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United States Department of Agriculture

Agricultural Research Service

Title: Comparison of methods to determine carbon and nitrogen in soil extractions


Submitted to: ASA-CSSA-SSSA Annual Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: November 8, 2007
Publication Date: November 8, 2007
Citation: Lachnicht Weyers, S.L., Wilts, A.R. 2007. Comparison of methods to determine carbon and nitrogen in soil extractions [abstract][CD-ROM]. ASA-CSSA-SSSA Annual Meeting Abstracts. ASA-CSSA-SSSA Annual Meeting. Nov. 4-8, 2007, New Orleans, LA.

Technical Abstract: Microbial biomass carbon (MBC) and nitrogen (MBN) in soil samples are routinely measured by the direct extraction method of Vance et al. (1989), which involves extraction with 0.5 M K2SO4 from paired fumigated and un-fumigated soil sub-samples. As widely reported in the literature, total organic carbon (TOC) analyzers are often used to directly measure carbon (C) in the extractions. An alternative C method involves digesting the extracted solutions in standardized chemical oxygen-demand (COD) capsules and measuring color development on a spectrophotometer. One nitrogen (N) analysis method that is similar to the COD method also involves a digestion, which converts organic N into the more easily measured form of nitrate (NO3). The NO3 is then measured by auto-analysis on a continuous flow injection system. For N analyses, several digestion methods have been proposed, one is a persulfate digestion using an autoclave (Cabrera and Beare, 1993) and another requires a laboratory-grade microwave (Kowalenko and Babuin, 2003). We compared five methods that may be used to determine MBC and MBN from soil extractions. The TOC and COD methods were used to measure C. The N was measured by persulfate digestion with an autoclave or a microwave and with a new system of total N (TN) analysis by direct combustion. The measurement of TN is achieved after direct combustion and detection with a solid state electrochemical detector (ECD) on a TOC/TN system. We found that all methods gave comparable results. However, the COD method was sensitive to inorganic carbon and the microwave assisted digestion was sensitive to NO3 contamination of the microwave chambers. We found that use of the TOC/TN method was most efficient because of the simultaneous determination of C and N. TOC/TN also reduced chemical waste and required a relatively small sample volume.

Last Modified: 8/3/2015
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