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.