A 15N-Tracing Study to Explore the Coupling Effects of Biochar and Nitrogen Fertilizer on Tomato Growth, Yield, Nitrogen Uptake and Utilization, and the Rhizosphere Soil Environment under Root-Divide Alternative Irrigation

To investigate the coupling effects of biochar and nitrogen fertilizer on tomato growth, nitrogen uptake and utilization (NUU), and the soil environment, a pot experiment was conducted using 15N-tracing technology from March to July 2021 and from September 2021 to January 2022. Three biochar application rates (B0, B1, and B2; 0, 3, and 6 t/hm2, respectively) and three nitrogen levels (N1, N2, and N3; 150, 300, and 450 kg/hm2, respectively) were set up. The results show that the growth, yield, rate of 15N uptake, nitrogen derived from soil (Ndfs), total nitrogen (TN), 15N utilization, and recovery rate of tomatoes were improved under biochar application, but nitrogen derived from fertilizer (Ndff) gradually decreased. A Pearson correlation analysis showed that the 15N uptake, Ndfs, TN, rhizosphere soil organic matter, soil organic carbon, and TN were significantly positively correlated with the yield and lycopene content of tomatoes. The comprehensive benefit to the tomatoes was evaluated based on combination weighting with the help of the technique for order preference by similarity to ideal solution (TOPSIS). This indicates that the best planting mode was the B2N2 treatment, with a biochar rate of 6 t/hm2 and nitrogen levels of 300 kg/hm2, under the alternative partial root-zone irrigation.