The contribution of agricultural lime to carbon dioxide emissions in the United States: dissolution, transport, and net emissions
Agricultural lime (aglime) is commonly applied to soils in the eastern U.S. to increase soil pH. Aglime includes crushed limestone (CaCO3) and crushed dolomite (MgCa(CO3)2). Following the supposition by the Intergovernmental Panel on Climate Change (IPCC) that all C in aglime is eventually released as CO2 to the atmosphere, the U.S. EPA estimated that 9 Tg (Teragram = 1012 g = 106 metric tonne) CO2 was emitted from an approximate 20 Tg of applied aglime in 2001. A review of historic data on aglime production and use indicates that 30 Tg may better represent the annual U.S. consumption of aglime. More importantly, our review of terrestrial and ocean C dynamics indicates that it is unlikely that all C from aglime is released to the atmosphere following application to soils. On the contrary, the primary pathway for aglime dissolution is reaction with carbonic acid (H2CO3) which results in uptake of CO2. Depending on soil pH and nitrogen fertilizer use, a fraction of aglime may react with strong acid sources such as nitric acid (HNO3), thereby releasing CO2. Data on soil leaching and river transport of calcium (Ca2+) and bicarbonate (HCO3−) suggest that a significant portion of dissolved aglime constituents may leach through the soil and be transported by rivers to the ocean. Much of the fraction transported to the ocean will precipitate as CaCO3. Bicarbonate remaining in the soil profile is expected to release CO2 following re-acidification of the soil over time. Our analysis indicates that net CO2 emissions from the application of aglime is 0.059 Mg C per Mg limestone and 0.064 Mg C per Mg dolomite. This is in contrast to IPCC estimates of 0.12 and 0.13 Mg C per Mg limestone and dolomite, respectively. Based on our best estimate, the application of 20–30 Tg of aglime in the U.S., consisting of 80% limestone and 20% dolomite, would have resulted in a net 4.4–6.6 Tg CO2 emissions in 2001.