Iron-acidity interactions in hydromorphic rice systems of south western coastal India

A comprehensive understanding of iron (Fe) dynamics in soil acidification processes is crucial for formulating effective and site-specific liming strategies in acid soils. The study investigated the dynamics of Fe fractions, interrelations with acidity and the stability of Fe minerals in the hydromorphic paddy soils of Kerala, India. Georeferenced composite soil samples were collected from rice-growing agroecological units of Kerala during April to May 2023. The soil samples were subjected to sequential extraction of Fe fractions: Fws, water-soluble and exchangeable; Fsp, specifically adsorbed lead (Pb) displaceable; Fas, acid-soluble; Fmn, manganese (Mn) oxide-occluded; Fom, organic matter occluded; Fam, amorphous Fe oxide-occluded; Fci, crystalline Fe oxide-occluded and Frs, residual. Besides, the forms of acidity were also analysed to determine Aex, exchange acidity; Aet, extractable acidity; Ata, total acidity; Atp, total potential acidity; and Apd, pH-dependent acidity. Path analysis (p = 0.05) revealed that Fas had a significant direct positive influence (0.82), and Fwa had a significant direct negative effect (− 0.53) on pH. The changes in Fas were directly controlled by Fmn and that of Fwa by Fsp. The Aet had a direct positive, and Atp had a significant negative control (− 1.16) on Aex in the soil. Structural equation modelling revealed a significant negative effect between Fsp and a positive direct effect between Fas, Fom and Fci on Aet. The Fas imposed a significant direct positive and Fam, a significant negative control on Atp. Thus, acid soluble Fe, extractable acidity, and total potential acidity were found to be the influencing factors of pH in the paddy soils of Kerala. Further, the mineral stability diagram of Fe revealed that the solubility of Fe was controlled by amorphous Fe minerals in the paddy growing hydromorphic wetlands of Kerala, India.