Complete ammonia-oxidizing bacteria dominate ammonia oxidation rates and N2O emissions after vegetation restoration in subtropical karst forest soils

Complete ammonia-oxidizing bacteria (comammox) are crucial for understanding soil N₂O emission mechanisms. Although comammox abundance and composition have been analyzed in various ecosystems, few studies have determined ammonia oxidation rates and the relative contributions of comammox, ammonia-oxidizing archaea (AOA), and ammonia-oxidizing bacteria (AOB) to N₂O emissions from subtropical karst forest soils, especially after vegetation restoration. To address this gap, we explored the total abundance, community structure, and ammonia oxidation rates of AOA, AOB, and comammox, as well as their relative contributions to N₂O emissions, in cropland (control) and subtropical forest soils in a karst ecosystem. Our findings showed that compared with cropland, the total abundance of comammox exhibited a significant increase (17.65 %) in forest at the RNA level. Moreover, the dominant genera in AOA, AOB, and comammox communities were Nitrososphaera, Nitrosospira, and Nitrospira, respectively, in both cropland and forest soils. Additionally, the ammonia oxidation rates of comammox increased by almost 4.4 times after vegetation restoration, which attributed to the high ammonia affinity in the low ammonia nitrogen (NH₄⁺-N) environment. The relative contribution of comammox to N₂O emissions was significantly higher in forest (40.87 %) than in cropland (11.25 %), which was attributed to soil texture differences. In conclusion, after vegetation restoration, low NH₄⁺-N increased comammox ammonia oxidation rates due to the high ammonia affinity, and coarse-textured soils stimulated more N₂O emission owing to the sluggish convective flow of slurry and limiting the redistribution of NH₄⁺. Our study highlights the important role of comammox in regulating ammonia oxidation in subtropical forest soils, which has important implications for improving soil nitrogen (N) cycling and ecosystem restoration in karst regions.