Interpretation of soil δ<sup>13</sup>C as an indicator of vegetation change in African savannas
Gillson L., Waldron S., Willis KJ.
Question: The relationship between carbon-13 in soil organic matter and C3 and C4 plant abundance is complicated because of differential productivity, litter fall and decomposition. As a result, applying a mass balance equation to δ13C data from soils cannot be used to infer past C3 and C4 plant abundance; only the proportion of carbon derived from C3 and C4 plants can be estimated. In this paper, we compare δ13C of surface soil samples with vegetation data, in order to establish whether the ratio of C 3:C4 plants (rather than the proportion of carbon from C3 and C4 plants) can be inferred from soil δ13C. Location: The Tsavo National Park, in southeastern Kenya. Methods: We compare vegetation data with δ13C of organic matter in surface soil samples and derive regression equations relating the δ13C of soil organic matter to C3:C4 plant abundance. We use these equations to interpret δ13C data from soil profiles in terms of changes in inferred C3:C4 plant ratio. We compare our method of interpretation with that derived from a mass balance approach. Results: There was a statistically significant, linear relationship between the δ13C of organic matter in surface soil samples and the natural logarithm of the ratio of C3:C4 plants in the 100m2 surrounding the soil sample. Conclusions: We suggest that interpretation of δ13C data from organic matter in soil profiles can be improved by comparing vegetation surveys with δ13C of organic matter in surface soil samples. Our results suggest that past C3 plant abundance might be under-estimated if a mass balance approach is used.