The Important Roles of Natural Fe-Oxide Nano-Phases in Nutrient Dynamics and Nutrient Acquisition by Plants.

Natural nano-minerals (commonly in the range of 4-10 nm particle size) occur in virtually all soils (wetland and dryland soils; acid and calcareous soils). The minerals that have received the greatest attention are the hydrous ferric oxides (principally ferrihydrite) and related substituted analogs. Though thermodynamically unstable in most soil environments, these minerals persist, in concentrations ranging from low to substantial, because of natural abiotic and biotic precipitation processes and kinetic inhibitions that prevent their crystal growth and transformation to more highly crystalline phases. These minerals are common products of rapid precipitation in alternating redox and wetland environments as in flooded rice culture or in rapidly oxidizing environments as in some mineland soils and associated tailings. Nano-sized hydrous oxides play important roles in soil processes because of their small particle size, high surface adsorption capacity, surface catalytic characteristics, and relative ease of dissolution under reducing conditions and in the presence of microbial and plant-root exudates such as citrate and the plant phytosiderophores. It is because of these characteristics that the nano-sized poorly crystalline hydrous Fe oxides often control nutrient availability to both plants and microbes, especially of Fe, Zn and phosphate, as well as the mobility and bioavailability of toxic species, e.g., of As. It is also because of these reactions that natural Fe-oxide nano-phases play important roles in overall ecological and agricultural sustainability. The emphasis in the current paper will be with the role of nano-sized hydrous oxides on nutrient availability of Fe, Zn and P in agricultural situations. The two primary examples will be of current research with (i) Fe and P bioavailability and acquisition by sorghum and cowpea in a dryland situation, and (ii) Fe, Zn, and P dynamics and bioavailability under flooded rice culture.