A comparison of extraction techniques on the stable carbon-isotope composition of soil humic substances

Because humic substances have varying turnover rates, their stable carbon-isotope compositions have been used to assess carbon residence time in soils. However, any study of this nature must ensure that extraction techniques do not fractionate the delta C-13 values of the humic acid extract and residual humin. This study examines the effects of different extraction procedures on the carbon-isotope signature of the humic fractions of soil organic matter. Humic acid and humin were isolated from soils using three different treatment methods: extraction with 0.5 M NaOH solution until completion, extraction with 0.1 M NaOH + Na4P2O7 solution until completion, and extraction with 0.1 M NaOH + Na4P2O7 solution for only one day. The delta C-13 values of the humin fractions were similar and the delta C-13 values of the humic acid fractions were similar regardless of extraction technique when the soil carbon content was greater than 0.5% prior to extraction and in many soils when carbon content was less than 0.5%. However, within the group of soils that had a low carbon content some soils exhibited shifts in the delta C-13 values of the humic acid or humin when different extraction techniques were used. In these samples, humic acids extracted after only one day of reaction were slightly enriched in C-13 relative to those that were treated until exhaustion with 0.1 M NaOH + Na4P2O7. Where shifts in delta C-13 values for humic acid or humin fractions were observed in low carbon fractions treated with different chemical solutions. those fractions extracted with 0.5 M NaOH were enriched in C-13 by 0.5 to 2.3 parts per thousand. relative to those extracted with the sodium pyrophosphate mixture, with one exception. Possible explanations for C-13 enrichment of humic acids produced with a NaOH extraction solution are discussed including contamination of the soluble humic acid with non-humified carbon or carbon associated with solubilised silica. Samples at the greatest depth in each of the soil profiles displayed widely variable behaviour for the delta C-13 values of humic acid or humin purified with the different chemical solutions. This study demonstrates that for some of the samples with low carbon contents delta C-13 values of humin or humic acid can be either enriched or depleted in C-13, when different extraction solutions are used. (C) 2009 Elsevier B.V. All rights reserved.