Water addition regulates the metabolic activity of ammonia oxidizers responding to environmental perturbations in dry subhumid ecosystems

<jats:title>Summary</jats:title><jats:p>Terrestrial arid and semi‐arid ecosystems (drylands) constitute about 41% of the <jats:styled-content style="fixed-case">E</jats:styled-content>arth's land surface and are predicted to experience increasing fluctuations in water and nitrogen availability. Mounting evidence has confirmed the significant importance of ammonia‐oxidizing archaea (<jats:styled-content style="fixed-case">AOA</jats:styled-content>) and bacteria (<jats:styled-content style="fixed-case">AOB</jats:styled-content>) in nitrification, plant nitrogen availability and atmospheric <jats:styled-content style="fixed-case"><jats:roman>N<jats:sub>2</jats:sub>O</jats:roman></jats:styled-content> emissions, but their responses to environmental perturbations in drylands remain largely unknown. Here we evaluate how the factorial combinations of irrigation and fertilization in forests and land‐use change from grassland to forest affects the dynamics of <jats:styled-content style="fixed-case">AOA</jats:styled-content> and <jats:styled-content style="fixed-case">AOB</jats:styled-content> following a 6‐year dryland field study. Potential nitrification rates and <jats:styled-content style="fixed-case">AOA</jats:styled-content> and <jats:styled-content style="fixed-case">AOB</jats:styled-content> abundances were significantly higher in the irrigated plots, accompanied by considerable changes in community compositions, but their responses to fertilization alone were not significant. <jats:styled-content style="fixed-case">DNA</jats:styled-content>‐stable isotope probing results showed increased <jats:styled-content style="fixed-case"><jats:roman><jats:sup>13</jats:sup>CO<jats:sub>2</jats:sub></jats:roman></jats:styled-content> incorporation into the <jats:styled-content style="fixed-case"><jats:italic>amoA</jats:italic></jats:styled-content> gene of <jats:styled-content style="fixed-case">AOA</jats:styled-content>, but not of <jats:styled-content style="fixed-case">AOB</jats:styled-content>, in plots receiving water addition, coupled with significantly higher net mineralization and nitrification rates. High‐throughput microarray analysis revealed that active <jats:styled-content style="fixed-case">AOA</jats:styled-content> assemblages belonging to <jats:styled-content style="fixed-case"><jats:italic>N</jats:italic></jats:styled-content><jats:italic>itrosopumilus</jats:italic> and <jats:styled-content style="fixed-case"><jats:italic>N</jats:italic></jats:styled-content><jats:italic>itrosotalea</jats:italic> were increasingly labelled by <jats:styled-content style="fixed-case"><jats:roman><jats:sup>13</jats:sup>CO<jats:sub>2</jats:sub></jats:roman></jats:styled-content> following irrigation. However, no obvious effects of land‐use changes on nitrification rates or metabolic activity of <jats:styled-content style="fixed-case">AOA</jats:styled-content> and <jats:styled-content style="fixed-case">AOB</jats:styled-content> could be observed under dry conditions. We provide evidence that water addition had more important roles than nitrogen fertilization in influencing the autotrophic nitrification in dryland ecosystems, and <jats:styled-content style="fixed-case">AOA</jats:styled-content> are increasingly involved in ammonia oxidation when dry soils become wetted.</jats:p>