Modeling Calcite's Sensitivity to Biogenic CO₂ Production: A Pathway to Soil CO₂ Efflux Partitioning

Soil inorganic carbon (SIC), primarily calcite, represents a potentially reactive carbon reservoir, influencing soil-atmosphere CO₂ exchange and acid-base buffering processes. Though often considered stable, SIC is sensitive to biogenic CO₂ and acidification, risking extra CO₂ emissions beyond soil organic matter (SOM) mineralization. This study investigates SIC reactivity using δ¹³C-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 CO₂ pressures, controlling porewater acid carrying capacity. Dissolution promoters (H₂O, H⁺, and H₂CO₃) 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. δ¹³C tracing showed SIC-sourced CO₂ 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 CO₂ sources, SC turnover, and climate feedbacks.