Persulfate Digestion and Simultaneous Colorimetric Analysis of Carbon and Nitrogen in Soil Extracts

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 NO − 3 , and DOC to CO 2 , which was trapped in the alkaline digest solution. We optimized the persulfate digestion for simultaneous analysis of dissolved CO 2 and NO − 3 with flow injection analysis (FIA). Dissolved CO 2 was diffused across a Gore‐Tex (W.L. Gore & Associates, Inc., Sunnyvale, CA) membrane and analyzed with a pH indicator, while NO − 3 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.