Identification and localization of key ammonia-oxidizing microorganisms in facility cucumber soils

China has the largest area of production facilities for vegetables in the world. The cucumber is an important crop of facility cultivation, which accounted for about 77% of the global production. Fertilization is an important production measure to improve facility cucumber stable yields, it has been reported that the amount of N fertilizer in cucumber production is as high as 684.0 kg ha-1. Excessive use of N fertilizer in facility cucumber has drastically deteriorated soil health. Soil microorganisms play an important role in the facilities agricultural production by helping to maintain or restore soil fertility by decomposing organic matter, influencing nutrient cycling, forming humus, and promoting biodiversity . The soil nitrogen cycle is driven by microorganisms, and when environmental factors such as pH change, the soil nitrogen cycle and nitrogen utilization by plants are subsequently altered responsively , and the composition and function of microbial communities driving the nitrogen cycle are also affected. By clarifying whether microorganisms involved in soil ammonia oxidation are affected by soil nitrogen nutrition, the key microorganisms driving ammonia oxidation in facility cucumber rhizosphere soil will be clarified. The key step of the soil nitrogen cycle is nitrification. Nitrification consists of two steps: Ammonia oxidation and Nitrite oxidation. The ammonia oxidation is the rate-limiting step, which is mainly participated by ammonia-oxidizing bacteria and ammonia-oxidizing archaea. Nitrogen fertilization is the main factor leading to the alteration of soil ammonia-oxidizing microbial communities, which synergistically affects the abundance and diversity of AOA and AOB communities together with the pH of the soil. NH4+-N, as an indirect substrate for AOA and AOB metabolism, has a direct relationship with community structure . There are some differences in the research of different scholars on the factors affecting nitrifying microorganisms. Most of them focused on soil depth, nitrogen fertilizer type, etc., and without resolving the responsive changes of microbial communities mediating soil nitrification to nitrogen application, ignoring the positioning of dominant ammonia-oxidizing microorganisms. In this experiment, we investigated the correlation between the diversity and community structure of AOA and AOB and soil physicochemical properties in the rhizosphere and non-rhizosphere soils of solar greenhouse cucumber, to clarify the changes in the responsiveness of AOA and AOB to nitrogen application, and explore the key microorganisms and major influencing factors driving the soil nitrogen cycle in solar greenhouse cucumber. To locate the advantageous ammonia-oxidizing microorganisms in solar greenhouse cucumber and provide a theoretical basis for creating a favorable microenvironment for microbial activity.