Calcium helps evaluate soil’s ability to retain earth’s carbon

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Source: Soil Science Society of America

Soils play a vital role in dealing with the environmental impacts of rising atmospheric carbon levels, primarily CO2, from natural and human activities. The Earth’s carbon budget is a dynamic process. As carbon is released through fossil-fuel burning and changing land use, scientists are seeking a more accurate understanding of carbon storage and cycling.

The Earth holds carbon in what scientists call pools, reservoirs of carbon stocks stored in and on the earth and oceans as organic and inorganic matter. Simplistically, organic carbon compounds are connected to plants or animals while inorganic carbon compounds are often linked to minerals or rocks. Soil is second only to the oceans as a carbon sink, pools into which more carbon flows in than out. Soil scientists have a better picture of soil organic carbon (SOC) – soil containing decaying plant and animal matter – than soil inorganic carbon (SIC). Scientists are now studying SIC, theorizing it may be a key area for forming and holding carbon, preventing it from returning to the atmosphere for eons.

A team of Experiment Station scientists from Clemson University and Virginia Tech analyzed the 12 major soil groups in the continental United States, ranking them for their potential ability to form new SIC based on average annual atmospheric wet deposition (AAAWD) of calcium (Ca) – that is, the amount of Ca2+ (ionic calcium) present in rainfall. The results were first presented at the Soil Science Society of America Annual Meeting in November 2007 in New Orleans, LA and recently have been published in the May-June 2009 issue of the Soil Science Society of America Journal.

The study evaluated AAAWD of Ca2+ from 1994 to 2003 within the continental United States by soil order, using spatial analysis of Ca2+ wet deposition data obtained from the National Atmospheric Deposition Program (NADP) and the State Soil Geographic (STATSGO) Database from the Natural Resources Conservation Service of the U.S. Department of Agriculture. Using geographic information system (GIS) software, spatial data layers were developed and averaged to create a final Ca2+ wet deposition map layer. The total Ca2+ wet deposition per soil order (in kg) was then calculated by combining the final average Ca2+ wet deposition map layer with the generalized soil order data layer.

Results from the study revealed that the total AAAWD of Ca2+ within the continental United States was 8.6 × 108 kg, which would be equivalent to the maximum theoretical formation of 2.6 × 108 kg of carbon as SIC, barring losses of Ca2+ due to competitive processes, such as plant uptake, erosion, and deep leaching.  The soil orders receiving the highest area-normalized total AAAWD of Ca2+ were Alfisols and Mollisols, non-arid soils that are typically associated with the “bread-basket” regions of the United States.

Research team member Elena Mikhailova, a soil scientist at Clemson who originally conceived the research approach, stated “Formation of new carbonate minerals in soils – what scientists call pedogenic carbonates – represent a pathway by which atmospheric CO2 can be sequestered. Maps of potential SIC formation and storage based on wet Ca2+ deposition can aid in understanding terrestrial ecosystem inorganic carbon dynamics and the way it can be manipulated to decrease CO2 concentrations in the atmosphere.”

The research is part of an ongoing project at Clemson to study soil carbon, particularly inorganic carbon stocks, and its role in the global carbon budget. Studies will measure, profile and identify the soil carbon characteristics and regional distribution to understand conditions and develop predictive models for future soil inorganic carbon research.

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