Fertilization and sampling time outweigh plant diversity in shaping soil bacterial communities in a long-term grassland experiment

Fertilization often reduces plant diversity and alters soil bacterial communities, while declines in plant diversity themselves also influence bacterial composition and functioning. Because these drivers co-occur in managed grasslands, their relative contributions, and how they vary across the growing season, remain poorly understood. We addressed this in an 11-year grassland experiment with factorial manipulations of plant diversity and NPK fertilization. Soils were sampled in early and late summer to assess bacterial diversity, community composition, and functional potential. Plant species richness and fertilization showed both independent and interactive effects on soil bacteria. Fertilization increased bacterial diversity in early summer but reduced it in late summer. Furthermore, fertilization negated positive plant diversity effects in early summer but not in late summer, indicating strong seasonal context dependency. In addition, fertilization effects on bacterial community composition differed seasonally, with fertilized plots showing greater similarity between early and late summer than unfertilized plots, indicating stronger seasonal dynamics in unfertilized communities. Structural equation models revealed that responses were largely explained by changes in plant biomass driven by fertilization, plant diversity and sampling time, together with fertilization- and time-induced changes in soil pH. Overall, fertilization and sampling time exerted stronger effects on soil bacterial communities than plant species richness, suggesting that nutrient inputs and temporal dynamics are dominant drivers of bacterial community patterns. Repeated within-season sampling is therefore essential for robust inference on how nutrient enrichment and plant diversity jointly shape soil bacterial communities.