Application of two contrasting rice-residue-based biochars triggered gaseous loss of nitrogen under denitrification-favoring conditions: A short-term study based on acetylene inhibition technique

Denitrification is the key microbial process that leads to gaseous loss of soil nitrogen in agricultural lands. Most alarmingly, the dominant gas species could be N₂O, which is a much stronger greenhouse gas than CO₂. In addition to the primary role as tool for soil carbon sequestration, biochar has the potential to suppress N₂O emissions. However, the mechanism for the suppression of N₂O emissions by biochar remains elusive. To address this, we performed a short-term incubation experiment targeting the impact of two contrasting biochars on gaseous loss of soil N under denitrification-favoring conditions including high load of NO₃ ⁻ (100 µg N/g soil), anoxia and high moisture content (70% WFPS). The acetylene inhibition technique was adopted to differentiate N losses as N₂ and N₂O. Two biochars produced from rice chaff (600 °C, pH > 10, C:N 70) or rice husk (300 °C, pH < 5, C:N 54) were applied at two rates (5 and 10% w/w). Results exhibited an increase in gaseous loss of N in all biochar treatments compared with an unamended control. The presence of a positive correlation between total denitrification and CO₂ emission rates suggested that biochar derived labile carbon played a pivotal role in triggering the loss of N in gaseous forms. Similarly, the abundances of denitrifying genes nosZ and nirS were considerably higher in biochar treatments, indicating denitrifier's heterotrophic nature. The biochar pyrolysis conditions and application rates played a decisive role in controlling N₂O emissions. Rice chaff biochar, primarily characterized by its alkaline pH, significantly suppressed N₂O emissions. Taken together, biochar amendment to agricultural soil can trigger N losses via denitrification and only the alkaline biochar suppressed N₂O emissions, probably because of the enhanced activity of N₂O reductase.