Water-based nitrogen fixation in lettuce cultivated by low-temperature plasma aeroponic system

Excessive use of nitrogen fertilizer has caused environmental issues, such as soil acidification, water eutrophication, and greenhouse gas emissions, all of which harm sustainable agriculture. Soil acidification can also degrade the soil health and fertility, thus reducing the cultivation capacity damage to agricultural land in the long term. Water eutrophication can also trigger the algal blooms to deplete oxygen and then harm aquatic life. Efficient and eco-friendly nitrogen fixation is highly required for sustainable agriculture against these negative impacts. The crop yields can also be enhanced for overall resource efficiency. In this study, nitrogen fixation was explored in fog cultivation using low-temperature plasma aeroponic technology. The suspended droplet solutes were selected as a source of hydrogen from the fog cultivation tank. A plasma generator was utilized with the fog cultivation system. Plasma Activated Mist (PAM) was produced to facilitate the interactions between micron-sized droplets and plasma. The numerical simulation was conducted under specific discharge conditions, namely a temperature of 300 K, a voltage of 15 kV, and an ambient air pressure of 1.01×10⁵ Pa. The electron continuity equation and the drift-diffusion equation of electron density were employed to better understand the behavior of the plasma under needle-needle electrode discharge. There were valuable insights into the fundamental process of the plasma in fog cultivation. A cultivation experiment was carried out to verify the effectiveness of the nitrogen fixation. The lettuce was taken as the test crop. The plasma generator was selected as an AIRNASA model, KJF04. A gas detection system was also employed to monitor the concentration trends of NH₃ and O₃. Two key substances were generated during plasma activation. The results revealed that the plasma was concentrated between the needle electrodes, with an electron density reaching 1.93e¹⁸ (1/m³). The concentrations of NH₃ and O₃ in the generated substances were measured to be 0.21, and 0.304 μmol/L, respectively. These plasma-activated substances were found to significantly enhance the efficiency of energy transfer for the high activity of chemical reactions. There was a positive impact on the plant growth and soil quality. Specifically, the plasma atomized cultivation system improved the nitrogen increment of lettuce leaves, stem length, and root length by 17.84%, 71.49%, and 42.36%, respectively, and the leaf area by 53.85%. Moreover, the energy consumption increased by 0.14 kW·h over a 42-day test period, compared with the conventional atomized cultivation. The crop yield and quality were improved with minimal additional production costs. The low-temperature plasma aeroponics can effectively excite and transform the hydrogen atoms in fog droplets and nitrogen atoms in the air, thereby improving the growth rate and nutrient uptake efficiency of crops. The promising potential can be found to significantly stimulate the growth potential of lettuce in the efficient utilization of aerosolized droplets. The crop yield and quality were greatly enhanced to reduce the need for chemical fertilizers and pesticides, compared with traditional fog cultivation. A promising technological pathway can be offered in agricultural production, particularly for cost-saving and efficiency. Overall, low-temperature plasma aeroponics can serve as a sustainable and efficient alternative to conventional nitrogen fixation in agriculture, especially with environmentally friendly and economically viable farming practices in the future.