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.