Yuanyuan Li, Hanjun Zhou, Chen Xu Nie, Yunjie Wu, Qian Zhang, Zhao Ben, Xiefeng YE
Soil bulk density (BD) acts as a fundamental physical constraint that regulates the rhizosphere microenvironment and plant development. In this study, we conducted an in-situ pot-in-field experiment with five distinct soil bulk density gradients (0.5, 0.8, 1.1, 1.4, and 1.7 g·cm⁻³), where pots were embedded into field furrows to ensure natural environmental conditions. We systematically investigated the effects of soil BD on flue-cured tobacco root morphology, anatomy, soil physicochemical properties, enzyme activities, and rhizosphere microbial community structure. Our results demonstrated a non-linear response to soil compaction: while extreme compaction (T5, 1.7 g·cm⁻³) maximized soil moisture, it severely restricted gas exchange and inhibited root expansion. Conversely, an optimal soil BD of 1.1 g·cm −3 (T3) balanced capillary porosity and aeration, facilitating the largest stele diameter, a highly developed phloem system, and vigorous root hair production. This optimal physical state (1.1 g·cm⁻³) significantly enhanced root morphology-characterized by a complex dichotomous branching pattern—and promoted the recruitment of beneficial rhizosphere microbiomes compared to compacted or excessively loose soils. Structural equation modeling (SEM) revealed that soil BD influences plant performance through a cascaded pathway: modifying soil physical and biological properties, which subsequently dictates root anatomical plasticity and topological architecture. These findings challenge the "lower is better" paradigm of soil compaction management and identify 1.1 g·cm −3 as the threshold for optimal tobacco growth. Our study provides a mechanistic framework for understanding how physical soil constraints modulate the plant-microbe-soil continuum, offering crucial insights for precision soil management in intensive tobacco production systems.