Small-scale interaction of iron and phosphorus in flooded soils with rice growth

In the rhizosphere of flooded paddy soils, the solubilization, efflux, and uptake of phosphorus (P) are highly intertwined with iron (Fe) redox cycling. However, the direct observation of Fe-P coupling in the rhizosphere is challenging. This study combined high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques to capture the one-dimensional distributions of soluble reactive P (SRP), soluble Fe(II), and labile P and Fe in the root zone of rice (Oryza sativa L.), respectively. The results show a depletion of soluble/labile P and Fe concentrations around the rice root zone, compared to anaerobic bulk soils that have two different soil Olsen-P levels. Two-dimensional (2D) measurements of DGT-labile P concentrations exhibited similar but stronger trends of P depletion due to uptake of P from soil solids. In low-P soil treatment, 97.8% soluble Fe(II) was depleted in the rice root zone relative to bulk soil, and a 540% enrichment of total Fe in Fe plaques appeared in comparison to that in high-P soil. This demonstrated that the rice plant showed an adaptive metabolic reaction to combat P deficiency in low-P soil by increasing Fe plaque formation. This reaction directly resulted in stronger depletion of P in low-P soil, as indicated by the results of 2D measurements of DGT-labile P concentrations. Moreover, the significant (P < 0.001, R² = 0.175-0.951) positive corrections between SRP vs. soluble Fe(II), and DGT-labile P vs. Fe were observed in combination with pronounced peaks at the same position in the rice root zone, thus verifying that the cycling of Fe dictated P depletion. A notably lower value of the DGT-labile Fe/P ratio was found in high-P soil, which indicates a relatively higher risk of P release compared to that in low-P soil.