Atmospheric Mg2+ wet deposition within the continental United States and implications for soil inorganic carbon sequestration

Little is known about atmospheric magnesium ion (Mg²⁺) wet deposition in relation to soil inorganic carbon sequestration. Understanding the conversion of carbon dioxide (CO²) or organic carbon to a form having a long residence time within the soil (e.g., dolomite, magnesian calcite) will greatly benefit agriculture, industry, and society on a global scale. This preliminary study was conducted to analyze atmospheric Mg²⁺ wet deposition within the continental United States (U.S.) and to rank the twelve major soil orders in terms of average annual atmospheric Mg²⁺ wet deposition. The total average annual Mg²⁺ wet deposition for each soil order was estimated with geographic information systems (GIS) using the following data layers: (1) atmospheric Mg²⁺ wet deposition data layers covering the continental U.S. for a 10-yr period (1994–²003) and (²) a soil order data layer derived from a national soils database. A map of average annual Mg²⁺ wet deposition for 1994–²003 reveals that the highest deposition (0.75–1.41 kg ha^-1) occurred in Oregon, Washington, parts of California, and the coastal areas of East Coast states due to magnesium enrichment of atmospheric deposition from sea salt. The Midwestern region of the U.S. received about 0.²5–0.75 kg ha-1 Mg²+ wet deposition annually, which was associated with loess derived soils, occurrence of dust storms and possibly fertilization. The soil orders receiving the highest average annual atmospheric Mg²⁺ wet deposition from 1994 to ²003 were: (1) Mollisols (3.7×10⁷ kg), (²) Alfisols (3.6×10⁷ kg) and (3) Ultisols (2.8×10⁷ kg). In terms of potential soil carbon sequestration, the average annual atmospheric Mg²⁺ wet deposition was equivalent to formation of the following theoretical amounts of dolomite: (1) Mollisols (2.8 × 10⁸ kg of CaMg(CO₃)₂), (2) Alfisols (2.7 × 10⁸ kg of CaMg(CO₃)₂) and (3) Ultisols (2.1 × 10⁸ kg of CaMg(CO₃)₂). The soil orders receiving the lowest average annual atmospheric Mg²⁺ wet deposition were: (1) Andisols (3.3 × 10⁶ kg), (2) Histosols (3.4 × 10⁶ kg) and (3) Vertisols (5.0 × 10⁶ kg). The methods proposed here to estimate soil inorganic carbon sequestration potential from atmospheric wet deposition data can be useful for preliminary carbon accounting on a global scale.