Combined use of olive mill waste compost and sprinkler irrigation to decrease the risk of As and Cd accumulation in rice grain

Abstract

Sprinkler irrigation has been successfully introduced in rice production as an alternative to the traditional flooding system, allowing water savings and the reduction of As accumulation in the grain. However, the same conditions can increase Cd mobility and grain accumulation, an effect that needs to be countered. A 3-year field experiment was set-up in a Mediterranean region (Extremadura, Spain), to evaluate how the application of compost from olive mill waste (single application, 80 t ha^-1), influences the accumulation of As and Cd in the grain under different irrigation regimes. Accumulation of As in the grain was always lower in the sprinkler irrigation when compared with the flooding irrigation, reaching a 5-fold difference in the third year. Compost application did not evidence a clear effect on the As accumulation in the rice grain, but highly significant negative correlations (p < 0.001) were obtained between As content in the grain (total, inorganic, and organic) and the humification parameters in the soil, evidencing the importance of using a mature and stable organic amendment to avoid As accumulation in the grain. Cadmium accumulation in the rice grain decreased in each treatment where compost was applied, relatively to the non-treated counterpart (e.g., from 0.080 to <0.010 mg kg^-1, in direct seeding with sprinkler irrigation, in the third year). There were no significant differences in the total inorganic As between treatments with or without compost application, but it was possible to observe an increase in the predominance of the organic As over the more toxic inorganic As, when compost was applied, allowing a decrease in the risk associated to As accumulation. Therefore, the aerobic cultivation of rice, with the simultaneous application of an adequate source of organic matter, can be considered a good solution to cope with the risk of accumulation of As and Cd in the rice grain.

Keywords: Arsenic speciation; Deficit irrigation; Flooding irrigation; Potentially toxic elements accumulation; Rice production; Soil organic amendment.