Persulfate digestion and simultaneous colorimetric analysis of carbon and nitrogen in soil extracts

• Published in: Soil Science Society of America journal Vol. 68; no. 2; pp. 669 - 676
• Main Authors: Doyle, A, Weintraub, M.N, Schimel, J.P
• Format: Journal Article
• Language: English
• Published: Madison Soil Science Society 01.03.2004; Soil Science Society of America; American Society of Agronomy
• Subjects: Agronomy. Soil science and plant productions; Biological and medical sciences; Carbon; Carbon dioxide; chemical analysis; Chloroform; colorimetry; Dissolved organic carbon; Earth sciences; Earth, ocean, space; Exact sciences and technology; extraction; extractors; flow injection analysis; forest soils; Fumigation; Fundamental and applied biological sciences. Psychology; Geochemistry; glycine (amino acid); High temperature; laboratory techniques; Low temperature; lysine; mass; Methods and techniques; nicotinamide; nitrates; Nitrogen; Oxidation; redox reactions; savanna soils; Soil and rock geochemistry; soil fumigation; soil microorganisms; soil pH; Soil science; soil temperature; Soil testing; sulfates; taiga soils; Temperature; tundra soils; Urea; yeast extract; Yeasts; Alaska; Massachusetts; California; Analytical chemistry; Color reaction; dissoved organic nitrogen; nitrogen; Property of soil; oxidation; peroxosulfate; Extract; Digestion; Oxidant; SOIL CHEMISTRY; Inorganic element; Microbial biomass; chemical analysis; Dissolved organic carbon; colorimetry; Analytical reagent; carbon; organic materials; SOIL TESTING; Soil science; quantitative analysis; soils; chemical composition
• ISSN: 0361-5995; 1435-0661
• DOI: 10.2136/sssaj2004.6690

Persulfate digestions have been used for analyzing dissolved organic carbon (DOC) and nitrogen (DON), but most existing methods do not simultaneously analyze the same digest. Persulfate oxidizes DON to NO3(-), and DOC to CO2, which was trapped in the alkaline digest solution. We optimized the persulfate digestion for simultaneous analysis of dissolved CO2 and NO3(-) with flow injection analysis (FIA). Dissolved CO2 was diffused across a Gore-Tex (W.L. Gore & Associates, Inc., Sunnyvale, CA) membrane and analyzed with a pH indicator, while NO3(-) was analyzed with a standard Griess-Ilosvay reaction after Cd reduction. Digestions of glycine, urea, and yeast extract solutions recovered 97 to 99% of C and 92 to 96% of N, while digestions of lysine and nicotinimide recovered slightly less (93 to 94% C, 87 to 93% N). Glycine N, used as a digested standard to which sample recoveries were scaled, improved calculated recovery 94 to 103%. Dissolved organic C concentrations were not significantly different for persulfate oxidation (PO) and high temperature catalytic oxidation (HTCO) for 13 organic and mineral soil extracts with and without prior chloroform fumigation. Nitrogen recovery was higher for PO than HTCO. The DOC blanks were 0.5 to 1.0 mg C L-1 and detection limit was 0.5 mg C L-1, and both could be lowered if needed. The upper limit of analysis was at least 2000 mg C L-1 with dilution. The digest was performed at temperatures as low as 80°C or as high as 125°C with equivalent results. We recommend the low temperature to reduce leakage. This analytical system is particularly convenient for the simultaneous analysis of microbial biomass C and N with the fumigation-extraction procedure.