Heterogeneous Turnover of Molecular Organic Substances from Crop Soils as Revealed by 13 C Labeling at Natural Abundance with Zea mays

Abstract

Understanding the dynamics of soil organic matter is of major importance for agronomical and environmental studies. In particular, soil organic matter holds soil physical stability and thus prevents erosion; soil organic matter stores pollutants; and soil organic carbon impacts climate change as either a source or sink of atmospheric CO2. However, knowledge on the dynamics of soil carbon is still incomplete, particularly at the molecular level. Here, using a recently developped analytical technique that allows measuring the 13C isotope composition of individual substances within a complex organic mixture I show that plant-wax derived soil n alkanes have a higher turnover than bulk soil organic matter. Soil samples were collected in a field experiment where maize was cropped 23 years. Since maize, a C4 plant, is 13C-enriched versus previous vegetation made of C3 plants, cropping maize introduces 13C-enriched carbon in the soil over time. This labelling at natural abundance thus allows to distinguish maize-derived carbon from initial C3 plant-derived carbon. The 13C composition of bulk soil carbon was measured by isotope ratio monitoring mass spectrometry (IRMS), whereas that of individual soil n-alkanes was measured by gas chromatography-IRMS. The results show that after 23 years of maize cropping the increase of 13C composition is faster for the C31 soil n-alkane (+7.6‰) than for bulk soil carbon (+5.6‰). This finding evidences the faster turnover of soil n-alkanes, which is rather unexpected because n-alkanes are chemically stable compounds. Using a first order kinetic model, it is estimated that the complete C turnover will last 192 years for bulk soil carbon and 148 years for the plant-derived soil n-alkane.