Transformation of organic matter in agricultural soils: radiocarbon concentration versus soil depth

Radiocarbon measured by accelerator mass spectrometry (AMS) was used as a tracer to study the origin and fate of organic materials in soils. Fractionation methods used to separate the heterogeneous organic material into functionally defined pools of different stability included acid-alkali-acid extraction and density fractionation. 14C values of the humin fraction, isolated from samples of different field trials at the agricultural station Rotthalmünster (Germany), yielded 14C decreases of about 30% to 54% from the surface soil to ca. 65 cm depth. These results indicate a progressive enrichment of stable organic compounds with increasing soil depth. In contrast, a minor decline in 14C concentrations of the humic acid fraction, which mostly showed higher 14C values than the humin, reflect the translocation of modern organic carbon towards greater depth. Low radiocarbon levels of the light occluded particulate organic matter (<1.6 g/cm3), obtained by density separation, suggest stabilization of organic carbon in soil aggregates. A comparison of 14C results for density fractions from field trials located in a rural and an industrialized region reflect their susceptibility to contamination by fossil fuel-derived carbon and their heterogeneous composition. As a consequence individual short-chain phospholipid fatty acids (PLFA), as indicators for viable soil microbial biomass, were isolated by preparative capillary gas-chromatography. Compound-specific radiocarbon analysis of the isolated PLFAs revealed the assimilation of different substrates for their synthesis.14C concentrations of the monounsaturated PLFAs (n-C16:1, n-C17:1, and n-C18:1), which were close to the modern atmospheric 14C level, suggest a high specificity to young carbon sources. The saturated PLFAs, isolated from the plough-horizon, were synthesized from sub-recent soil organic carbon (SOC) as shown by a higher contribution of bomb-14C. A considerable 14C decrease from the surface to 30-45 cm soil depth of the saturated PLFAs indicates the incorporation of more stabilized SOC particularly in subsoil i/a-C15:0, n-C16:0, and n-C17:0 PLFAs.