Effects of Silicon Application on Nitrogen Migration in Soil–Rice Systems under Cadmium Stress

Cadmium (Cd) contamination is a serious threat to plants and humans, which subsequently impairs sustainable agricultural production and ecosystem service. Silicon (Si) has been applied to mitigate Cd toxicity, but inevitably has direct and indirect impacts on nitrogen (N) behaviors in soil and plants. However, what role plants play in the N response to Si in soil–rice systems under Cd stress is not known. Therefore, the effects of Si on N migration through different pathways in the soil-rice system were systematically investigated in a rice-cultivation lysimeter experiment. The rice was planted in Cd-contaminated (5 mg kg −1 ) and uncontaminated soils with three levels of Si application (0, 100, and 200 kg SiO 2 hm −2 ), and the contents of N and Cd in different forms in plants and soils were measured. The group without Cd and Si was set as CK. The study reported that Cd stress caused Cd accumulation in plants, inducing a decrease of 26.0~83.4% in plant dry weights and a decrease of 15.7~46.6% in N concentration compared with CK. Moreover, the leaching of N in soils was increased by Cd, in which the NO 3 − -N rather than the NH 4 + -N was leached out. These adverse effects on the plant growth and soil N loss were significantly alleviated by Si application in two ways: (1) the Cd availability in soils was reduced with the acid-extractable Cd (the Cd form with high mobility), decreasing from 1.07 to ~0 mg kg −1 ; (2) the Cd uptake and translocation in plants were restricted, with the Cd content decreasing by 59.1~96.4% and the translocation index decreasing from 17.7% to 2.2%. The combination of the two mechanisms consequently increased the N absorption of plants from 1.35 to 2.75~3.5 g. The results of the N mass balance calculation showed that, compared with soil N flux, plant-absorbed N contributed predominantly (43.9~55.6%) to the soil total N variation. Moreover, there is a significant trade-off between plant-absorbed N and soil N flux. The magnitude and direction of the soil N flux were greatly and negatively affected by plant-absorbed N during the flooding period. Hence, we conclude that Si application could reduce the leaching of N in soil–rice systems under Cd stress, mainly due to the promotion of the N absorption of plants rather than N immobilization in soils. This study provided new evidence that plants played a dominant role in N response to Si in soil-rice systems under Cd stress.