Abstract: Microbially induced carbonate precipitation (MICP) is an emerging soil improvement technique. However, its practical application often relies on multiple injection cycles, leading to prolonged construction periods, high material consumption, and potential environmental risks. To achieve efficient solidification under single-injection conditions, this study introduces sodium bicarbonate (NaHCO3) to regulate the reaction nucleation pathway and crystal evolution. The effects of NaHCO3 on precipitation behavior, crystal structure, interfacial cementation strength, and mechanical properties were systematically investigated in different calcium source systems (CaCl2, Ca(NO3)2, Ca(CH3COO)2). The results indicate that sodium bicarbonate, by providing an exogenous carbonate source and buffering the system pH, inhibits homogeneous nucleation and promotes the transformation of amorphous calcium carbonate into stable calcite. This significantly optimizes the spatial distribution of precipitates and cementation efficiency. In the CaCl2 system, the unconfined compressive strength of sand columns reached 814 kPa after a single injection—approximately 120% higher than that achieved with conventional MICP after five injection cycles. Moreover, disturbance resistance was markedly enhanced, with the mass loss rate under ultrasonic disturbance decreasing from 46.5% to 5.4%. Microstructural characterization further revealed that the strength of the sand columns depends primarily on the stability of the precipitate structure and the effective cementation proportion at particle contacts, rather than on the total amount of precipitate generated. This study demonstrates that through precise regulation of the nucleation pathway and spatial deposition mechanisms, efficient, rapid, and low-environmental-impact soil reinforcement via MICP can be achieved under single-injection conditions. The findings provide new theoretical support and a technical approach for the engineering application of this technology.