Water amount, rather than watering frequency, structures root‐associated microbial communities and their links to plant performance

Climate change is altering both the amount and temporal distribution of precipitation, yet drought is often simplified to a single gradient of water reduction. As a result, how experimentally imposed variation in water amount and watering frequency jointly shape root‐associated microbial communities and their links to plant performance remains poorly understood. Using a 3‐year full‐factorial drought experiment in a dry grassland system, we disentangled the individual and interactive effects of water amount, watering frequency and plant composition (conspecific stands versus two‐species mixtures) on root‐associated microbiome and tested whether microbial variation was linked to plant performance. Water amount emerged as the dominant driver of microbial structure, strongly shifting bacterial and fungal composition and reducing bacterial diversity. Watering frequency had weaker but detectable effects, acting mainly on bacterial communities and partly interacting with water amount, whereas the specific contrast between conspecific stands and two‐species mixtures had limited effects. Bacterial community composition was associated with root trait variation and plant fitness, suggesting that drought‐driven bacterial reassembly formed part of the pathway through which water regime influenced plant performance. Together, our results show that water amount and watering frequency represent distinct dimensions of experimental drought with partly independent effects and highlight bacterial reassembly as an important component of grassland responses to increasingly severe and temporally variable drought. Read the free Plain Language Summary for this article on the Journal blog.