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United States Department of Agriculture

Agricultural Research Service

Title: A Volumetric Inorganic Carbon Analysis System

Authors
item Wagner, Steve
item Hanson, Jay
item Olness, Alan
item Voorhees, Ward

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 28, 1997
Publication Date: N/A

Interpretive Summary: Soil productivity is related to the amount of organic matter in the soil. Soil organic matter appears to be reduced over time with tillage. This could effect the ability of future generations to produce food. It is important to measure the amount of organic matter in the soil to determine if land management practices are contributing to a long-term decline. In some soils almost all of the carbon in the soil is organic matter, and the amount can be measured with standard procedures. In other soils, including some of the most productive soils in the Northern Corn Belt, there are two components to the total carbon; organic matter and inorganic carbon (typically in calcium or magnesium carbonates). Standard techniques allow us to measure the total carbon in these soils, but before measuring the organic matter component we must rid the soil sample of the inorganic carbon. This requires costly techniques and equipment, and requires the use of hazardous chemicals. To avoid these problems, we divide the soil sample into two uniform subsamples. With one subsample, we measure the total carbon with the standard techniques. We use a pressure calcimeter technique to determine the inorganic carbon in the other subsample. Using a personal computer and pressure transducer, we developed an automated system that quickly, accurately, safely, and inexpensively determines the inorganic carbon of soils. The organic matter component is determined by subtracting the inorganic component from the total carbon. The technique allows scientists to efficiently determine soil organic matter content of soils containing both organic and inorganic carbon. The benefits to society include completing this research with fewer tax dollars and without generating hazardous waste.

Technical Abstract: Measurement of soil organic matter is important to discern long term trends resulting from land management. Methods of measurement of soil organic carbon (Co) are slow and expensive when inorganic carbon (Ci) is present. Our objective was to develop an automated volumetric technique for determination of Ci. A Ci analysis system was developed by combining a pressure transducer with a personal computer (PC), data acquisition board, and appropriately developed software. The PC software, InCarbS, conducts linear regression of standards, converts pressure measurements to % Ci, and automatically records data in a spreadsheet file. An intuitive PC graphic user interface (GUI) display aids calibration and operation of the instrument. Soil samples with pH >7.0 and containing up to 6% Ci were selected for analysis by combustion, neutralization, and volumetric methods. In the combustion method, Ci was determined as a difference between total C and organic C. In the acid neutralization method, Ci was determined with an automated titration to an endpoint pH of 7.0. The volumetric method used sample acidification in a closed vessel and determination of the pressure of CO2 released. Correlations of Ci content by the volumetric method and combustion or neutralization methods show close agreement (r**2 of 0.994 and 0.997, respectively). The determination of Ci by combustion involves the difference between total C and organic C, and coefficients of variation (CV) became >10% at Ci contents of <0.5 to 0.75%. The CV were >5% for Ci <0.2% by volumetric determination and >10% for Ci <0.5% by neutralization. Using the modified volumetric method, samples can be processed at the rate of 20 hr**-1 including setup and cleanup.

Last Modified: 9/2/2014
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