Review on iron availability in soil: interaction of Fe minerals,plants, and microbes

Purpose

The rationale of this paper is to review the state of the art regarding the biotic and abiotic reactions that can influence Fe availability in soils. In soil, the management-induced change from oxic to anoxic environment results in temporal and spatial variations of redox reactions, which, in turn, affect the Fe dynamics and Fe mineral constituents. Measuring the Fe forms in organic complexes and the interaction between bacteria and Fe is a major challenge in getting a better quantitative understanding of the dynamics of Fe in complex soil ecosystems.

Materials and methods

We review the existing literature on chemical and biochemical processes in soils related with the availability of Fe that influences plant nutrition. We describe Fe acquisition by plant and bacteria, and the different Fe–organic complexes in order to understand their relationships and the role of Fe in the soil carbon cycle.

Results and discussion

Although total Fe is generally high in soil, the magnitude of its available fraction is generally very low and is governed by very low solubility of Fe oxides. Plants and microorganisms can have different strategies in order to improve Fe uptake including the release of organic molecules and metabolites able to form complexes with Fe^III. Microorganisms appear to be highly competitive for Fe compared with plant roots. Crystalline Fe and poorly crystalline (hydro)oxides are also able to influence the carbon storage in soil.

Conclusion

The solubility of crystalline Fe minerals in soil is usually very low; however, the interaction with plant, microbes, and organic substance can improve the formation of soluble Fe^III complexes and increase the availability of Fe for plant growth. Microbes release siderophores and plant exudates (e.g., phytosiderophores, organic acids, and flavonoids), which can bind and solubilize the Fe present in minerals. The improved understanding of this topic can enable the identification of effective solutions for remedying Fe deficiency or, alternatively, restricting the onset of its symptoms and yield’s limitations in crops. Therefore, development and testing of new analytical techniques and an integrated approach between soil biology and soil chemistry are important prerequisites.     

Hydrolysis lignin as a multifunctional additive in Atlantic salmon feed improves fish growth performance and pellet quality and shifts gut microbiome

The objective of this study was to evaluate hydrolysis lignin (H‐lignin), derived from wood biomass, as a multifunctional component of aquafeeds. Atlantic salmon (28.8 ± 1.1 g) were fed diets for 16 weeks, which included two H‐lignin types (HL1 and HL2) at 15, 30 or 50 g/kg (wt/wt) or a control diet (no added H‐lignin). HL1 was extracted with water such that no soluble sugar and oligosaccharides remain, while HL2 contains a higher fraction of water‐soluble sugars and oligosaccharides. Pellet durability and density were measured. After 16 weeks, salmon were measured for weight and length, and whole carcass, hindgut and digesta contents were sampled. Pellet durability increased from the control to 30 g/kg H‐lignin but decreased at 50 g/kg. Salmon fed diets with HL1 at 15 and 30 g/kg showed higher weight gain and lower feed conversion ratio compared with salmon fed the control diet and 50 g/kg HL1. There were no significant differences in whole‐body composition or intestinal morphology. Microbial characterization (16S) revealed lower abundance of Proteobacteria, higher abundance of Mycoplasmataceae and increasing Lactobacillaceae abundance with higher HL1 inclusion. This study demonstrates that HL1 (at 15 and 30 g/kg) shows potential as a functional feed additive for salmon.