• Decreased microbiome diversity and stability due to enhanced leaf litter leaching. • Expansion of the anoxic zone into the unsaturated zone due to increased organic carbon supply. • Decreased soil microbiome metabolic potential for cellulolysis and N 2 fixation in unsaturated soil. • Depth-dependent response of microbial community to increased organic carbon availability. • Implications for soil response to climate change. Climate change is expected to affect precipitation intensity and soil temperature and indirectly impact the release of leached dissolved organic carbon (LDOC) from leaf litter during the early stages of its decomposition, which could affect the health and function of forest soil ecosystems. Here, we experimentally investigate the spatially-explicit impact of LDOC on the forest soil microbiome and the associated biogeochemical processes. Homogenized soil columns were subjected to realistic artificial precipitation for 3 months with the initial level of LDOC adjusted by the number of times the leaf litter was flushed in preparation for the experiment. Hydrological and geochemical parameters (redox potential, pH, dissolved oxygen, soil moisture, matric potential, chemical speciation) were measured continuously as a function of time and depth. The same initial microbial community developed into distinct communities under different LDOC and above and below the water table. The LDOC from leaf litter increased the availability of carbon (C) and nitrogen (N) in porewater four-fold and two-fold respectively in the first two weeks. This resulted in the expansion of the anoxic zone above the water table and a decrease in the soil microbial metabolic potential for cellulolysis and N 2 fixation in unsaturated soil along with an increase of soil microbial metabolic potential for fermentation at all depths. Finally, increased LDOC decreased the stability, phylogenetic diversity, and complexity of the soil microbiome, limiting its functional diversity. Thus, management of leaf litter should receive more attention due to its indirect role in the impact of climate change on the soil microbiome.