Availability of Mn, Zn and Fe in the rhizosphere

This review paper critically assesses the literature on soil-microbe-plant interactions influencing availability of micronutrients in the rhizosphere. The emphasis is placed on Zn and Mn, but Fe is also covered to some extent. Micronutrient availability in the rhizosphere is controlled by soil and plant properties, and interactions of roots with microorganisms and the surrounding soil. Plants exude a variety of organic compounds (carboxylate anions, phenolics, carbohydrates, amino acids, enzymes, etc.) and inorganic ions (protons, phosphate, etc.) to change chemistry and biology of the rhizosphere and increase micronutrient availability. Increased availability may result from solubilization and mobilization by short-chain organic acid anions, amino acids and other low-molecular-weight organic compounds. Acidification of the rhizosphere soil increases mobilization of micronutrients (eg. for Zn, 100-fold increase in solubility for each unit of pH decrease). For diffusion-supplied micronutrients, the uptake rate is governed by the soil nutrient supply. Fertilisation with micronutrients (more so in case of Zn than Fe) can be effective in increasing the concentration of micronutrients at the soil-root interface. In addition, micronutrient-efficient crops and genotypes can increase an available nutrient fraction and hence increase micronutrient uptake. Our understanding of the physiological processes governing exudation and the soil-plant-microbe interactions in the rhizosphere is currently inadequate, especially in terms of spatial and temporal variability in root exudation as well as the fate and effectiveness of organic and inorganic compounds in increasing availability of soil micronutrients and undesirable trace elements. The interactions between microorganisms and plants at the soil-root interface are particularly important as well obscure.