Rice paddy redox controls root iron plaque: (trans)formation, kinetics, and elemental fate

• Rice roots form Fe-rich plaque, which accumulates numerous elements. • Root plaque increases with time and Fe(II) in the porewater. • Occasional dry downs favored lepidocrocite formation over goethite. • Plaque concentrations of As and Mn tended to increase over time. • The plaque is an important reservoir of nutrients and contaminants. Rice thrives in flooded environments partly due to the formation of Fe-rich plaque on the roots. The main Fe plaque mineral, ferrihydrite, is a dynamic Fe oxyhydroxide mineral with high reactivity, surface area, and propensity to transform into more crystalline minerals, including goethite and lepidocrocite. However, ferrihydrite formed in situ is more complex than pure, lab-synthesized ferrihydrite due to interactions with porewater constituents. This is the first field-based kinetic analysis of iron plaque transformation and investigation of how irrigation management affects elemental composition. Rice was grown under six different irrigation managements to expose Fe plaque to differing redox conditions, and root plaque was collected at six developmental stages each year during the two-year field study. The amount of Fe plaque increased with both porewater Fe(II) concentration and plant age. While ferrihydrite was the dominant phase, more flooded conditions formed goethite, while periodic dry downs formed lepidocrocite. The amount of plaque goethite was strongly correlated with porewater Fe(II) levels. We also quantified Al, As, Ca, Cd, Cr, Ga, Mg, Mn, P, Si, and Zn in the plaque, finding that their concentrations relative to Fe generally declined over time, except for As and Mn. Thus, irrigation practices strongly affect the mineral composition of the Fe plaque, which is likely to have long-term implications for elemental cycling.