Organic matter carbon, nitrogen, and phosphorous from a single persulfate digestion

• Published in: Limnology and oceanography, methods methods, 2015-04, Vol.13 (4), p.202-211
• Main Authors: Gibson, C. A., O'Reilly, C. M., Conine, A. L., Jobs, W., Belli, S.
• Format: Article
• Language: English
• ISSN: 1541-5856; 1541-5856
• DOI: 10.1002/lom3.10023

Ecological stoichiometry requires accurate measurement of carbon : nitrogen : phosphorus ratios in organic matter. Currently, carbon : nitrogen : phosphorus ratios are typically determined by analyzing one sample for carbon : nitrogen and another sample for percent phosphorus. Not only does this approach require multiple samples and accurate sample weights, but it also obscures natural variability. We developed a method which allows us to obtain carbon : nitrogen : phosphorus ratios from a single, small sample of organic matter. The analysis consists of a single persulfate digestion in sealed vials that converts the organic carbon (C), nitrogen (N), and phosphorus (P) to carbon dioxide gas (CO2), nitrate ion, and phosphate ion. The combination of 3% persulfate in 0.15 M NaOH provides a digestion that is initially alkaline but becomes acidic after about 30 min. In the final acidic solution, CO2 is driven into the vial headspace and is measured on a gas chromatograph‐mass spectrometer. Nitrate and phosphate are measured on an ion chromatograph and by using standard colorimetric methods, respectively. We assessed the method by evaluating recovery of C, N, and P from chemical standards and by comparing values obtained with this method to those obtained by current, conventional methods. We used cultured algae and pond seston on filters, leaf litter, and zooplankton samples for the method comparisons. The method is precise at low mass; we were able to confidently measure C : N : P ratios on individual Daphnia magna. The method showed high recovery for standards and produced C : N : P ratio values similar to those found by other analytical approaches and to values in the literature.