Anaerobic digestate (AD) is an increasingly prominent component of circular agriculture, yet its rapid biogeochemical effects require nuanced evaluation across laboratory and field settings. This study integrated a fully controlled 28-day soil incubation with an in situ longitudinal field tracking trial (15 m³·ha⁻¹ AD applied prior to winter wheat cultivation) to assess organic carbon mineralization kinetics and seasonal nutrient dynamics. In the incubation experiment, AD addition triggered an immediate acceleration of carbon mineralization, doubling the mineralization rate constant (k = 0.41 day-1 vs. 0.19 day-1 in the unamended control) and significantly reducing the biological carbon half-life from 3.7 to 1.7 days. In the field, the immediate post-application phase (Day 14, Term II) was marked by a transient flush of available N (+25%) and S (+67%), confirming rapid early-stage nutrient mobilization. Across the 295-day seasonal cycle (culminating after crop harvest in Term IV), the soil exhibited significant increases in organic matter, available P, and exchangeable K, alongside a pH shift from 7.0 to 7.5 and a reduction in bioavailable Fe and Mn. Because the field component employed a longitudinal baseline-tracking design, late-season nutrient and carbon increments represent the combined legacy of AD conditioning, ambient seasonal dynamics, and crop rhizosphere turnover. These findings demonstrate that AD serves as an active organic-mineral conditioner, optimizing early nutrient supply while supporting the progressive restructuring of the soil sorption complex over the cropping season.