Abstract
Soil inorganic carbon (SIC), primarily calcite, represents a potentially reactive carbon reservoir, influencing soil-atmosphere CO(2) exchange and acid-base buffering processes. Though often considered stable, SIC is sensitive to biogenic CO(2) and acidification, risking extra CO(2) emissions beyond soil organic matter (SOM) mineralization. This study investigates SIC reactivity using δ(13)C-enriched calcite (11.9 t ha(-1), +102.02‰) under organic residue decomposition, examining the effects of residue type (maize vs wheat), degradability (leaves vs roots), and placement (mixing vs mulching). Incubations at 25 °C with 80% soil-water saturation coupled high-resolution pH optodes and HYDRUS-PHREEQC simulations to quantify SIC reactivity. Mixed applications of labile maize leaves (C:N = 17.3) intensified topsoil (∼50% of the 10 cm column) acid loading (pH 7.9 → 5.7), promoting decarbonation and deepening acidification front (>3.2 cm). Soil respiration emerged as a key influencer of CO(2) pressures, controlling porewater acid carrying capacity. Dissolution promoters (H(2)O, H(+), and H(2)CO(3)) drove topsoil decarbonation (0.84 t C ha(-1) in mixed profiles vs 0.06 t C ha(-1) in mulched) and subsoil (5-10 cm) bicarbonate accrual. δ(13)C tracing showed SIC-sourced CO(2) peaks (+25 to +51‰, 40-60% contribution) during incubation's first quarter (∼day 16-24) prior to SOM-domination (0 to -12‰, 20-10%), reflecting a mixed continuum of CO(2) sources, SC turnover, and climate feedbacks.