Divergent response of soil nitrogen and ammonia-oxidizing microorganisms to alfalfa-wheat rotation system under precipitation variability in semi-arid region

Legume rotation is a well-established management practice for enhancing soil nitrogen availability. Ammonia-oxidizing microorganisms play a key role in soil nitrogen cycle process by regulating nitrification process. However, the impact of soil nitrogen variations induced by perennial legume rotation on ammonia-oxidizing microorganisms, particularly their sensitivity to precipitation variability, remain poorly understood in semi-arid region. We investigated the changes in soil nitrogen level and community characteristics of ammonia-oxidizing microorganisms based on a field experiment with 3-year alfalfa ( Medicago sativa L.)-wheat ( Triticum aestivum L.) rotation under three precipitation treatments (normal precipitation; increased precipitation 30 %; decreased precipitation 30 %) on the Loess Plateau of China, with wheat monoculture as the control. The results showed that alfalfa-wheat rotation reduced 0–10 cm soil total nitrogen (TN), nitrate nitrogen (NO 3 − -N) and ammonium nitrogen (NH 4 + -N) contents by 19 %, 45 % and 14 %, compared with wheat monoculture. Additionally, alfalfa-wheat rotation decreased the operational taxonomic units (OTUs) and alpha diversity of the ammonia-oxidizing bacteria (AOB) community, whereas the OTUs and alpha diversity of the ammonia-oxidizing archaea (AOA) community were increased under both increased and decreased precipitation 30 %. A key-stone ecological cluster hereafter from co-occurrence network analysis exhibited a strong correlation with soil available nitrogen, with Nitrosospira (AOB) and Nitrosopumilus (AOA) emerging as the central genera in ammonia oxidation. Our findings demonstrated that the AOB community was more sensitivity to soil nutrient alterations induced by alfalfa-wheat rotation. In contrast, the AOA community was primarily regulated by soil water content. Our results highlighted the importance of AOA community in sustaining ammonia oxidation in semi-arid region under future climate scenarios.