Extremely dry calcareous soil AOB
In extreme drought conditions, the activity and community structure of ammonia-oxidizing bacteria (AOB) in calcareous soils exhibit unique characteristics. AOB are key players in the nitrogen cycle, responsible for the oxidation of ammonia to nitrite. In calcareous soils, which are typically alkaline and rich in calcium, AOB often dominate over ammonia-oxidizing archaea (AOA) due to their higher affinity for ammonia and ability to thrive in nutrient-rich environments. Under extreme drought, soil moisture decreases significantly, which can directly affect microbial growth by reducing the soil water potential. Despite these harsh conditions, AOB have been shown to maintain or even increase their transcription levels of the *amoA* gene, which encodes the enzyme responsible for ammonia oxidation. This suggests that AOB may be more resilient to drought compared to AOA, which typically show a decline in activity under similar conditions. The resilience of AOB in drought conditions can be attributed to their ability to utilize the accumulated ammonia (NH4+) and organic nitrogen that becomes available as plants reduce their nitrogen uptake during drought. This accumulation of nitrogen compounds creates favorable conditions for AOB, allowing them to continue their metabolic activities even when other microbial groups may struggle. Furthermore, the community structure of AOB in calcareous soils under drought conditions may shift towards specific taxa that are better adapted to osmotic stress and high inorganic nitrogen levels. This shift can have broader implications for nitrogen cycling and greenhouse gas emissions, as AOB are known to produce higher yields of nitrous oxide (N2O) compared to AOA. In summary, AOB in calcareous soils demonstrate a remarkable ability to adapt and thrive under extreme drought conditions, maintaining their role in nitrogen cycling and potentially influencing the overall soil ecosystem dynamics.