Silica-Biochar (Si-char) control of nitrate leaching in a soil box experiment

Nitrate leaching is one of the major challenges in agricultural soils in variable climate conditions, causing changes in rainfall patterns influence soil water behavior and nutrient losses. Soil amendments such as biochar and silica have been introduced as an effective strategy to mitigate nitrate leaching by modifying soil water dynamics. However, their combined effects on water percolation and nitrate transport under repeated natural rainfall events are still not well understood. Therefore, in this study, we evaluated the hydrological behavior and nitrate leaching of soils amended with biochar and silica using a soil box experiment (60cm×30cm×20cm) conducted under natural conditions. Four treatments were selected for this project: a control, biochar (2% w/w), silica (1% w/w), and a combined biochar–silica (Si-char) treatment with three replications for each treatment. After preparing the soil, amendments, and soil boxes, the experiment started with an initial saturation phase to establish the same soil moisture conditions in different treatments. Five natural rainfall events were recorded in this phase. Percolation volumes and nitrate concentrations were measured after each rainfall event.

The results showed that across all five rainfall events, biochar and silica treatments reduced nitrate leaching compared to the control. However, the Si-char treatment consistently showed the lowest nitrate leaching, which is the most effective mitigation of nutrient losses. These results highlight the synergistic effects of amendments compared to single application of amendments. Statistical analysis using one-way ANOVA confirmed that nitrate leaching differed significantly among treatments during all rainfall events (p<0.05). In addition, ANOVA results for individual treatments across the events revealed contrasting response patterns. In the control treatment, the difference in nitrate leaching between rainfall events was significant, indicating high sensitivity to rainfall variability. On the other hand, the Si-char treatment showed fewer statistical groups between rainfall events, which indicates that it was more stable and has more predictable behavior in the face of changing rainfall conditions. The better performance of Si-char suggests that biochar and silica acted as a complement. Biochar likely increased soil nitrate retention, while silica contributed to more stable soil water retention and reduced sensitivity to rainfall variability. Together, these processes increased the residence time of water in the soil and limited nitrate transport beyond the root zone. Overall, the results showed that Si-char is the most effective treatment compared to others for reducing nitrate leaching under repeated rainfall events. This approach may be considered by land managers for sustainable nitrogen management in dry farming systems.