Carbon isotopic composition of plant waxes, bulk organics and carbonates from soils of the Serengeti grasslands

Abstract

The carbon isotopic composition of different carbon-bearing materials, organic and inorganic, is commonly used to reconstruct distributions of tropical C4 grasslands. However, no study of modern soils has yet combined carbon isotopic compositions (δ13C) of bulk soil organic matter (SOM), soil carbonate (SC), plant wax n-alkanes (alk) and n-alkanoic acids (acid) to allow for comparison between all four materials. Here, we studied carbonate-precipitating, grassland soils across the Serengeti ecosystem, Tanzania to directly compare δ13C values of these four materials using samples from 11 sites along a NW–SE transect and depth profiles at each location. Among the soil carbonates found at 9 sites, the δ13C results (δ13CSC mean = 0.5 ‰, 1σ = 0.8‰, n = 70 nodules collected), denote C4 grasses exclusively, but soil carbonates are absent from two of the three sites with trees. Organics were measured at all 11 sites including bulk (δ13CSOM −13.1 ± 1.8‰) and plant wax biomarkers (e.g., δ13C29alk −23.5 ± 3.1, δ13C31alk −22.2 ± 2.0, δ13C33alk −21.9 ± 1.8, δ13C28acid −19.1 ± 2.1‰, δ13C30acid −21.1 ± 2.6‰, and δ13C32acid −20.1 ± 1.7‰). δ13C values in organic materials record the dominance of C4 grasses at the sampled sites. Surface samples are more negative than those at depth at most sites, reflecting changes in atmospheric composition and surficial inputs in recent decades, and degradation at depth over centuries. We found significant correlations between each of the organic proxies, but the δ13C29alk is overly responsive to tree inputs (greater C29 concentration and 13C-depletion relative to other homologues), whereas the δ13C31alk, δ13C33alk and δ13C32acid are robust recorders of vegetation cover. Of all materials considered (plant wax, SOM and soil carbonates), plant wax n-alkanes and n-alkanoic acids have the broadest applicability, extending to non-carbonate bearing soils, and to lacustrine and marine sedimentary archives.