Microbial Activation of Terrestrial Phytogenic Silica Cycling

Phytogenic silica ( p Si) is a bio‐reactive phase of the terrestrial silicon (Si) cycle; however, the mechanisms governing its fate and mobility remain elusive. Plants sequester Si to form p Si, which is inherently embedded within the organic matrix of their biomass, resulting in organic matter‐associated p Si ( Op Si). Following biomass decomposition, Op Si and p Si are released into soil pools where they may transition into colloidal phases that serve as pivotal drivers for the transport and biogeochemical transformation of associated elements. Here, we identify a previously unrecognized microbial‐driven engine governing Op Si and p Si mobility. We hypothesized that bacteria release Op Si and p Si by enzymatically consuming the organic matrix and degrading the particle coatings. This process triggers a shift in interfacial reactivity that markedly increases the surface charge and colloidal mobility of p Si via electrostatic repulsion. To test this hypothesis, we utilized a model Op Si and p Si colloid pair prepared from rice straw and Bacillus velezensis to quantify evolving biophysical interactions. Micro‐morphological and surface charge analyses confirmed the presence of an intact organic sheath shielding the p Si core. This organic shield maintains a low surface charge density on Op Si (i.e., 5.7 ± 0.5 site nm −2 ) and negligible bacterial repulsion, effectively keeping p Si stagnant. When the organic sheath is consumed by microbes, the exposure of the high‐charge silica core causes the surface charge density to escalate dramatically, reaching 34.1 ± 0.7 site nm −2 upon the complete conversion of Op Si to p Si. This microbially driven “activation” transforms stagnant p Si into a highly mobile phase, thereby increasing its mobility in the environment.