Effects of Irrigation and Drainage Modes on the Residual Characteristics of Heavy Metals in Soil

Effects of irrigation and drainage modes on the residual characteristics of heavy metals (As, Cd, Cr, Cu, Mn, and Pb) in soil were investigated. The results showed that different water-saving modes in farmland had a notable effect on the residual concentration of heavy metals: Water-saving irrigation > flooding irrigation > dry field. As for bioavailability fractions (exchangeable and carbonate fractions) of heavy metals, the difference under water-saving modes was unobvious. The bioavailability fractions of Cd showed the most significant difference between water-saving irrigation (8.55%) and flooding irrigation (6.16%), and for bioavailability fractions of non-rhizosphere (10.53–10.78%) it was higher compared with rhizosphere fractions (3.26–7.10%). Therefore, bioavailability fractions of heavy metals in soil were closely associated with the input intensity of pollutants and the leaching intensity of different water-saving modes. The potential bioavailability fractions (Fe/Mn oxide fraction and organic matter fraction) of heavy metals in paddy fields (28.89%) were lesser than in dry fields (30.37%), the potential bioavailability fractions of heavy metals in flooding irrigation (30.14%) were greater than in water-saving irrigation (27.64%), and for rhizosphere soils (25.73–37.64%) it was greater compared with non-rhizosphere soils (27.64–31.18%); the most significant response on different water-saving modes showed Cu. Thus, the increase of crystalline forms of Fe/Mn oxide induced by a high redox potential and dissolved oxygen in dry fields and the rhizosphere soils played an important role for the high percentage of potential bioavailability fractions of heavy metals.