Molecular insight into soil carbon turnover

Curie-point pyrolysis-gas chromatography coupled on-line to mass spectromet ry (Py-GC/MS) and isotope ratio mass spectrometry (Py-GC/IRMS) were used to determine the individual turnover rate of specific carbohydrates, lignin, lipids and N-containing compounds from French arable soils. The analysed so ils were cultivated, either continuously with a C3 plant (wheat delta(13)C- value = -25.2 parts per thousand), or transferred to a C4 plant (maize delt a(13)C-value = -11.4 parts per thousand) cropping 23 years ago. Most pyroly sis products identified were related to carbohydrates (furans), lipids (hyd rocarbons and derivatives of benzene), proteins (nitriles and pyrrole) and lignins (phenols), The relative yield of all individual pyrolysis products was similar in the samples from the maize and control wheat soil. The isoto pic enrichment between identical pyrolysis products from the two soils vari ed from 1 to 12 delta (delta) units, indicating that after 23 years of cult ivation 7 to 90% of their C was derived from maize. This suggests a slow me an turnover time varying from 9 to 220 years. Based on the differences in i sotopic enrichment of chemical structures after vegetation change the pyrol ysis products could be divided into three groups: (i) pyrolysis products wi th a nearly complete C4 signal, e.g. phenol, derived from lignin degradatio n products, (ii) pyrolysis products with an intermediate isotopic enrichmen t of 6-8 parts per thousand, most likely to be a composite of remaining (po ssibly physically protected) fragments derived from both maize and native w heat, and (iii) pyrolysis products showing only low enrichments in C-13 of 1-3 parts per thousand. Most of their precursors were found to be proteinac eaous materials. This indicates that proteins or peptides are indeed preser ved during decomposition and humification processes occurring in the soil. Our study highlights the potential of Py-GC/MS-C-IRMS to further novel insi ghts into the dynamics of soil organic constituents. Copyright (C) 1999 Joh n Wiley & Sons, Ltd.