The steep moisture gradient induced by biochar amendment in the capillary zone significantly reduced N₂O emissions in...

Achieving synchronous high total nitrogen (TN) removal and low nitrous oxide (N₂O) emissions remains a key challenge in constructed wetlands (CWs). We incorporated high water holding capacity biochar into both the capillary zone and the surface layer of CWs to investigate its impact on nitrogen transformation and N₂O emissions. Results demonstrated that biochar added to the capillary zone achieved synchronous TN removal and N₂O emission reduction. Compared to the CW without biochar, the TN removal efficiency increased by 28.1 % and N₂O/removed TN value decreased by 92.6 %. Biochar created a steep moisture gradient in the capillary zone, thereby promoting the partitioning and balance of redox reactions in CW, which suppressed N₂O generation. Meanwhile, biochar enhanced the electron transfer efficiency of N₂O reducing bacteria in the capillary zone, leading to a 102.0 % increase in cytochrome C content. This significantly promoted the enrichment of Rhodanobacter, Comamonas, Flavobacterium, Flavihumibacter and Simplicispira carrying nosZ, which further capture N₂O generated in both the capillary zone and saturated zone, reducing it to nitrogen (N₂) and thereby forming an N₂O emission barrier. In contrast, biochar application in the surface layer enhanced localized nitrification but disrupted the overall redox balance. This study provides a crucial theoretical foundation for optimizing biochar configuration to achieve synchronous nitrogen removal and N₂O emission reduction in CWs.