Abstract: Reducing heavy metal inputs is one of the key measures for the safety of agricultural production. We investigated heavy metal pollution sources and excessive irrigation water status at watershed scale in a typical Cd polluted Huanjiang section of Great Huanjiang River Basin. We purified irrigation water by the grit chamber and 3-level constructed wetland system, and simultaneously coupled with adding slaked lime and biochar materials into paddy soil as Cd activity control measures. The main objective of this study was to understand the combined reduction effects of Cd contaminated irrigation water purification with soil Cd activity control on Cd accumulation in rice. Results showed that Cd concentration in irrigation water displayed obvious fluctuation during April to November 2020, with the maximum and minimum inflow concentration of 8.38  and 2.15 μg·L^-1 into the constructed wetland system, respectively. The mean Cd concentration declined from 5.88±3.27 to 0.69±0.35 μg·L^-1 after purification through the constructed wetland system, while no reduction effect was observed in the grit chamber process. The best Cd removal result was observed in the firstclass wetland, the total Cd removal rate of the wetland system reached 88.3%, and the effluent water quality satisfied the irrigation water standard. The addition of hydrated lime, biochar and lime plus biochar reduced Cd accumulation in rice grain by 9.2%, 1.5% and 20.0%, respectively, compared to the single irrigation water purification treatment. Single lime treatment with the same addition amount performed better than biochar treatment. The reduction rate of Cd in rice grain under the treatment of irrigation water purification coupled with passivation materials reached 45.8% compared with the local traditional rice production mode. Therefore, irrigation water purification by constructed wetland system, combined with soil Cd activity control, could effectively reduce Cd input into the paddy soil and Cd accumulation in rice, and thus effectively improved rice production security coefficient. The results can provide scientific support and practical reference for the safety production of Cd-contaminated rice fields in southern China.

Key words: irrigation water, artificial wetland, lime, biochar, rice grain Cd accumulation.