Biochar impacts on soil nitrogen and carbon dynamics in a Spodosol amended with biosolids and inorganic fertilizer

Despite evidence suggesting that biochar can retain nutrients, particularly nitrogen (N), and reduce the risks of transport, research on the co-application of biochar with organic and inorganic fertilizer sources is limited. Three laboratory studies (herein referred to as static incubation, column leaching, and batch sorption) were conducted to evaluate the impacts of two biochar materials (pine and grass biochar generated at temperatures of 800 and 400°C, respectively) on N and carbon (C) dynamics in soils amended with different N sources (ammonium nitrate, Class B, and Class AA biosolids). Nitrogen sources were applied at an equivalent rate of 180 kg N ha^-1 while biochar was applied at a 1% (w/w) rate. Biochar effects on soil N and C dynamics were variable and dependent on biochar and N sources. A negligible but significant effect of pine biochar in reducing NH₄ leaching was observed; however, both biochar materials were ineffective in reducing NO₃ or inorganic N leaching. Reductions in leachate NH₄ by pine biochar were attributed to relative greater ability of this material to retain NH₄ than grass biochar. Both biochar materials exhibited no ability to sorb NO₃ . Similarly, biochar had no effect on soil N₂ O emissions. Class B biosolids resulted in greater N leaching and soil N₂ O emissions compared to other fertilizers. This response was likely due to inherently high levels of inorganic N and moisture, which possibly favored denitrification. Further research is warranted to better understand the underlying mechanisms controlling soil N and C dynamics and responses to co-application with fertilizer sources.