Nitrogen fertilization raises CO₂ efflux from inorganic carbon: A global assessment

Nitrogen (N) fertilization is an indispensable agricultural practice worldwide, serving the survival of half of the global population. Nitrogen transformation (e.g., nitrification) in soil as well as plant N uptake releases protons and increases soil acidification. Neutralizing this acidity in carbonate-containing soils (7.49 × 10⁹  ha; ca. 54% of the global land surface area) leads to a CO₂ release corresponding to 0.21 kg C per kg of applied N. We here for the first time raise this problem of acidification of carbonate-containing soils and assess the global CO₂ release from pedogenic and geogenic carbonates in the upper 1 m soil depth. Based on a global N-fertilization map and the distribution of soils containing CaCO₃ , we calculated the CO₂ amount released annually from the acidification of such soils to be 7.48 × 10¹²  g C/year. This level of continuous CO₂ release will remain constant at least until soils are fertilized by N. Moreover, we estimated that about 273 × 10¹²  g CO₂ -C are released annually in the same process of CaCO₃ neutralization but involving liming of acid soils. These two CO₂ sources correspond to 3% of global CO₂ emissions by fossil fuel combustion or 30% of CO₂ by land-use changes. Importantly, the duration of CO₂ release after land-use changes usually lasts only 1-3 decades before a new C equilibrium is reached in soil. In contrast, the CO₂ released by CaCO₃ acidification cannot reach equilibrium, as long as N fertilizer is applied until it becomes completely neutralized. As the CaCO₃ amounts in soils, if present, are nearly unlimited, their complete dissolution and CO₂ release will take centuries or even millennia. This emphasizes the necessity of preventing soil acidification in N-fertilized soils as an effective strategy to inhibit millennia of CO₂ efflux to the atmosphere. Hence, N fertilization should be strictly calculated based on plant-demand, and overfertilization should be avoided not only because N is a source of local and regional eutrophication, but also because of the continuous CO₂ release by global acidification.