Differential incorporation of plant waxes and organic carbon into neotropical rainforest and savanna soils

Abstract:

Plant wax biomarkers are an important tool in paleoenvironmental research, and the stable carbon and hydrogen isotope compositions of these compounds can be used to reconstruct past vegetation and hydrology. Soils are repositories of plant organic material, and the major stock of biomarkers eroded by rivers into depositional basins. A detailed understanding of the processes controlling the incorporation of plant waxes into soils is therefore crucial for the interpretation of paleoenvironmental records. While the incorporation of plant waxes into soils from forest biomes such as the Amazon rainforest have been well studied, neotropical savanna and shrubland areas have not been analyzed. To elucidate the processes governing the input of plant waxes into savanna soils, we analyze long-chain n-alkanes, long-chain fatty acids and bulk organic matter from soil depth profiles along a transect from the Atlantic rainforest across the Cerrado savanna to the Amazon rainforest. Our results show that down profile, leaf-derived long-chain n-alkane and fatty acid concentrations decrease faster compared to bulk organic matter. We find that δ¹³C of bulk organic carbon increases down-profile in forested soil profiles but decrease in grasslands dominated Cerrado vegetation types. We also find systematic differences in the relative distribution of plant waxes between grasslands and forest. The ratio of mid- to long-chain n-alkanes is for instance consistently higher in the grassland soil profiles than under forests, indicating an enhanced relative contribution of microbial compounds in grassland soils. Overall, our results indicate differences in plant wax and organic carbon incorporation in neotropical forest and savanna vegetation types that likely reflect differences in rooting, carbon turnover times and microbial activity.