During the last decade, compound-specific hydrogen isotope analysis of plant leaf-wax and sedimentary
n-alkyl lipids has become a promising tool for paleohydrological reconstructions. However, with the exception of several previous studies, there is a lack of knowledge regarding possible effects of early diagenesis on the δD values of
n-alkanes. We therefore investigated the
n-alkane patterns and δD values of long-chain
n-alkanes from three different C3 higher plant species (
Acer pseudoplatanus L.,
Fagus sylvatica L. and
Sorbus aucuparia L.) that have been degraded in a field leaf litterbag experiment for 27 months.We found that after an initial increase of long-chain
n-alkane masses (up to ∼50%), decomposition took place with mean turnover times of 11.7 months. Intermittently, the masses of mid-chain
n-alkanes increased significantly during periods of highest total mass losses. Furthermore, initially high odd-over-even predominances (OEP) declined and long-chain
n-alkane ratios like
n-C
31/C
27 and
n-C
31/C
29 started to converge to the value of 1. While bulk leaf litter became systematically D-enriched especially during summer seasons (by ∼8‰ on average over 27 months), the δD values of long-chain
n-alkanes reveal no systematic overall shifts, but seasonal variations of up to 25‰ (
Fagus,
n-C
27, average ∼13‰).Although a partly contribution by leaf-wax
n-alkanes by throughfall cannot be excluded, these findings suggest that a microbial
n-alkane pool sensitive to seasonal variations of soil water δD rapidly builds up. We propose a conceptual model based on an isotope mass balance calculation that accounts for the decomposition of plant-derived
n-alkanes and the build-up of microbial
n-alkanes. Model results are in good agreement with measured
n-alkane δD results. Since microbial ‘contamination’ is not necessarily discernible from
n-alkane concentration patterns alone, care may have to be taken not to over-interpret δD values of sedimentary
n-alkanes. Furthermore, since leaf-water is generally D-enriched compared to soil and lake waters, soil and water microbial
n-alkane pools may help explain why soil and sediment
n-alkanes are D-depleted compared to leaves.
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