Italian ryegrass (Lolium multiflorum L.)-rice (Oryza sativa L.) rotation promotes the nitrogen cycle in the rice rhizosphere through dominant ammonia-oxidizing bacteria

The dominant ammonium bacteria established in different crop rotations are directly related to the conversion and utilization of nitrogen (N) in paddy soil. This study investigated the ammonia oxidation process in the rice rhizosphere of Italian ryegrass (Lolium multiflorum L.)-rice (IRR), celery (Apium graveolens)-rice (CLR) and winter fallow-rice (WFR) rotations in South China through the amoA functional gene analysis based on Miseq high-throughput sequencing platform and qPCR methods. The abundance of amoA gene in IRR was increased by 116.67 % (P < 0.05) and 42.76 % (P > 0.05) compared with CLR and WFR, respectively. Nitrosomonas became the dominant genus in the rhizosphere of IRR, with the highest relative abundance (59.30 %). The contents of soil organic matter, total N and nitrate nitrogen (NO3<overline>-N) in the rice rhizosphere soil in IRR were significantly higher than other two treatments (P < 0.05) by 12.47-14.31 %, 16.88-25.00 %, and 97.48-159.60 %, respectively. Although lower microbial diversity was observed in IRR, the soil fluorescein diacetate hydrolysis enzyme activities and ammonia monooxygenase activities in the rice rhizosphere were higher in IRR than those in CLR and WFR (P < 0.05). Also, the soluble sugar and crude protein of milled rice were highest in IRR among the three rotation treatments (P < 0.05). The ammonia oxidation efficiency of ammonium-oxidizing bacteria was largely dependent on key taxa, such as Nitrosomonas (dominant genus in IRR), rather than the dynamics of AOB community structure. The key amoA-nitrifier taxa of the AOB community developed in IRR are capable of promoting rice production by improving soil N cycle efficiency and reducing the loss of N. Our findings illustrate the regulatory mechanism of IRR on the rice rhizosphere AOB community and reveal the important role of winter ryegrass in promoting soil nitrogen cycling during nutrient transformation of agricultural ecosystems in paddy fields.