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

Abstract

The addition of biochar to soil can effectively increase the efficiency of nitrogen (N) use and reduce N losses in agricultural systems. This study evaluates the effects of adding non-labeled ammonium sulfate and ¹⁵N-labeled ammonium sulfate with rice husk-derived biochars produced at 400 °C, 600 °C, and 700 °C on ¹⁵N uptake in tall fescue (Festuca arundinaceous) during a 90-day greenhouse experiment. The uptake and retention of ¹⁵N ammonium sulfate were significantly increased after treatment with 400 °C biochar plus ammonium sulfate fertilizer. This treatment maximized the percentage of N derived from the fertilizer and the cumulative recovery of ¹⁵N in plants (77.7%), while minimizing the potential N losses (22.1%), while treatment with the 600 °C or 700 °C biochar moderately improved the uptake. In contrast, the treatment with ammonium sulfate alone (without biochar), ¹⁵N recovery was only 47.9% and the N losses were the highest (51.9%) among the treatments. The superior performance of the 400 °C biochar treatment is likely explained by its high cation exchange capacity and the presence of oxygen-containing functional groups, which favor ammonium adsorption. The biochars produced at higher temperatures were more aromatic with larger specific surface areas and higher degrees of crystallinity but had lower cation exchange capacities than 400 °C biochar, possibly because their fewer polar functional groups reduced their nutrient retention. Mechanistic interpretations inferred from comparable literature highlight the need for direct characterization by X-ray diffraction and N₂ adsorption analysis. The research indicates that biochar formed at an appropriate carbonization temperature can optimize nitrogen retention and improve the efficiency use of fertilizers in pasture production systems.