Influence of preparation methods on C and N concentrations and δ13C composition of terrestrial and aquatic organic materials

The accurate determination of the concentrations of carbon [C] and nitrogen [N] (from which C/N is derived) and carbon isotopic composition (δ13C) of organic matter (OM) have been regarded as key proxies for our interpretation of environmental processes. C/N ratios are widely used to examine OM source mixing and alteration in sediments and δ13C to identify carbon sources and photosynthetic pathways in plants; assess carbon reservoir turnover times and soil C dynamics; determine trophic levels in environmental systems; estimation of carbon burial rates; and to understand mineralisation processes. These interpretations are grounded in the assumption that we can reliably determine C/N and δ13C concentrations in OM. This relies upon the complete removal of inorganic carbon from the sample total carbon and is achieved by acid treatment. An inherent assumption of these acid treatment methodologies is that any offsets in C/N and δ13C are linear and proportional. This paper represents the results of the first systematic comparison of the effect of acid treatment methodologies on the reliability of organic C and N, and δ13C values on a range of terrestrial and aquatic, modern and geological environmental materials. We investigated the 3 most common methodologies; (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) %C, %N, C/N and δ13C showed significant within and between method variability and that the precision of any one method varies significantly between sample materials, and above analytical precision; (ii) disproportionate and non-linear offsets of %C, %N and C/N values after acidification within and between methodologies and within and between sample materials; (iii) that alterations in %C and %N did not necessarily manifest themselves in shifts in δ13C, and vice-versa; (iv) small (~90 µg C) sample sizes showed consistent overestimations and inaccuracies after acidification; (v) 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. The response of δ13C within and between methodologies and environments to acidification appears to be relatively disproportionate, and raises cause for concern on the interpretative nature of C/N ratios and its support for carbon isotopic concentrations. The comparability between laboratories (different preparation methods) and environmental settings (amount, type and nature of OM) are also likely to be problematic. We conclude that the response of C and N concentrations in OM to acid treatment in environmental materials is neither negligible nor systematic. Keywords: δ13C, C/N ratio, Method comparison, environmental reconstruction.