chelates. Development and application of new fertilizers using nanotechnology are one of the potentially effective options of enhancing the iron humates, according to the sustainable agriculture. Particle size, pH, and kinetics constrain the iron humate efficiency. Thus, it is relevant to understand the iron humate mechanism in the plant-soil system linking their particle size, characterization and iron distribution in plant and soil using 57Fe as a tracer tool. Three hybrid nanomaterials (F, S, and M) were synthesized as iron-humic nanofertilizers (57Fe-NFs) from leonardite potassium humate and 57Fe used in the form of 57Fe(NO3)3 or 57Fe2(SO4)3. They were characterized using Mossbauer spectroscopy, X-ray diffraction (XRD), extended X-ray absorption fine structure spectroscopy (EXAFS), transmission electron microscopy (TEM) and tested for iron availability in a calcareous soil pot experiment carried out under growth chamber conditions. Three doses (35, 75, and 150 mu mol pot-1) of each iron-humic material were applied to soybean iron deficient plants and their iron nutrition contributions were compared to 57FeEDDHA and leonardite potassium humate as control treatments. Ferrihydrite was detected as the main structure of all three 57Fe-NFs and the plants tested with iron-humic compounds exhibited continuous long-term statistically reproducible iron uptake and showed high shoot fresh weight. Moreover, the 57Fe from the humic nanofertilizers remained available in soil and was detected in soybean pods. The Fe-NFs offers a natural, low cost and environmental option to the traditional iron fertilization in calcareous soils.
Iron (Fe) is an essential micronutrient for humans and plants. Iron deficiency is very common in the human diet and affects an estimated two billion people in the world (Briat et al., 2015). Iron chlorosis is a widespread agricultural problem occurring in about 30–50% of cultivated soils (Cakmak, 2002) and one of the major limiting factor of crop production in calcareous soils. Farmers apply iron synthetic chelates to alleviate iron deficiency in cash crops. Despite the high costs of these fertilizers, they tend to lixiviate and the chelating agents may avoid the precipitation and enhance mobilization of heavy metals (Ylivainio, 2010). Many crops are sensitive to the iron chlorosis, such as citrus and fruit trees but soybean (Glycine max L.) is one of the most studied iron Strategy I plant (Fuentes et al., 2018). Moreover, soybean production reaches levels of about 230 million metric tons per year across the world (Vasconcelos and Grusak, 2014) and this legume is a highly nutritious crop which contains more protein (40%) and oil (20%) than any other ordinary food source (Bolon et al., 2010).
According to the United Nations [UN] (2013), the rapidly growing world population is projected to reach 9.6 billion by the year 2050 and Food and Agriculture Organization of the United Nations [FAO] (2017) has predicted that the global grain production is required to increase by 70% to meet these demands. Therefore, new approaches should be developed for alleviation of iron deficiency in plants and new ecofriendly fertilizers are needed in order to enhance crop environmental quality. Iron fertilizers based on HSs extracted from lignites, such as leonardite, are used in the Mediterranean area (as liquid concentrates) in drip irrigation (Kovács et al., 2013). This kind of iron fertilizers is more ecofriendly than synthetic iron chelates but they are less efficient in correcting iron chlorosis. Moreover, field experiments have demonstrated that the synthetic chelate has a fast effect while the iron humate fertilizers provide increasing iron availability in the root–soil interface resulting in slow uptake of Fe by the plants (Cieschi et al., 2017). Kulikova et al. (2017) have demonstrated that only iron from very small and amorphous nanoparticles of ferric polymers incorporated into humic matrix is readily taken up by plants. Therefore, the synthesis of iron humates should be optimized for developing efficient NFs.
