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Iron (Fe) is essential for the maintenance of processes such as respiration and photosynthesis, thus being an important element for the plant production of cultivated or natural species. However, the combination of Fe and the acidity of acid rain creates favourable environments for the occurrence of oxy-reduction reactions, which transform Fe3+ into soluble forms (Fe2+), increasing the availability of this metal in the soil, causing direct and indirect effects on plant growth, development, and productivity. Fe can also affect the availability of other nutrients in the soil like nitrogen (N) and phosphorus (P), directly influencing plant growth. At an optimum concentration, Fe can increase the availability of N, while in high concentrations, it does not alter N content. However, Fe has a high affinity for P and in acid soils this metal is commonly found in high concentrations, thus Fe usually binds to P, making this macronutrient unavailable for uptake by the plant, thereby negatively affecting the plant’s metabolism. To promote the recovery of environments contaminated by toxic levels of Fe, the use of mycorrhizal associations, such as ectomycorrhizae, is considered a promising practice. Ectomycorrhizal (ECM) fungi developed three Fe absorption mechanisms to prevent its insolubility and toxicity: acidification of the medium, reduction of ferric to ferrous form, and secretion of iron-chelating molecules (melanins and siderophores). However, for host plants to obtain the benefits of fungi under conditions of Fe toxicity, it is necessary to carry out in vitro studies, using several sources of this metal. It is also extremely important to analyse the relationship of Fe with other nutrients in the medium, mainly N and P, since ECM fungi have intra and interspecific variations, and their growth is influenced by several abiotic factors.

Nitrogen (N) is one of the essential elements for life on Earth and is mainly distributed in an inert form in the atmosphere, so it cannot be used by plants. Therefore, the N found in the atmosphere must undergo several transformations to be fixed in the soil and then used by plants. With the presence of N in several organic molecules, the acquisition of this element by plants has an impact on the growth and development of several agricultural crops. Understanding the metabolic mechanisms related to N and the methods of uptake, transport and use of this nutrient by plants have always been a topic of interest in several types of research. N also influences the soil microbiota, because microorganisms participate in the N cycle, especially in relation to the process of biological application of N. Thus, with the discovery of N-fixing microorganisms, it was possible to explore the impact that the microbiota would have on the plant’s N metabolism. In this context, we have gathered in this chapter, information about the ecophysiology of N in plants and how some fungi and bacteria are able to influence the metabolism of this element. We also explain a little how the association between plants and endophytic bacteria occurs, since this symbiotic relationship is very important to make N available to the plants.