Influence of biochar pyrolysis temperature on <sup>15</sup>N fertilizer uptake at different harvest times in tall fescue (<i>Festuca arundinacea</i>) production

Biochar soil amendment has gained attention as a strategy to enhance nitrogen (N) use efficiency and reduce N losses in agricultural systems. This study evaluated the effects of co-applying non-labelled ammonium sulphate and 15N-labelled ammonium sulphate with rice husk-derived biochar produced at 400 °C, 600 °C, and 700 °C on the 15N uptake in tall fescue (Festuca arundinacea) during a 90-day greenhouse experiment. The results showed that the treatment using 400 °C biochar in combination with ammonium sulphate fertilizer significantly enhanced the uptake and retention of 15N ammonium sulphate. This treatment resulted in the highest percentage of nitrogen derived from the fertilizer, the greatest cumulative recovery of 15N in plants (77.7%), and the lowest potential N losses (22.1%). In contrast, treatments with biochars produced at 600 °C and 700 °C showed intermediate performance, while the treatment with ammonium sulphate alone (without biochar) exhibited the lowest 15N recovery (47.9%) and the highest N losses (51.9%). The superior performance of the 400 °C biochar treatment is likely attributed to its higher cation exchange capacity, and, the presence of oxygenated-containing functional groups, which favor ammonium adsorption. Conversely, the biochars produced at higher temperatures showed increased aromaticity, larger specific surface areas, and higher degree of crystallinity. However, they exhibited lower cation exchange capacity, which was likely due to the loss of polar functional groups. As a result, their ability to retain nutrients was limited. Some mechanistic interpretations were inferred from comparable literature, highlighting the need for direct characterization through X-ray diffraction and N₂ adsorption analysis. Overall, this research highlights the importance of selecting biochar with an appropriate carbonization temperature to optimize nitrogen retention and improve fertilizer efficiency in pasture production systems.