Contrasting Temporal Dynamics of Iron and Aluminium under Prolonged Submergence of an Acidic Paddy Soil in Lower Kuttanad, Kerala, India

Abstract

Submergence induces major changes in the chemical environment of paddy soils and can alter the mobility of redox- and pH-sensitive elements. The present study characterised the temporal dynamics of soil acidity and iron (Fe) and aluminium (Al) fractions during 90 days of continuous submergence in an acidic paddy soil of Lower Kuttanad, Kerala, India. A 40 m² field area was maintained under continuous submergence and soil samples were collected from 0–15 and 15–30 cm depths at 0, 15, 30, 45, 60, 75 and 90 days after submergence, together with concurrent floodwater samples. Soil pH, electrical conductivity, potential acidity, cation exchange capacity, HCl-extractable Fe and Al, water-soluble Fe and Al, and total Fe and Al were determined, while floodwater pH, EC, total Fe and total Al were analysed. Soil pH increased during the initial and middle stages of submergence, whereas potential acidity declined. Reactive Fe fractions increased, with HCl-extractable Fe reaching 733.00 and 883.50 mg kg⁻¹ at 75 days in the 0–15 and 15–30 cm layers, respectively. In contrast, HCl-extractable and water-soluble Al declined progressively, while total Fe and Al showed comparatively limited temporal variation. Floodwater Fe increased from 11.43 to 15.30 mg L⁻¹ by 75 days, whereas floodwater Al declined from 5.56 to 0.43 mg L⁻¹ by 90 days. The contrasting Fe and Al responses indicate that partial alleviation of soil acidity under prolonged submergence was accompanied by reduced Al mobility but enhanced mobilisation of reactive Fe. The observations provide a temporal baseline for understanding Fe–Al partitioning in submerged acidic soils of Lower Kuttanad and for designing replicated studies on the timing and management of amelioration.