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 δ13C 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 δ13C values of the humin fractions were similar and the δ13C 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 δ13C 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 13C relative to those that were treated until exhaustion with 0.1 M NaOH + Na4P2O7. Where shifts in δ13C 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 13C by 0.5 to 2.3‰ relative to those extracted with the sodium pyrophosphate mixture, with one exception. Possible explanations for 13C 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 δ13C 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 δ13C values of humin or humic acid can be either enriched or depleted in 13C, when different extraction solutions are used.