Elevated atmospheric CO2 promotes the contribution of autotrophic nitrification to N2O emissions in a typical summer maize field

Elevated atmospheric carbon dioxide (CO2) concentration generally stimulates nitrous oxide (N2O) emissions from upland soils, leading to a promoted feedback on climate change. However, the underlying mechanisms on how elevated CO2 (eCO2) stimulates N2O emissions from nitrogen (N)-fertilized upland soils remain poorly known. Here, we investigated the effects of eCO2 on soil N2O production pathways and associated gross N transformation rates, as well as on nitrifying and denitrifying microbes. Based on a 13 years of free-air CO2 enrichment (FACE) platform, we found that N2O emissions were significantly increased by 46.7% under eCO2 in a typical summer-maize field. The 15N tracing experiment showed that autotrophic nitrification rate was significantly increased by 11.6% under eCO2, coincided with the significantly higher abundance and shift composition of ammonia-oxidizing bacteria (AOB) under eCO2. Also, eCO2 enhanced gross ammonium (NH4+) immobilization rate but reduced gross nitrate (NO3−) immobilization rate. Autotrophic nitrification accounted for approximately 78% of N2O emissions, followed by heterotrophic nitrification (about 19%) and denitrification (about 3%). It was found that eCO2 increased carbon (C) deposition in soil, which could affect the availability of soil labile organic C and N and alter the composition and activity of nitrifiers. These changes promoted the N2O production derived from autotrophic nitrification, resulting in higher N2O emissions under eCO2. We could deduce that nitrifier denitrification contributed to the increased N2O emissions greatly under eCO2, derived by strongly stimulated ammonia oxidation and microbial respiration (i.e. higher soil CO2 emissions) caused suboxic conditions (i.e. lower soil oxygen (O2) concentration), along with higher abundance ratios of (nirK+nirS)/nosZ. Our results suggest that controlling autotrophic nitrification appropriately is crucial for mitigating N2O emissions from upland soils under rising atmospheric CO2 concentration.