Nutrient availability in the rhizosphere: A review

The rhizosphere is the volume of soil where the chemical (pH, redox potential, nutrient concentration and root exudates), physical (temperature, water availability and soil structure) and biological characteristics (microbial and fungal associations) are shaped by plant-microbe-soil interactions. Communications between roots and between soil microorganisms and roots i.e., rhizosphere processes, occur and change continuously influencing for instance the nutrients solubility, their soil transport and plant uptake. The main driving force of these mechanisms seems to be related to root exudation processes. Root exudates comprise in fact a number of different organic (e.g., low molecular weight and high molecular weight organic compounds) and inorganic (e.g., protons and HCO3-) chemicals and can reach concentrations up to 250 mg C (photosynthetically fixed carbon) g-1 root. Several nutrients, in particular trace elements, are mainly adsorbed or are structural components of soil minerals, i.e., the quantitatively largest fraction in soil. Root exudates might interact with the minerals leading to mineral weathering and thus to the mobilization of trace elements with a consequent increase in their plant available fraction. However, root exudates have short half-lives because of the large microbial activity at the soil-root interface, which might limit their effects on nutrient mobility and acquisition. In addition, exudates also have a selective effect on the microbial community present in the rhizosphere. The review provides an overview of the rhizosphere processes involved in nutrients acquisition by soil organisms (plants and microorganisms) focusing on some macro-(e.g., phosphorus) and micronutrients (e.g., iron). In particular, the review focuses on i) the role of root exudates in mobilizing nutrients (including the mineral weathering processes and the plant-soil interactions underlying the phenomenon); ii) microbe-plant interactions, focusing on beneficial microbial communities and their association with plants; iii) relationships between nutrients unraveling synergisms and antagonisms between elements; and iv) rhizosphere processes triggered when nutrients reach toxic concentrations focusing on copper (Cu) toxicity. These aspects are discussed considering also the contribution of the metabolic changes triggered by nutrient shortage on root exudation phenomenon