Maize–peanut intercropping and N fertilization changed the potential nitrification rate by regulating the ratio of AOB to AOA in soils

Maize–peanut intercropping could potentially mitigate nitrogen (N) loss from the soil, a process primarily governed by the net nitrification rate. However, the impact of maize–peanut intercropping on the potential nitrification rate (PNR) and its relationships with key players, such as ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), are not well understood. Herein, we conducted a field experiment involving two management systems and two crops, namely, maize (MP_m) and peanut (MP_p) intercropping, maize monoculture (MM), and peanut monoculture (PM), under three N fertilization rates (no N fertilization, 150 ​kg ​N ​ha⁻¹, and 300 ​kg ​N ​ha⁻¹). Under intercropping (MP_m and MP_p), the abundance of AOA amoA gene increased by 64.8 ​% and 60.3 ​% and the abundance of AOB amoA gene increased by 63.2 ​% and 68.2 ​% compared to the MM and PM monoculture systems, respectively. Furthermore, the abundances of AOA and AOB decreased in MP_p and MM, while AOB increased in MP_m and PM across the N fertilization gradient. The PNR increased corresponding to the N fertilization rates, with intercropping enhancing the PNR in peanut-planted soil but reducing the PNR in maize-planted soil compared to monocropping. Notably, no significant positive relationship between the abundances of AOA or AOB and the PNR. Random forest analysis indicated that the AOB/AOA ratio was an important predictor of the PNR. N fertilization and intercropping regulated the AOB/AOA ratio mainly through controlling the ammonia content and the soil C/N, respectively. These findings highlight the substantial impacts of N fertilization and intercropping on PNR, with the AOB/AOA ratio emerging as a valuable predictive indicator for the PNR.