Compound drought-heat legacies promote the contribution of comammox Nitrospira to N2O emissions: as evidenced from both acidic and alkaline soils

The responses of complete ammonia oxidization (comammox) to compound drought and heat and their contributions to post-drought nitrous oxide (N₂O) emissions remain unclear. Through selective inhibition coupled with qPCR quantification, we partitioned N₂O production from ammonia-oxidizing archaea (AOA), bacteria (AOB), and comammox Nitrospira in acidic red and alkaline calcareous soils under three treatments: (i) control (CK): 60 % water holding capacity (WHC), 25 °C; (ii) drought (D): 3 % WHC, 25 °C; and (iii) compound drought and heat (CDH) stress: 3 % WHC, 45 °C. Comammox Nitrospira exhibited faster recovery rates than AOA and AOB during the 28-day rewetting phase. AOA dominated N₂O emissions in red soil, contributing 34.41 % in CK, 43.91 % in D, and 42.97 % in CDH. AOB dominated N₂O emissions in calcareous soil, accounting for 81.78 % in CK, 76.46 % in D, and 69.77 % in CDH. D stress elevated comammox-driven N₂O contributions by 10.43 % in red soil and 1.67 % in calcareous soil compared with CK, while CDH stress increased them by 18.24 % and 4.27 % in these soils, respectively. These results highlight comammox Nitrospira as pivotal and non-negligible regulators in post-drought nitrification, particularly when heat and drought conditions coincide.