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

The carbon isotopic composition of different carbon-bearing materials, organic and inorganic, is commonly used to reconstruct distributions of tropical C-4 grasslands. However, no study of modern soils has yet combined carbon isotopic compositions (delta C-13) 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 delta C-13 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 delta C-13 results (delta C-13(SC) mean = 0.5 parts per thousand, 1 sigma = 0.8 parts per thousand, n = 70 nodules collected), denote C-4 grasses exclusively, but soil carbonates are absent from two of the three sites with trees. Organics were measured at all 11 sites including bulk (delta C-13(SOM) -13.1 +/- 1.8 parts per thousand) and plant wax biomarkers (e.g., delta C-13(29alk) -23.5 +/- 3.1, delta C-13(31alk) -22.2 +/- 2.0, delta C-13(33alk) -21.9 +/- 1.8, delta C-13(28acid) -19.1 +/- 2.1 parts per thousand, delta C-13(30acid) -21.1 +/- 2.6 parts per thousand, and delta C-13(32acid) -20.1 +/- 1.7 parts per thousand). delta C-13 values in organic materials record the dominance of C-4 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 delta C-13(29alk) is overly responsive to tree inputs (greater C-29 concentration and C-13-depletion relative to other homologues), whereas the delta C-13(31alk) delta(13)C(33alk )and delta C-13(32acid) 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. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study demonstrates that comparing the carbon isotope δ13C values can determine the distribution of C-4 grasslands in different types of carbon-bearing materials. Soil samples from the Serengeti ecosystem show that plant wax and organic material δ13C values are most accurate in recording vegetation cover, and they have broad applicability in various environments.