Laboratory and numerical modelling of irrigation infiltration and nitrogen leaching in homogeneous soils

Nitrogen (N) plays a key role in crop growth and production; however, data are lacking especially regarding the interaction of biochar, grass cover, and irrigation on N leaching in saturated soil profiles. Eighteen soil columns with 20-cm diameter and 60-cm height were designed to characterize the effects of different grass cover and biochar combinations, i.e., bare soil + 0% biochar (control, CK), perennial ryegrass + 0% biochar (C1), Festuca arundinacea + 0% biochar (C2), perennial ryegrass + 1% biochar (C3), perennial ryegrass + 2% biochar (C4), perennial ryegrass + 3% biochar (C5), F. arundinacea + 1% biochar (C6), F. arundinacea + 2% biochar (C7), and F. arundinacea + 3% biochar (C8), on periodic irrigation infiltration and N leaching in homogeneous loess soils from July to December 2020. Leachates in CK were 10.2%–35.3% higher than those in C1 and C2. Both perennial ryegrass and F. arundinacea decreased the volumes of leachates and delayed the leaching process in the 1%, 2%, and 3% biochar treatments, and the vertical leaching rate decreased with biochar addition. The N leaching losses were concentrated in the first few leaching tests, and both total N (TN) and nitrate (NO₃^-)-N concentrations in CK and C1–C8 decreased with increasing leaching test times. Biochar addition (1%, 2%, and 3%) could further reduce the leaching risk of NO₃^--N and the NO₃^--N loss decreased with biochar addition. However, compared to 1% biochar, 2% biochar promoted the leaching of TN under both grass cover types. The N leaching losses in CK, C1, C2, C3, C4, C6, and C7 were primarily in the form of NO₃^--N. Among these treatments, CK, C1, and C2 had the highest cumulative leaching fractions NO₃^--N (> 90%), followed by those in C3, C4, C6, and C7 (> 80%). The cumulative leaching fraction of NO₃^--N decreased with increasing leaching test times and biochar addition, and 3% biochar addition (i.e., C5 and C8) reduced it to approximately 50%. The one-dimensional advective-dispersive-reactive transport equation can be used as an effective numerical approach to simulate and predict NO₃^--N leaching in saturated homogeneous soils. Understanding the effects of different biochar and grass combinations on N leaching can help us design environmentally friendly interventions to manage irrigated farming ecosystems and reduce N leaching into groundwater.