Tropical soil profiles reveal the fate of plant wax biomarkers during soil storage

Abstract

The waxy coating that protects the leaves and other soft tissues of plants includes n-alkane and n-alkanoic acid compounds that are commonly used as biomarkers to reconstruct past environment. Plant waxes have geological relevance given their persistence in soils and paleosols, as well as in lake and marine sediments, yet diagenesis may alter their molecular and isotopic signatures from synthesis to deposition. This study seeks to understand the fate of plant wax biomarkers in soils after leaf-fall as characterized by a series of tropical soil profiles. We investigate the changes in abundance, molecular distributions, and hydrogen (δD) and carbon isotopic compositions (δ13C) of plant waxes (n-alkanes and n-alkanoic acids) in six litter-to-soil profiles along a 2740 m elevation transect from the eastern flank of the Andes mountains down to the lowland Amazon floodplain in Peru. From litter to soil, we find acid/alkane ratios increase while absolute abundances decrease. In contrast, within each soil, acid/alkane ratios are roughly constant, and we find an equivalent exponential decline in concentration in both compound classes with depth, with molecular distributions indicating some new production. We observe a 4–6‰ 13C-enrichment from litter to deeper soils for both C29n-alkanes and C30n-alkanoic acids, of which the Suess effect accounts for ≤2‰. We infer that microbial degradation and production (or ‘turnover’) processes influence the δ13C of plant waxes that survive in soils; in contrast, no systematic change in δD values is observed. The plant wax signal in soils includes averaging of inputs and diagenetic effects, so this signature is particularly relevant for the interpretation of plant wax archives in paleosols and the plant waxes eroded from soils and exported to downstream sedimentary archives. We show that soils represent the major stock of plant wax under living ecosystems, suggesting that soils may be a quantitatively-important source of plant waxes available for fluvial erosion, with implications for studies of carbon cycling and paleoenvironmental reconstructions from downstream geological archives.