Nitrogen fertilization raises CO2 efflux from inorganic carbon: a Global assessment.

Nitrogen (N) fertilization is a worldwide indispensable agricultural practice serving the survival of half of global population. Nitrogen transformation in soil as well as plant N uptake leads to release of protons and enhances soil acidification. Neutralization of this acidity in carbonate-containing soils (7.94 109 ha; ca. 54% of global land surface area) results in CO2 release corresponding to 0.21 kg C per kg applied N. Based on the global N fertilization map and the distribution of soils containing CaCO3, we calculated the annual amount of CO2 released from carbonates as 7.48 1012 g C y-1. These baseline levels of continuous CO2 release will remain until N fertilization occurs. In the second half of the study, we estimated that about 273 1012 g C are released annually in the same process of CaCO3 neutralization by liming of acidic soils, a common agricultural practice to revitalize over-fertilized fields. These two CO2 sources correspond to 3% of global CO2 emissions by fossil fuel combustion or 30% of CO2 by land use changes. However, the duration of CO2 release after land use changes usually takes only 1-3 decades before a new C equilibrium is reached in soil. In contrast, the CO2 released by CaCO3 acidification cannot reach equilibrium until it is completely neutralized, which will take centuries or even millennia. This promotes limiting soil acidification as an effective strategy for inhibiting millennial CO2 efflux to the atmosphere. Hence, N fertilization should be plant demand-specific to prevent over fertilization, not only because of its role in local eutrophication, but also this newfound role of continuous CO2 release by global acidification.Key words: CO2 efflux, global acidification assessment, global warming, mitigation policy, nitrogen fertilization, soil acidification mechanisms, soil inorganic carbon [ABSTRACT FROM AUTHOR]