Arsenic, Cadmium, and Nickel Accumulation in Rice: Surveys and Mitigation Strategies in Italy

Ensuring high-quality rice with minimal arsenic (As) and heavy metal concentrations in the grain has long been a priority for the rice sector in Italy. The recent reductions in cadmium (Cd) and inorganic arsenic limits for rice, established by the European Commission, along with the proposed introduction of a new limit for nickel (Ni), present additional challenges for rice cultivation. Italy is the largest rice producer in Europe with more than 210,000 hectares under cultivation and rice is grown in uncontaminated soils, mainly with moderately to strongly acid pH and low clay content. Acidic soil condition is known to enhance the availability of heavy metals for plant uptake. Water management practices, influencing the soil oxidation status, have a large impact upon the bioavailability of As and Cd in the soil, showing an opposite behavior. Regarding Ni, research has demonstrated that the bioavailability of the metal in the soil is connected to the dissolution and recrystallization cycles of iron (oxyhydr)oxides, implying a resemblance to the behavior of Cd. However, the global literature on the impact of agronomic practices and the understanding of the bioavailability of Ni in soil, as well as the processes of metal accumulation in the rice grain, remain relatively constrained. In this complex scenario, rice farmers need to implement strategies aimed at reducing the presence of As and heavy metals in rice grain, to ensure compliance with European standards and enhance the overall quality of Italian rice. To address these concerns, several surveys have been undertaken in recent years to assess the levels of As, Cd, and Ni in both rice grain and paddy soil across Italy. Different experimental trials were conducted to develop agronomic strategies aimed at mitigating the presence of As, Cd, and Ni in rice grain. These strategies include (i) water management, (ii) liming, and (iii) rice variety selection. The surveys were carried out in 2021, 2022, and 2023 on 160 samples, including both rice grain, collected in the field or farm warehouses, and soil. The analysis focused on determining the content of inorganic As, Cd, and Ni in rice. The effect of water management on As, Cd, and Ni was investigated through a combination of field and mesocosm trials. Different water management practices, including Alternate Wetting and Drying (AWD), continuous flooding, and aerobic rice, were examined. Field trials were conducted to assess the effect of liming in mitigating the presence of Cd and Ni. Additionally, the influence of genotype was assessed by comparing different Italian rice varieties in dedicated field trials. The monitoring results provided an overview of the conditions of both Italian soils and rice. It was observed that the concentrations of Cd and Ni in rice were significantly affected by water availability in the cultivation areas, which was contingent upon the climatic conditions during the growing season, as well as the agronomic management practices applied by the farmers. The trials conducted to assess the effect of water management revealed that the levels of Ni and Cd in rice grains increased significantly when dry periods occurred in the irrigation practices, compared to the continuous flooding. The effect intensified with prolonged dry periods, as seen in aerobic cultivation or with an increase in the severity of dry periods in AWD management. Dry periods seemed to have a greater impact on increasing the concentration of Ni in the rice grain compared to Cd. Conversely, As concentrations in rice were higher with continuous flooding. Field trials demonstrated that the application of lime to increase pH of paddy soils resulted in a strong reduction of Cd levels in the rice grain. While liming influenced both Cd and Ni, it was notably more effective in reducing Cd contents than Ni levels. Nevertheless, liming has proven to be an important strategy for farmers in mitigating Cd and Ni levels in rice, even under condition