Modelling Sorption and Leaching Behaviour of Sulphate in Light Sierozem (Calcids) Columns with Rape Straw Biochar Amendments with Steady Flow

Biochar amendments could enhance retention of nutrients such as ammonium (NH4+), nitrate (NO3-), and phosphate (PO43-) in soils. However, the situation for sulphate (SO42-), which is an indispensable nutrient element for crop growth, is unclear. In this paper, the effects of biochar derived from rape (Brassica campesstris L.) straw at 600°C on the sorption and leaching of SO42- in light sierozem (Calcids) was studied in columns, where biochar amendment rate, column soil height, solution pH value and initial sulphate concentration were selected as factors. It is shown that the transport of sulphate was a significant non-equilibrium process and the sorption and leaching curves (SLCs) of sulphate were asymmetrical. The breakthrough time would be increased by increasing biochar amendment and soil column height, and by decreasing solution pH value and initial sulphate concentration. The SLCs of bromide trace were fitted to determine dispersion coefficient (D) using equilibrium convection dispersion equation (CDEeq). The non-equilibrium (two-site) model (CDEnon-eq) with the results from CDEeq was used to simulate the transport processes of sulphate in the soil column, with good fitness, using software CXTFIT 2.1 fitting. The results could supply an implication for biochar application in loess areas.