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.