Soil salinization threatens soil organic carbon (SOC) sequestration. Although microbial necromass carbon (MNC) is crucial for SOC formation and stability, how biochar affects MNC in saline–alkaline soils remains unclear. This study assessed the impact of biochar amendment (0, 10, 20, and 30 t ha⁻¹) on SOC and MNC dynamics in saline–alkaline soils cultivated with Arundo donax cv. Lvzhou No. 1 across tillering, jointing, and maturity stages. Biochar amendment significantly enhanced SOC and the soil C/N ratio, with the highest dose (30 t ha⁻¹) raising SOC by 47.21% at jointing and 34.64% at maturity. Biochar significantly increased MNC at all growth stages, with increases ranging from 22.74% to 30.81%. From the jointing to the maturity stage, SOC exhibited a decline (20.03 to 27.77%), in contrast to the minimal change in MNC (–6.37% to 9.80%). This divergent trend consequently led to a peak in the MNC/SOC ratio at maturity. It directly demonstrates the relative stability of MNC and indicates its role as a persistent carbon reservoir within the topsoil. Biochar also elevated soil pH and nutrient availability, which reshaped microbial community structure and enhanced bacterial diversity. Partial least squares path modeling revealed that biochar facilitates MNC accumulation directly and indirectly by modifying soil chemical properties and thereby enhancing microbial diversity. These findings show that biochar enhances stable SOC storage in saline–alkaline soils primarily through the formation and stabilization of microbial necromass, thus revealing its potential for climate change mitigation.