Impact of iron-modified biochars on soil nitrous oxide emissions: Variations with iron salts and soil fertility

Nitrous oxide (N₂O) emissions from soils are a significant environmental concern due to their contribution to greenhouse gas emissions. Biochar has been considered as a promising soil amendment for its potential to influence soil processes. Iron modification of biochar has been extensively discussed for its ability to enhance adsorption of pollutants, yet its impact on mitigating soil N₂O emissions remains poorly understood. In the present study, corn straw (CB) and wood (WB) biochars were treated with FeSO₄/FeCl₃ (SCB and SWB) and Fe(NO₃)₃ (NCB and NWB). The effects of these biochars on soil N₂O emissions were investigated using soils with varying fertility levels over a 35-day incubation period at 20 °C. Results revealed significant variations in biochar surface chemistry depending on biochar feedstock and iron salts. Compared to pristine biochars, NWB and NCB exhibited higher pH, total N content, and dissolved NO₃-N concentrations (246 ± 17 and 298 ± 35 mg kg^-1, respectively), but lower bulk and surface C content. In contrast, SWB and SCB demonstrated acidic pH and elevated dissolved NH₄-N concentrations (5.38 ± 0.43 and 4.19 ± 0.22 mg kg^-1, respectively). In forest soils, NWB and NCB increased cumulative N₂O emission by 28.5% and 67.0%, respectively, likely due to the introduction of mineral nitrogen evidenced by significant positive correlation with NO₃-N or NH₄-N. Conversely, SWB and SCB reduced emissions in the same soil by 28.5% and 6.9%, respectively. In agricultural soil, most biochars, except SWB, enhanced N₂O emissions, possibly through the release of labile organic carbon facilitating denitrification. These findings underscore the significance of changes in biochar surface chemistry and the associated potential risk in triggering soil N₂O emissions. This study highlights the need for a balanced design of biochar that considers both engineering benefits and climate change mitigation.