Soil ammonia oxidation process and its driving factors in the riparian zone of drainage ditch in saline-alkali area of Ningxia, Northwest China.

Ammonia oxidation plays a critical role in nitrogen cycling within riparian zones. To investigate this process in saline-alkali soils of the Yinbei region, northern Yinchuan, Ningxia, we selected five distinct riparian types along the Third Drainage Ditch: gravel-reed mixed zone, reed zone, high-salt zone, embankment zone and bare soil zone. We quantified soil potential nitrification rates (PNR), environmental factors, and analyzed ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) communities via me-tagenomics and qPCR targeting genes. The results showed that the total potential nitrification rate (PNR) in the riparian zones ranged from 0.47 to 1.37 μmol N·g·d, with PNR (potential nitrification rate of AOA) being higher than PNR (potential nitrification rate of AOB). The copy number of AOA genes (2.63×10-2.06×10 copies·g) was significantly higher than AOB genes (7.14×10-9.55×10 copies·g). The PNR and gene copy number in the reed zone, gravel-reed mixed zone, and embankment zone were higher than those in the high-salt zone and bare soil zone, indicating that nitrification in the riparian zones was dominated by AOA. AOA were affiliated with the phylum Nitrososphaerota, with the dominant genera being and . The physicochemical factors, including ammonium, nitrite, nitrate, electrical conductivity, total organic carbon, and total nitrogen exhibited significant differences among different riparian zones. PNR and PNR were significantly influenced by ammonium, total nitrogen, total organic carbon and pH, and PNR showed a highly significant positive correlation with gene copy number. Structural equation modeling (SEM) results indicated that pH and total organic carbon were the primary factors affecting nitrification in the riparian zones and that AOA gene copy number showed significant positive correlation with nitrification rate.