Mobility, Bioavailability, and Enrichment of Soil-Bound Phosphorus in Flood-Prone Paddy Fields: A Case Study of Kunnukara, South India.

Bioavailability, enrichment, and risk of phosphorus (P) and its fraction composition were monitored in the paddy soils of Kunnukara during the pre-cultivation and post-harvest periods in the years 2020 and 2021. Iron-bound P (≥ 105.56 ± 0.05 mg/kg) was found highest among the P fractions. The bioavailability of P was recorded at peak value during the post-harvest period, contributed by organic P, Iron bound P, and loosely bound P. Principal component analysis inferred that loosely bound P was pH-dependent and significantly influenced by cation exchange, particle density, soil aggregate stability, and total organic carbon (TOC) in the post-harvest soil, whereas TOC, aluminium-bound P, and calcium-bound P in the pre-cultivation soils. Additionally, physico-chemical parameters like electrical conductivity, bulk density, specific gravity, TOC, and soil aggregate stability have a significant impact on the composition of P fractions in the soil. Bioavailable phosphorus (BAP) ranged from 642.78 ± 0.49 to 594.20 ± 1.23 mg/kg during the post-harvest period. Moreover, the contribution of BAP to total P ranged from 99.45 to 99.54%, indicating the fact that soil is sufficient in BAP. Pollution indices revealed that the paddy soils are at risk of eutrophication. Phosphorus Pollution Index (PPI) > 1 exhibited moderate pollution (1.06 to 1.07) at the topsoil (0-15 cm) and PPI < 1, mild pollution (0.92 to 0.99) at 15-30 cm depths. The organic nitrogen index ≥ 0.133 indicates severe soil pollution in the study site. An extended fertilizer application in the field contributes to nutrient enrichment and warrants the risk of contamination in nearby riverine systems (River Periyar and River Chalakkudy).