Structure, variation and assembly processes of bacterial communities in different root-associated niches of tomato under periodic drought and nitrogen addition

Root-associated bacteria play a vital role in the growth and adaptation of host plants to drought stress. These bacteria can be classified as rhizoplane and rhizosphere bacteria based on their distance from the root surface. Tomato plants are often exposed to periodic drought and nitrogen (N) addition throughout their life cycle, but the impacts of these factors on the plant and root-associated bacteria are not well understood. To gain insight into this relationship, we conducted an experiment to monitor the effects of periodic drought and N addition on rhizoplane and rhizosphere bacteria of tomato plants. Drought and N addition had interactive effects on plant and soil properties, which varied with the timing of drought. There were clear divergences in community traits such as alpha diversity, beta diversity, and network topological features between the two types of bacteria. The rhizoplane bacteria showed lower alpha diversity but higher beta diversity and were more sensitive to drought and N addition than the rhizosphere bacteria. Nitrogen addition could downsize the effects of drought on rhizoplane bacterial community compositions. The higher proximity to the root might induce a community to develop more cooperation between different members to cope with plant metabolites, as revealed by the more connected and modularized community network of the rhizoplane bacteria. Drought at the seedling stage had great legacy effects on plant and soil properties. It may enhance selection, cause the dominance of deterministic processes in the assembly of rhizoplane bacteria, and reduce bacterial community network complexity. In conclusion, N addition could interact with drought in affecting tomato plants and their root-associated bacteria, depending on the timing of drought and the fineness of root niches. The higher sensitivity of rhizoplane bacteria to drought and N addition calls for more research due to their higher proximity and importance to plants in future environmental changes.