Combined Atmospheric Ca2+ and Mg2+ Wet Deposition within the Continental United States and Implications for Soil Inorganic Carbon Sequestration.

The relationship between combined atmospheric calcium ion (Ca²⁺) and magnesium ion (Mg²⁺) wet deposition and the sequestration of carbon in soils as pedogenic carbonates has not received adequate attention in the scientific literature. This preliminary study quantified combined average annual atmospheric Ca²⁺ and Mg²⁺ wet deposition from 1994 to 2003 within the continental United States (U.S.) and ranked the twelve major soil orders in terms of combined Ca²⁺ and Mg²⁺ wet deposition.  The average annual combined atmospheric Ca²⁺ and Mg²⁺ wet deposition for each soil order was estimated using geographic information systems (GIS) and data layers derived from atmospheric Ca²⁺ and Mg²⁺ wet deposition and a national soils database.  The soil orders receiving the highest combined average annual atmospheric Ca²⁺ and Mg²⁺ wet deposition from 1994 to 2003 were:  1) Mollisols (1.85 × 10¹⁰ eq), 2) Alfisols (1.39 × 10¹⁰ eq), 3) Entisols (6.39 × 10⁹ eq), and 4) Aridisols (4.61 × 10⁹ eq). In terms of potential soil carbon sequestration, these deposited Ca²⁺ and Mg²⁺ values would be equivalent to formation of the following theoretical amounts of calcite or sequestered carbon:  1) Mollisols (9.26 × 10⁸  kg CaCO₃; 1.11 × 10⁸  kg C), 2) Alfisols (6.97 × 10⁸  kg CaCO₃; 8.36 × 10⁷  kg C), 3) Entisols (3.19× 10⁸  kg CaCO₃; 3.83 × 10⁷  kg C), and 4) Aridisols (2.30 × 10⁸  kg CaCO₃; 2.76 × 10⁷  kg C). Studies such as this one can be useful for examining carbon cycling and accounting on a global scale by estimating the potential for soil carbonate formation from atmospheric wet deposition data.