High spatial variations of potential nitrification and N₂O production across the Yangtze River and associated major tributaries

Globally, rivers can yield a large amount of nitrous oxide (N₂O) by microbial nitrogen processes. However, how the nitrification mediated N₂O production in river sediments remains poorly understood. Here, we investigated potential nitrification and N₂O production rates, and ammonia-oxidizing bacteria (AOB) communities in the mainstream and eight major tributaries of the Yangtze River. Potential nitrification and N₂O production rates varied significantly, which were both significantly lower in the middle reach than in the upper and lower reaches. Potential nitrification and N₂O production rates also varied significantly across the tributaries, in which Wujiang and Hanjiang showed higher rates. Nitrification dominated N₂O production in the upper reach, whereas denitrification was mainly responsible for N₂O production in the middle and lower reaches. Moreover, denitrification was the dominant pathway for N₂O production in the tributaries (except for Ganjiang). AOB diversity varied significantly across the upper, middle and lower reaches. Microbial complexity and stability of AOB were significantly higher in the lower reach than the middle and upper reaches. Sediment size, NH₄⁺, Fe(II), and DOC significantly correlated with AOB abundance, potential nitrification, and N₂O production. These results suggest that sediment characteristics greatly affect N₂O production by modifying nitrogen availability and microbial processes. In addition, enhanced nutrient availability can increase microbial complexity and further facilitate nitrification activity and N₂O production, representing a positive climate warming feedback loop.