Applying Nitrogen Fertilizer at the Full Heading Stage Has the Potential to Decrease Brown Rice Cd Accumulation

Soil cadmium (Cd) contamination has become a serious problem in China. In this study, the goals of the field experiments were to understand how the application of nitrogen fertilizer at the full heading stage and filling stage reduced the uptake of Cd in rice by affecting the distribution of Cd in iron plaques on the root surfaces and subcellular in the root and flag leaf. The hydroponic culture experiments aimed to explore the effect of interaction or deficiency of nitrogen and cadmium on cadmium accumulation in rice at the late growth stage. The results showed that under the condition of having enough nitrogen supply during the early growth stage, applying nitrogen fertilizer during the full heading stage and filling stage resulted in a significant increase in the concentration of Fe and Cd on the root plaques at the milk stage and mature stage. Additionally, it increased the concentration and proportion of Cd in the soluble fraction of subcellular in the flag leaves at the milk stage. On the other hand, when there was a deficiency of nitrogen, the concentration of Fe in the root plaques increased significantly, while the concentration of Cd in the root plaques decreased significantly. Moreover, the proportion of Cd in the flag leaf cell walls increased significantly. Regardless of whether there was sufficient or deficient nitrogen supply during the early growth stage, applying nitrogen at the full heading stage reduced the Cd concentration in brown rice by 35.11% and 57.25%, respectively. Under hydroponic culture conditions with both Cd exposure and later-stage nitrogen supply significantly increased the Cd concentration in brown rice. However, in the absence of Cd, nitrogen supply significantly reduced the Cd concentration in brown rice. The accumulation of Cd in brown rice was significantly correlated with the accumulation of Fe and Mn at the late growth stage. Our findings indicate that when there is sufficient nitrogen nutrition during the early growth stage of rice, topdressing nitrogen fertilizer at the full heading stage combined with lime with mineral element fertilizers such as Fe and Mn is an effective approach to reduce the presence of cadmium in rice grains.