Additive effects of nitrogen deficiency and water stress on reassembly of the rhizosphere microbiome

Rhizosphere microbial symbioses confer advantages to host plants under environmental stress. However, the effect of multiple stressors on triggering rhizomicrobiome reassembly remains poorly understood, as stress effects can be additive, antagonistic, or synergistic. Tomato was used as a model plant to investigate the interactions between nitrogen deficiency and two contrasting water stresses (drought and waterlogging) on the reassembly of rhizosphere bacterial and fungal communities through quantitative microbiome profiling. The combination of nitrogen deficiency with drought or waterlogging increased α-diversity, except for bacteria under the combined stress of nitrogen deficiency and drought. Nitrogen deficiency generally reduced the abundance of taxonomic groups, with this reduction exacerbated under drought but reversed under waterlogging, particularly for bacteria. Similar results were observed for the abundance of bacterial functional guilds and dominant microbial taxa. The interactive effects of nitrogen deficiency and water stress on microbial α-diversity, as well as on the abundance of taxonomic groups, functional guilds, and dominant taxa were predominantly additive. Additive interactions dominated rhizomicrobiome reassembly in response to nitrogen deficiency and water stress, with distinct response patterns observed between bacterial and fungal communities. This study provides new insights into rhizomicrobiome dynamics under multiple abiotic stressors.