Multi-walled carbon nanotubes improve nitrogen use efficiency and nutritional quality in Brassica campestris

In order to explore the effects of addition of multi-walled carbon nanotubes (MWCNTs) to soil on the nutritional quality and nitrogen assimilation and utilization by Brassica campestris, a pot experiment was conducted with five treatments: conventional fertilization (CK), conventional fertilization + MWCNTs (C), 10% nitrogen (N) reduction + MWCNTs (N₁ + C), 20% N reduction + MWCNTs (N₂ + C), and 30% N reduction + MWCNTs (N₃ + C). The concentration of MWCNTs was 10 mg per kg air-dried soil in each pot. Soil and plant samples were collected on the 15th, 30th and 45th days after treatments. Soil characteristics and activities of enzymes related to nitrogen metabolism were determined. The N assimilation, ¹⁵N abundance and the nitrogen utilization efficiency (NUE) in soil and plants were determined using the ¹⁵N stable isotope tracer technique. The results showed that the plant height, fresh weight, dry weight, leaf width and leaf area in N₂ + C treatment increased by 13.16%, 53.97%, 35.11%, 55.09% and 66.63%, respectively, while there was no significant difference between the other treatments and CK. The contents of soluble sugar and protein increased by 71.36% and 26.39%, respectively, after N₂ + C treatment. N₂ + C treatment significantly improved the NUE by 85.92%. Further tests showed that N₂ + C treatment enhanced the activities of soil enzymes (27.10% for urease, 12.06% for sucrase, 55.64% for protease, 33.62% for nitrate reductase) and N metabolism enzymes (70.65% for nitrate reductase, 60.89% for glutamine synthetase and 67.29% for glutamate synthetase) in plants, which may contribute to the improved NUE. This study for the first time suggests that MWCNTs can be potentially applied in agriculture to improve the NUE of plants.