Two-year and multi-site field trials to evaluate soil amendments for controlling cadmium accumulation in rice grain

Highlights: • Soil Mn amendment can effectively decrease Cd accumulation in rice grain on acidic soils low in Mn. • Non-additive interactions between soil liming and MnO₂ amendment on grain Cd were discovered using a factorial treatment design. • Mn-loaded biochar was most effective in decreasing grain Cd by simultaneously increasing extractable Mn and decreasing extractable Cd of field-moist soils at rice grain-filling stage. Representing the staple crop for half of the world population, rice can accumulate high levels of cadmium (Cd) in its grain, posing concerns on food safety. Different soil amendments have been proposed to decrease Cd accumulation in rice grain by either decreasing soil Cd availability, introducing competitive ions on Cd uptake, or down-regulating the expression of transporters for Cd uptake. However, the effectiveness of soil amendments applied alone or in combinations needs to be tested under field conditions. Here, we present results of field trials with two rice cultivars differing in Cd accumulation grown at three field sites in southern China in two years, to investigate the effects of two Mn-containing soil amendments (MnO₂, Mn-loaded biochar (MB)), Si fertilizer (Si), limestone, and K₂SO₄, as well as interactions among MnO₂, Si, and limestone on decreasing Cd accumulation in rice grain. We found that single applications of MnO₂ or MB to acidic soils low in Mn decreased grain Cd concentrations by 44–53 % or 78–82 %, respectively, over two years without decrease in performance. These effects were comparable to or greater than those induced by limestone liming alone (45–62 %). Strong interactions between MnO₂ and limestone resulting from their influence on soil extractable Cd and Mn led to non-additive effects on lowering grain Cd. MB addition minimized grain Cd concentrations, primarily by increasing extractable and dissolved Mn concentrations, but also by decreasing Cd extractability in soil. In comparison, Si and K₂SO₄ amendments affected grain Cd levels only weakly. We conclude that the amendments that decrease labile Cd and increase labile Mn in soils are most effective at reducing Cd accumulation in rice grain, thus contributing to food safety.