What shapes nitrifiers in drylands? Global drivers of AOA and AOB abundance across microsites

Nitrifying microbes play a central role in soil N cycling by controlling N transformations and the potential for N losses (including the greenhouse gas N₂O). Yet, the drivers of nitrifier communities remain poorly resolved across global drylands, which cover more than 40% of terrestrial surface. A mechanistic, global scale understanding of the controls on nitrifiers is thus critical for forecasting dryland N cycling and N loss pathways under climate change and land-use pressures. Using a standardized global dryland survey spanning 98 dryland rangelands in 25 countries, we quantified the abundance of ammonia-oxidizing bacteria (AOB), and ammonia-oxidizing archaea (AOA) across contrasting vegetated and bare microsites (a defining feature of dryland landscapes). We applied (structural/linear) equation modeling to assess climatic, edaphic, geographic, grazing, and vegetation controls. Controls on nitrifier abundance differed between microsites. Vegetated microsites were mainly driven by soil resources: ammonium showed a positive relationship with AOB and the total abundance of nitrifiers, whereas soil organic C had a consistent negative effect. Bare microsites showed stronger climatic control, with AOA and total nitrifiers exhibiting a U-shaped response to mean annual temperature. We didn’t see any effects of increased grazing pressure on total nitrifiers. Overall, these results highlight microsite context as a key regulator of nitrifier communities across global dryland rangelands. They indicate that changes in vegetation cover and patch structure through their effects on vegetated–bare soil balance and canopy buffering are likely to be a key pathway by which ongoing global change restructures nitrifier abundance and nitrogen cycling in drylands.