Nitrification-induced acidity controls CO2 emission from soil carbonates

Nitrification acidifies soil, and the produced H⁺ are neutralized by inorganic carbon (C) in soil leading to irreversible CO₂ emissions. CO₂ released by nitrogen (N) fertilizer-induced acidification is partitioned between solid (CaCO₃ re-precipitation), liquid (dissolved HCO₃⁻ and CO₃²⁻) and gaseous (CO₂) phases. Therefore, quantifying the effects of N fertilization on CO₂ emissions from soil inorganic C is an enormous challenge. ¹⁴C-labeled CaCO₃ was used as a model inorganic C to trace the released CO₂ caused by acidification by five fertilizers: chicken manure, urea, KNO₃, NH₄NO₃, and (NH₄)₂SO₄ added at three N rates. Cropland soil was homogenously mixed with Ca¹⁴CO₃ powder and fertilizers, and the emitted CO₂ was trapped in NaOH solution to determine total CO₂ and ¹⁴CO₂ efflux originated from inorganic C. Fertilization, particularly ammonium-based fertilizers ((NH₄)₂SO₄, NH₄NO₃), strongly decreased soil pH by 0.35 units over 40 days. All fertilizers except KNO₃ increased total CO₂ emissions by 21%–490% compared to the unfertilized control soil. The fertilization effects on cumulative ¹⁴CO₂ emission induced by CaCO₃ neutralization, corresponded to acidification and decreased in the order (NH₄)₂SO₄ > NH₄NO₃ > urea > KNO₃ > chicken manure. Ammonium-based fertilizers induced the strongest CO₂ emissions originated from inorganic C, emitting 1.6–4.5 times more ¹⁴CO₂ than non-fertilized soils during the first nine days. The total CO₂ emissions from SIC were proportional to the fertilizer dose applied. Therefore, we conclude that both the choice of N fertilizers and their application rates need to be considered to control CO₂ emissions originated from inorganic C. The soil inorganic C losses should be prevented not only because of their irreversible contribution to atmospheric CO₂, but also to safeguard ecosystem services of CaCO₃, such as organic matter preservation, soil structure stabilization, and C sequestration.