Soil Acidification by Nitrification Controls Co 2 Emission from Soil Carbonates: 14 C Labeling of Caco 3

Nitrification contributes to soil acidification, and therefore strongly and irreversibly to the neutralization of soil inorganic carbon (SIC), e.g., CaCO3, and CO2 emissions. CO2 released by nitrogen (N) fertilizer-induced acidification is partitioned between solid (CaCO3 re-precipitation), liquid (dissolved HCO3-) and gaseous (CO2) phases. Therefore, quantifying the effect of N fertilization on SIC-originated CO2 emissions is an enormous challenge. Here, 14C-labeled CaCO3 was used as a model SIC compound to trace its neutralization products, including CO2 emissions, caused by a range of fertilizers (chicken manure, urea, KNO3, NH4NO3, and (NH4)2SO4). Cropland soil was homogenously mixed with Ca14CO3 powder (11.3 kBq pot-1) and fertilized with 0, 0.1, 0.15, and 0.25 g N kg-1 soil, and the emitted CO2 was trapped in NaOH to determine total CO2 and SIC-originated 14CO2 effluxes. 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. The effects of fertilization on cumulative 14C-CO2, i.e. SIC dissolution, corresponded to fertilizers’ acidification potential and increased in the order chicken manure < KNO3 < urea < NH4NO3 < (NH4)2SO4. Among all fertilizers, SIC-originated CO2 emissions were highest for ammonium-based fertilizers, emitting 1.6-4.5 times more 14CO2 than nonfertilized soils during the first nine days. The CO2 emissions from SIC were proportional to the fertilizer dose applied. Therefore, we conclude that both the choice and application rates of N fertilizers need to be considered for plant nutrition and to control SIC-originated CO2 emissions. The SIC losses should be prevented not only because of their irreversible contribution to atmospheric CO2, but also to safeguard ecosystem services, such as organic matter persistence, soil structure stability, and C sequestration.