Gradual CO₂ Increase More Strongly Boosts Nitrification and Ammonia Oxidizers in Paddy Soils Than Abrupt Increase

Elevated atmospheric CO₂ can alter nitrogen availability through the regulation of ammonia oxidation in paddy soils. However, most previous research has focused on abrupt CO₂ enrichment, which does not reflect the gradual increase in atmospheric CO₂ under future climate change scenarios. Consequently, the effects of progressive versus abrupt CO₂ enrichment on ammonia oxidizers remain underexplored. Three CO₂ treatments were established using an automated CO₂ regulation system: ambient CO₂ (CK), progressive CO₂ increase (PI; +40 ppm yr.^-1 to +200 ppm), and abrupt CO₂ increase (AI; +200 ppm). Compared with CK, PI increased nitrification potential by 22.28%, whereas AI showed no significant effect. The abundance of ammonia-oxidizing archaea (AOA) remained unchanged across CO₂ enrichment treatments. However, PI significantly elevated the abundances of ammonia-oxidizing bacteria (AOB) and complete ammonia oxidizers (Comammox) by 41.26% and 208.64%, respectively. The CO₂ enrichment treatments significantly altered the community structure of AOB but had no significant effects on those of AOA or Comammox. Soil dissolved organic carbon and NH₄ ⁺-N were the key factors influencing ammonia-oxidizer communities and nitrification potential. Overall, PI exerted stronger stimulatory effects than AI. Additionally, nitrification potential was more closely associated with AOB abundance and community structure than with those of AOA or Comammox.