Molecular turnover time of soil organic matter in particle‐size fractions of an arable soil

The composition and molecular residence time of soil organic matter (SOM) in four particle-size fractions (POM >200 microm, POM 63-200 microm, silt and clay) were determined using Curie-point pyrolysis/gas chromatography coupled on-line to mass spectrometry. The fractions were isolated from soils, either continuously with a C(3) wheat (soil (13)C value = -26.4 per thousand), or transferred to a C(4) maize (soil (13)C value = -20.2 per thousand) cropping system 23 years ago. Pyrograms contained up to 45 different pyrolysis peaks; 37 (ca. 85%) were identifiable compounds. Lignins and carbohydrates dominated the POM fractions, proteins were abundant, but lignin was (nearly) absent in the silt and clay fractions. The mean turnover time (MRT) for the pyrolysis products in particulate organic matter (POM) was generally <15 years (fast C pool) and 20-300 years (medium or slow C pools) in silt and clay fractions. Methylcyclopentenone (carbohydrate) in the clay fraction and benzene (mixed source) in the silt fraction exhibited the longest MRTs, 297 and 159 years, respectively. Plant-derived organic matter was not stored in soils, but was transformed to microbial remains, mainly in the form of carbohydrates and proteins and held in soil by organo-mineral interactions. Selective preservation of plant-derived OM (i.e. lignin) based on chemical recalcitrance was not observed in these arable soils. Association/presence of C with silt or clays in soils clearly increased MRT values, but in an as yet unresolved manner (i.e. 'truly' stabilized, or potentially still 'labile' but just not accessible C).