Microbial changes induced by defoliation: Stronger associations with root traits than soil properties in annual ryegrass.

Microorganisms associated with belowground carbon availability play a critical role in the functions and sustainability of grazing grassland ecosystems. However, research on microbial responses to varying grazing intensities at different soil depths remains limited. This study examined rhizosphere bacterial communities under varying defoliation intensities in a 24-week pot experiment involving repeated defoliation of annual ryegrass (Lolium rigidum). We assessed bacterial diversity, composition, co-occurrence networks, and influencing factors across three soil layers (depths of 0-5, 5-10 and 10-15 cm). Results revealed significant reductions in root growth and root sugar concentrations (fructose, glucose, and sucrose) with increasing defoliation intensity, leading to decreased dissolved carbon and microbial biomass carbon in the rhizosphere. Defoliation influenced bacterial diversity, community composition, and complexity of co-occurrence networks, with effects shifting from positive to negative with increasing soil depth. The magnitude of these effects varied with defoliation intensity. Increased defoliation intensity and shallower soil depth were associated with simplified bacterial networks and a reduced abundance of keystone taxa. The negative effects of defoliation on bacterial co-occurrence network complexity were primarily linked to root variables, with root fructose being the most important predictor according to random forest analysis. Overall, bacterial community structure was more closely associated with root traits than soil properties, and buffered defoliation-induced reductions in root traits at the soil surface but not at greater depths. These findings provide new insights into the response of soil microbes to moderate grazing and highlight root traits as key indicators for steering the functions of grassland ecosystems.