Afforestation promotes soil nitrous oxide production by ammonia oxidizers

The contribution of ammonia–oxidizing archaea (AOA), bacteria (AOB), and complete ammonia oxidizers (comammox) to nitrous oxide (N2O) production following afforestation remains poorly understood, hindering our ability to assess the sustainability of rehabilitated ecosystems. We investigated the factors influencing aerobic N2O in cropland and plantation forest soils in southwest China using molecular techniques and inhibitor experiments. Our findings reveal that afforestation significantly increased N2O production derived from AOA, AOB, and comammox. This increase can be attributed to enhanced N mineralization and ammonia availability, likely driven by elevated calcium concentration, alleviated phosphorus limitation, and aggravated microbial nitrogen limitation. Furthermore, we observed that N2O production was predominantly mediated by AOB at both land–use types, accounting for 42–47%, followed by AOA (34–37%). Comammox contributed 10–11% to total N2O production, with an N2O yield of approximately 0.04–0.22%. Such positive impacts of afforestation on ammonia oxidizers produced N2O should be fully considered when assessing afforestation's climate change mitigation outcomes and for the sustainability of ecosystems.