Iron oxide‐humic acid coprecipitates as iron source for cucumber plants

In soil, iron (Fe) solubility depends on complex interactions between Fe minerals and organic matter, but very little is known about plant availability of Fe present in Fe oxides associated with humic substances. For this purpose, this study investigates the effect of Fe mineral crystallinity in the presence of humic acids (HA) on Fe availability to plants. Four Fe–HA mineral coprecipitates were prepared, either in the presence or absence of oxygen, i.e., two goethite (G)‐HA samples containing large amounts of Fe as nanocrystalline goethite and ferrihydrite mixed phases, and two magnetite (M)‐HA samples containing crystalline magnetite. Bioavailability studies were conducted in hydroponic systems on cucumber plants (Cucumis sativus L.) grown under Fe deficient conditions and supplied with the Fe–HA coprecipitates containing goethite or magnetite. Results showed that plants grown in the presence of Fe–HA coprecipitates exhibited a complete recovery from Fe deficiency, albeit less efficiently than plants resupplied with Fe‐chelate fertilizer used as control (Fe‐diethylene triamine penta acetic acid, Fe‐DTPA). However, the supply with either G‐ or M–HA coprecipitates produced different effects on plants: G–HA‐treated plants showed a higher Fe content in leaves, while M–HA‐treated plants displayed a higher leaf biomass and SPAD (Soil–Plant Analysis Development) index recovery, as compared to Fe‐DTPA. The distribution of macronutrients in the leaves, as imaged by micro X‐ray fluorescence (µXRF) spectroscopy, was different in G–HA and M–HA‐treated plants. In particular, plants supplied with the poorly crystalline G–HA coprecipitate with a lower Fe/HA ratio showed features more similar to those of fully recovered plants (supplied with Fe‐DTPA). These results highlight the importance of mineral crystallinity of Fe–HA coprecipitates on Fe bioavailability and Fe uptake in hydroponic experiments. In addition, the present data demonstrate that cucumber plants can efficiently mobilize Fe, even from goethite and ferrihydrite mixed phases and magnetite, which are usually considered unavailable for plant nutrition.     

Iron Mobilization and Mineralogical Alterations Induced by Iron-Deficient Cucumber Plants (Cucumis sativus L.) in a Calcareous Soil

Dicotyledons cope with ion (Fe) shortage by releasing low-molecular-weight organic compounds into the rhizosphere to mobilize Fe through reduction and complexation mechanisms. The effects induced by these root exudates on soil mineralogy and the connections between Fe mobilization and mineral weathering processes have not been completely clarified. In a batch experiment, we tested two different kinds of organic compounds commonly exuded by Fe-deficient plants, i.e., three organic acids (citrate, malate, and oxalate) and three flavonoids (rutin, quercetin, and genistein), alone or in combination, for their ability to mobilize Fe from a calcareous soil and modify its mineralogy. The effect of root exudates on soil mineralogy was assessed in vivo by cultivating Fe-deficient and Fe-sufficient cucumber plants (Cucumis sativus L.) in a RHIZOtest device. Mineralogical analyses were performed by X-ray powder diffraction. The batch experiment showed that citrate and, particularly, rutin (alone or combined with organic acids or genistein) promoted Fe mobilization from the soil. The combinations of rutin and organic acids modified the soil mineralogy by dissolving the amorphous fractions and promoting the formation of illite. These mineralogical alterations were significantly correlated with the amount of Fe mobilized from the soil. The RHIZOtest experiment revealed a drastic dissolution of amorphous components in the rhizosphere soil of Fe-deficient plants, possibly caused by the intense release of phenolics, amino acids, and organic acids, but without any formation of illite. Both batch and RHIZOtest experiments proved that exudates released by cucumber under Fe deficiency concurred to the rapid modification (on a day-scale) of the mineralogy of a calcareous soil.     

Preliminary evaluation of eggshells as a source of phosphate on hydroponically grown tomato (Solanum lycopersicum L.) seedlings

Abstract

Phosphorus (Pi) is one of the most limiting factors in plant nutrition as it is the least mobile and available nutrient to plants in most soil conditions. The management of Pi fertilization in agriculture raises ecological, economic, and social issues, since phosphate rock minerals are the only significant global resources of Pi and they will be rapidly depleted. Eggshell waste is a big problem for food companies producing different types of egg products, since the eggshell waste is very often simply discarded and disposed at landfills, with high costs related to their disposal. The aim of this work was the characterization of eggshells as a Pi source for plants, using tomato (Solanum lycopersicum L cv Marmande) as a model species. Plants were grown hydroponically being exposed to adequate and limited Pi availability, with or without eggshell powder. Plant growth performance was characterized by analyzing changes in fresh weight, protein, chlorophyll concentration, carotenoid content, and measuring the plant’s capability to accumulate phosphate. The addition of eggshell powder to the nutrient solution significantly improved plant growth and increased protein and chlorophyll concentration, not only with respect to P-deficient control, but also with P-sufficient ones. Furthermore, eggshell powder significantly increased Pi accumulation in P-deficient plants, suggesting that eggshell waste could be a suitable material as Pi source for tomato plants, thus contributing to the environmentally friendly disposal of this waste.     

Silicon dynamics in the rhizosphere: Connections with iron mobilization

Silicon (Si) is a beneficial element for plants as it increases their resistance to several biotic and abiotic stresses. In the rhizosphere, root exudates, especially when released by nutritionally stressed plants, promote the mineral weathering and, consequently, influence Si biogeochemistry. This study aims at evaluating the mineralogical alterations in the rhizosphere of Fe‐deficient or Fe‐sufficient barley plants grown either in a natural or in an artificial calcareous soil, focusing on the dynamics of both Fe and Si. After 6 d of soil–plant contact, X‐ray diffraction (XRD) analysis of rhizosphere soil samples of Fe‐deficient plants revealed, for both natural and artificial soil, a decrease of amorphous phases and an increase of smectite compared to the unplanted soil. Root exudates released by Fe‐deficient plants were most likely the main responsible for the weathering of the amorphous phases by a ligand controlled dissolution mechanism. When the soil–plant contact was prolonged up to 12 d, plants overcame Fe nutritional stress and their effect on soil mineralogy completely changed, as proved by the considerable increase of amorphous and decrease of smectite. Smectite decrease might evidence the effort of plant to mobilize Si and micronutrients other than Fe from the soil through the exudation of organic ligands. When the artificial soil was treated with Fe‐sufficient barley plants, the mineral weathering trend appeared reversed compared to the experiments with Fe‐deficient plants. Plant nutritional status regulates the root exudation pattern and, consequently, drives mineral weathering processes in the rhizosphere. Barley has shown to be able to mobilize Si from smectite, yet depending on its Fe supply and proving the strict connection between Si and Fe dynamics in the rhizosphere.