According to Naderi and Danesh-Shahraki (2013), NFs are the most important products of nanotechnology with regard to agriculture. Nanosized active ingredients (from 1 to 100 nm in diameter) have a large specific surface area that can result in significantly enhanced reactivity, and this feature increases absorption of nutritional elements and essential compounds for plant growth and plant metabolism (Janmohammadi et al., 2016). Many attempts has been made to prepare inorganic Fe nanofertilizers. As example Sánchez-Alcalá et al. (2012) synthesized nanosiderite (FeCO3) and demonstrated that it was highly effective in preventing iron chlorosis in chickpea and had a great residual effect. Ghafariyan et al. (2013) reported that low concentrations of superparamagnetic Fe-NPs significantly increased the chlorophyll contents in sub-apical leaves of soybeans in a greenhouse test under hydroponic conditions, suggesting that soybean could use this type of Fe-NPs as source of Fe and reduce chlorotic symptoms of Fe deficiency. However, the research on natural Fe nano-humate complexes is now in progress. Dholakia (2016) developed the preparation of nanoparticulate liquid organic fertilizers employing humic acids. In addition, Kulikova et al. (2017) have synthesized well-defined iron (hydr)oxide NPs of feroxyhyte stabilized by traces of HS (a model of iron-based engineered NPs) and water-soluble Fe-HS complexes of the proven high availability to plants tested their iron materials in wheat plants under hydroponic conditions. These promising results motivated us to follow the research on Fe NFs stabilized with humates.
According to Dimkpa and Bindraban (2017), up to now, the bulk of research in plant nanoscience either consists of experiments conducted in artificial media, such as nutrient solutions, agar, sand, or other non-soil media. Moreover, Liu and Lal (2015) recommend that micronutrient research should focus on enhancing the bioavailability (plant-uptake rate) of NFs to address the field leaching associated with the conventional micronutrient fertilizers and compare the beneficial effects of these micronutrient NFs with commercially available micronutrient counterparts [e.g., FeNPs vs. FeCl3 or Fe(EDTA) as Fe sources] under the field condition. Therefore, it is of particular importance to test the 57Fe-NFs in a soil system in a long-term experiment which would enable for completion of the full growth cycle crop in order to be closer to agronomical conditions. Since the efficacy of an iron fertilizer is related to the iron that the plants can take from the fertilizer, the use of iron isotopes is highly beneficial for monitoring iron uptake by plants (Cesco et al., 2002; Nikolic et al., 2003; Tomasi et al., 2013). The use of stable Fe isotopes instead of radioactive ones gives a high flexibility in the experimental designs and can include field studies, because special safety measurements and trained staff are not required. Moreover, long-term assays can be carried out without taking care of radioactivity decay over time. In addition, the generation of radioactive wastes is avoided (Benedicto et al., 2011). Many studies about 57Fe application in soils experiments (Nadal et al., 2012; Martín-Fernández et al., 2017a,b) were reported, but this work is the first one in preparing 57Fe-NFs and applying them in a calcareous soil.
Here, three 57Fe-labeled humic nanomaterials (F, S, and M) were synthesized using potassium humate as a parent humic material and 57Fe in the form of 57Fe(NO3)3 (product F) and 57Fe2(SO4)3 (products S and M), characterized for iron speciation and phase composition of nanoparticles, and tested for bioavailability to soybean iron deficient plants grown in calcareous soils under growth chamber conditions. This was to establish a link between the Fe-NPs characteristics and their behavior in the soil–plant system using 57Fe as a tracer tool.
According to Lal (2008), in the context of sustainable agriculture, applying innovative nanotechnology in agriculture is regarded as one of the promising approaches to significantly increase crop production. The Fe-NFs can be considered as a part of a novel technology in line with the precision and sustainable agriculture. They are iron-natural complex NPs synthetized from leonardite and they contain ferrihydrite in their structures which was properly and widely characterized. Moreover, the 57Fe-NFs used in this paper are capable of supplying Fe to the plants, transport it from root to shoot and reach the soybean pods. The slow and continuous iron release of these 57Fe-NFs confirms their long-term effect in providing iron in calcareous conditions while in soil, they tends to remain available to the plant requirements for the different growth stages.
Although further research is needed about the contribution of iron nanoparticles in plant nutrition, the Fe-NFs offers a natural, low cost and environmental option to the traditional iron fertilization in calcareous soils.