Source and turnover of organic matter in agricultural soils derived from n-alkane/ n-carboxylic acid compositions and C-isotope signatures

• Published in: Organic geochemistry 2004-01, Vol.35 (11), p.1371-1393
• Main Authors: Wiesenberg, Guido L.B., Schwarzbauer, Jan, Schmidt, Michael W.I., Schwark, Lorenz
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochemistry
• ISSN: 0146-6380; 1873-5290
• DOI: 10.1016/j.orggeochem.2004.03.009

Agricultural soils are regarded as one potential sink for atmospheric CO 2 via photosynthetic fixation in plant biomass and subsequent transformation into soil organic matter upon soil diagenesis. The difference in C-isotope signatures of C 3- vs. C 4-plants allows for a natural isotopic labelling of soil organic matter after changes from C 3- to C 4-cropping. In this study, we demonstrate that isotopic shifts are paralleled by molecular signatures of C 3- vs. C 4-crop alkyl lipids. Turnover times vary significantly, based on cropping techniques. For grain-maize cropped soils at steady state average turnover times of 40 years for bulk SOC, 35 years for n-alkanes and 21 years for n-carboxylic acids were determined. Turnover times for silage-maize cropped soil at steady state were on average 250 years for bulk SOC, 60 years for n-alkanes and 49 years for n-carboxylic acids. Turnover times reported here for silage-maize cropped soils may be taken as maximum values only, because they derive from a single trial, which was affected by addition of anthropogenic refractory carbon. Discrimination of input from various plant parts (roots, stems and leaves) based on bulk C-isotopes is not feasible but can easily be achieved using compositions of carboxylic acids, especially the ratio of n-C 24 vs. n-C 22+26 and their respective C-isotope values. This enables delineation of the influence of different cropping techniques, e.g., silage- or grain-maize, on carbon storage in soils. Admixture of external sources of organic matter to the soil organic carbon pool of an urban site in Halle, Germany was identified based on alkyl lipid distributions. Nearby lignite mining was identified as a source for non-crop-derived alkyl lipids, primarily based on the elevated n-C 26-carboxylic acid content and heavier carbon isotopic signatures.