Evidence for bias in C and N concentrations and δ¹³C composition of terrestrial and aquatic organic materials due to pre-analysis acid preparation methods

• Published in: Chemical geology Vol. 282; no. 3-4; pp. 67 - 83
• Main Authors: Brodie, Chris R, Leng, Melanie J, Casford, James S.L, Kendrick, Christopher P, Lloyd, Jeremy M, Yongqiang, Zong, Bird, Michael I
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 23-03-2011
• Subjects: acid treatment; acidification; carbon; fumigation; isotopes; nitrogen; nitrogen content; organic matter; silver; tin; vapors
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2011.01.007

This study is the first systematic comparison of the effect of acid treatment methods on the reliability of organic carbon [C] and nitrogen [N], and carbon isotope (δ¹³C) values on a range of terrestrial and aquatic, modern and geological environmental materials. We investigated the 3 most common methods; (i) acidification followed by sequential deionised water rinses (“rinse method”); (ii) acidification in silver capsules (“capsule method”); and (iii) acidification by exposure to an acid vapour (“fumigation method”). We also investigated the effect of sample size and capsule type (silver and tin) on C/N ratio and δ¹³C values. We find (i) that %C, %N, C/N and δ¹³C showed significant within and between method variability; (ii) disproportionate and non-linear offsets of %C, %N and C/N values after acidification within and between methods and within and between sample materials; (iii) that alterations in %C did not necessarily manifest themselves in shifts in δ¹³C, and vice-versa; (iv) that small (~90μgC) sample sizes showed consistent overestimations and inaccuracies after acidification; (v) that the effect of capsule type was not significant on most samples, but did show a notable effect on our aquatic materials, generally increasing %C and %N, and producing depleted δ¹³C values. These findings raise cause for concern on the interpretative nature of C/N ratios and their support for carbon isotope values. The comparability between laboratories (different preparation methods) and environmental settings (amount, type and nature of OM) is also likely to be problematic. We conclude that the response of C and N concentrations in organic matter to acid treatment in environmental materials is neither negligible nor systematic.