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 C 3 and C 4 plant abundance is complicated because of differential productivity, litter fall and decomposition. As a result, applying a mass balance equation to δ 13 C data from soils cannot be used to infer past C 3 and C 4 plant abundance; only the proportion of carbon derived from C 3 and C 4 plants can be estimated. In this paper, we compare δ 13 C of surface soil samples with vegetation data, in order to establish whether the ratio of C 3 :C 4 plants (rather than the proportion of carbon from C 3 and C 4 plants) can be inferred from soil δ 13 C. Location: The Tsavo National Park, in southeastern Kenya. Methods: We compare vegetation data with δ 13 C of organic matter in surface soil samples and derive regression equations relating the δ 13 C of soil organic matter to C 3 :C 4 plant abundance. We use these equations to interpret δ 13 C data from soil profiles in terms of changes in inferred C 3 :C 4 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 δ 13 C of organic matter in surface soil samples and the natural logarithm of the ratio of C 3 :C 4 plants in the 100m 2 surrounding the soil sample. Conclusions: We suggest that interpretation of δ 13 C data from organic matter in soil profiles can be improved by comparing vegetation surveys with δ 13 C of organic matter in surface soil samples. Our results suggest that past C 3 plant abundance might be under-estimated if a mass balance approach is used.