Compound-specific stable-isotope (δ13C) analysis in soil science

This review provides current state of the art of compound-specific stable-isotope-ratio mass spectrometry (delta C-13) and gives an overview on innovative applications in soil science. After a short introduction on the background of stable C isotopes and their ecological significance, different techniques for compound-specific stable-isotope analysis are compared. Analogous to the delta C-13 analysis in bulk samples, by means of elemental analyzer-isotope-ratio mass spectrometry, physical fractions such as particle-size fractions, soil microbial biomass, and water-soluble organic C can be analyzed. The main focus of this review is, however, to discuss the isotope composition of chemical fractions (so-called molecular markers) indicating plant- (pentoses, long-chain n-alkanes, lignin phenols) and microbial-derived residues (phospholipid fatty acids, hexoses, amino sugars, and short-chain n-alkanes) as well as other interesting soil constituents such as "black carbon" and polycyclic aromatic hydrocarbons. For this purpose, innovative techniques such as pyrolysis-gas chromatography-combustion-isotope- ratio mass spectrometry, gas chromatography-combustion-isotope- ratio mass spectrometry, or liquid chromatography-combustion-isotope-ratio mass spectrometry were compared. These techniques can be used in general for two purposes, (1) to quantify sequestration and turnover of specific organic compounds in the environment and (2) to trace the origin of organic substances. Turnover times of physical (sand < silt < clay) and chemical fractions (lignin < phospholipid fatty acids < amino sugars approximate to sugars) are generally shorter compared to bulk soil and increase in the order given in brackets. Tracing the origin of organic compounds such as polycyclic aromatic hydrocarbons is difficult when more than two sources are involved and isotope difference of different sources is small. Therefore, this application is preferentially used when natural (e.g., C3-to-C4 plant conversion) or artificial (positive or negative) C-13 labeling is used.