[Effect of Biochar on NO₃^--N Transport in Loessial Soil and Its Simulation]

The objective of this study was to explore the effects of different amounts of biochar on the migration process and characteristics of NO₃^--N in loessial soil. In this study, six groups of mixed soil samples with biochar and loessial soil mass ratios of 0% (T0), 1% (T1), 2% (T2), 3% (T3), 4% (T4), and 5% (T5) were used as research objects. NO₃^--N was used as the tracer. Through the indoor soil column solute transport simulation tests, the effects of different biochar application amounts on the NO₃^--N transport process in loessial soil were simulated and studied. The results showed that the breakthrough curve of NO₃^--N in loessial soil shifted to the right with the increasing of biochar application, and the peak value gradually decreased. The initial penetration time, complete penetration time, and total penetration time increased with the increasing of biochar application amount. The total penetration time of NO₃^- in the T1, T2, T3, T4, and T5 treatments was 1.26, 2.31, 2.72, 3.22, and 3.57 times that of T0, respectively. The R² was > 0.997 and RMSE was < 2.083 of the two-zone model (TRM). Compared with the convection-dispersion equation (CDE), the TRM model had higher fitting accuracy and could better simulate the NO₃^--N migration process in loessial soil after the application of different contents of biochar. The analysis of the fitting parameters of the TRM model showed that the average pore velocity, hydrodynamic dispersion coefficient, and water content ratio in the movable zone gradually decreased with the increasing of biochar application, whereas the dispersion and mass exchange coefficient showed an increasing trend. The results showed that biochar application could effectively enhance the ability of loessial soil to fix NO₃^--N, reduce the leakage of NO₃^--N to groundwater, and play an important role in maintaining soil fertility and preventing groundwater pollution.