Improving Water Management to Reduce Cd Accumulation in Rice in Lightly Cd-Polluted Paddy Soils

Abstract

【Background】 Soil pollution by cadmium has become an increasing concern not only because of its toxicity after entering the food chain but also due to its detrimental impact on soil quality and developing sustainable agriculture. As all biogeochemical processes controlling Cd bioavailability in soil are modulated by soil moisture, we conjecture that improving water management should be able to immobilize Cd and reduce its root uptake and translocation in crop. 【Objective】 The purpose of this paper is to experimentally explore the appropriate water management to reduce Cd uptake and translocation in rice grown in lightly Cd-polluted paddy soils. 【Method】 The experiment was conducted in pots and consisted of seven treatments: Keeping the soil moist at a predefined level during the whole growth season (CK); keeping the pot flooded during the whole growth season (T1), flooding from the stem elongation stage onwards (T2), flooding from the panicle initiation to booting stage onwards (T3), flooding from the heading and flowering stage onwards (T4), flooding from milk grain stage onwards (T5), and flooding from dough grain stage onwards (T6). All treatments were drained one week before harvest. In each treatment, we measured Cd accumulation in different plant tissues, as well as the changes in redox potential and soluble Cd in the rhizosphere. 【Result】 Both redox potential and soluble Cd in soil were positively correlated at significant level (p<0.01). Both redox potential and soluble Cd in soil dropped steadily after flooding, making the soil more reductive. The longer the flooding lasted, the lower the redox potential was. The redox potential in T1 could drop as low as -159.78 mV, and T3 reduced the soluble Cd in the rhizosphere by 75.0%. The cadmium content in the rice grains did not exceed the criterion set in the national food safety standard (GB2762—2017) regardless of the treatments. Compared with CK, T1 reduced Cd content in the rice by 76.9% while in the meantime increasing rice yield by 19.2%. T6 impeded Cd translocation from the roots to the shoots, by increasing the Cd fraction in the roots by 15.2% and reducing the Cd accumulation in the stems by 13.0%. On average, flooding impeded Cd translocation from leaves to grains. Compared with CK, T5 increased the fraction of Cd in leaves by 5.0% while T3 reduced the fraction of Cd in grains by 4.3%. 【Conclusion】 Considering the balance of yield, Cd content in grain and ease of implementation, the best water management for the lightly Cd-contaminated paddy soils was to flood the fields from the heading and flowering stage until one week prior to the harvest.

Keywords

• water management
• rice growth stage
• cadmium
• redox potential
• rhizosphere
• soluble cd