Objectives Investigate the effects of different nitrogen fertilizer application rates on cadmium (Cd) accumulation in maize and its physiological response mechanism. This research aims to lay the foundation for the reasonable use of nitrogen fertilizer, reduce the cost of restoration and environmental pollution in future plant remediation by using Zea mays L. (maize) and Solanum nigrum L. intercropping system.
Methods The Cd-hyperaccumulator S. nigrum and maize were used as test materilas, soil with Cd concentration of 2.79 mg/kg, the pot experiment was conducted to study the three treatments of different nitrogen fertilizers rates, the treatments were monoculture and non-fertilizer, N 0.1 g/kg, and N 0.2 g/kg in competition mode, and study the effects of the nitrogen fertilizer application rate on the growth and Cd accumulation in various organs of maize and S. nigrum.
Results The research showed that the application of nitrogen fertilizer could increase the biomass of various organs of maize and S. nigrum, the biomass of S. nigrum and maize increased significantly with the increase of nitrogen application rate. The biomass increment of the root, stem, leaf and grain was the largest in the high application rate of nitrogen fertilizer, which increased by 47.2%, 51.0%, 25.3% and 63.2% for S.nigrum, respectively. The biomass of root, stem, leaf and grain of maize increased by 35.5%, 17.0%, 76.2% and 112%, respectively. Under different application rates of nitrogen fertilizer treatments, the concentration of Cd in the organs of S. nigrum increased significantly, while the concentration of Cd in the organs of maize was decreased significantly. Under maize and S. nigrum competition mode, with the high application rate of nitrogen fertilizer treatment, the concentration of Cd in root, stem, leaf and grain of S. nigrum increased by 23.2%, 41.2%, 12.3% and 45.3%, respectively. The Cd concentration in root, stem, leaf and grain of maize decreased by 49.2%, 38.0%, 42.8% and 19.5%, respectively. Under high application rate of nitrogen fertilizer treatment, Cd accumulation in organs of S. nigrum increased significantly, and Cd accumulation in root, stem, leaf and grain increased by 85.6%, 88.4%, 131% and 159%, respectively; the Cd accumulation in organs of maize was significantly reduced, and the Cd accumulation in stem, leaf and grain decreased by 12.2%, 34.8% and 79.5%, respectively. Under high application rate of nitrogen fertilizer, the enrichment coefficient of S. nigrum increased by 113% and the translocation coefficient increased by 15.1% under the competition mode between maize and S. nigrum; the enrichment coefficient of maize decreased by 25.7%, and the transport coefficient decreased by 15.2%. Under the competition mode between maize and S. nigrum, as the amount of nitrogen fertilizer increased, the Cd uptake and translocation of S. nigrum increased, and the Cd absorption and transportion of maize would be inhibited. Therefore, in the Cd contaminated soil, through the maize and S. nigrum intercropping treatment and the appropriate application of nitrogen fertilizer, the growth of S. nigrum and the Cd accumulation of shoots could be increased. The results of this study were intended to provide a theoretical basis for remediation of contaminated soil, improvement of remediation efficiency, and assurance of quality and safety of agricultural products.
Conclusions Under maize and S. nigrum competition mode, the high nitrogen application rate of N 0.2 g/kg could promote the uptake and translocation of Cd by hyperaccumulator S. nigrum, reduce the accumulation of Cd in various organs of maize, and improve the remediation efficiency of Cd contaminated soil, thus achieve the goal of the production and remediation at the same time.
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