Impact of organic farming practices on toxic metal accumulation in topsoil in paddy areas: A comparative study in Sri Lanka

Abstract

Given the increasing importance of sustainability in agriculture, this study investigates how organic farming influences soil toxicity, specifically the accumulation of toxic metals in paddy fields within the Anamaduwa Divisional Secretariat area. Collections of paddy soil samples were made both from an organically cultivated field and a chemically fertilized field. Twenty composite topsoil samples were collected from each paddy field according to the random sampling method. Soil quality parameters such as soil pH, conductivity, organic matter, total phosphate, potassium, ammonium nitrogen, and ash content were analyzed, and toxic metal concentrations (Pb, Cd, As, Ni, and Cr) were analyzed by Inductively Coupled Plasma–Mass Spectrometry (ICP-MS). Results indicate the soil collected from paddy areas treated with chemical fertilizers has high average pH, conductivity, total phosphate content, and potassium content. The ammonium nitrogen and ash content were enhanced in the soil collected from organic land, and organic content fluctuated slightly between the two areas (p>0.05). Soil samples from the field using chemical fertilizers displayed higher average concentrations of Pb (7.15±1.57 mg/kg), As (0.87±0.17 mg/kg), Cr (20.09±4.91 mg/kg), and Ni (8.58±2.51 mg/kg) compared to the organically farmed paddy field. The mean concentrations of Pb, As, Cr, and Ni in the soil samples of the organically cultivated field were reported as 2.84±1.06 mg/kg, 0.49±0.18 mg/kg, 12.19±3.72 mg/kg, and 2.72±1.58 mg/kg respectively. Statistical analysis indicates a significant difference in mean heavy metal concentrations (As, Ni, Cr) between organic and chemically fertilized soil samples (p<0.05). The concentration of Cd in the paddy soil has not been reported in either of the selected paddy areas. In conclusion, the sustained reduction of chemical fertilizer application over a long-term period may contribute to the decrease in toxic metal contamination observed in the paddy soil within organically cultivated areas. This reduction in contamination subsequently mitigates the risk of the transfer of these toxic metals to the paddy root and grain.