Modifying functional groups of straw-based hydrogel provide soil N₂O mitigation potential by complete denitrification

Farmed soil is considered to contribute more than 60% anthropogenic N₂O emission which plays critical role in global warming potential. Straw-based hydrogels with abundant functional groups are commonly used in environmental and agronomic applications primarily as adsorbents. Those functional groups could mediate electron transfer in the process of adsorbates and might also regulate N₂O emission. However, it was still unclear whether and how these functional groups affect the process. To address this knowledge gap, we employed spectral analysis and a robotized incubation system to determine mechanisms of different functional groups in influencing soil N₂O. The results indicated that the straw-based hydrogel was capable of significantly reducing the emission of N₂O in denitrification, and the electrochemical properties of the hydrogel had promoting effect of the microbial genetic potential for N₂O reduction. Specifically, carboxymethyl (R-COO) and primary amine groups (R-NH₂) promoted complete denitrification through electron supply. Furthermore, R-NH₂ had a significant negative correlation with N₂O emissions. To further verify the role of specific structures and functional groups in denitrification, we conducted an experiment using pure lignin as a template. This experiment resulted in an increase in R-NH₂ content to 13.2% and a decrease in N₂O emissions by 49.1% compared with straw hydrogel. These results prove the feasibility of straw-based hydrogel as a redox system to accelerate complete denitrification, and the electron-donating functional groups were significantly related to the reduction rate of N₂O. Finally, based on these mechanisms, functional group targeted grafting technology was proposed to improve its ability to mitigate N₂O emission reduction.