Variability in soil and foliar stable carbon and nitrogen isotope compositions in the winter rainfall biomes of South Africa

We present the stable carbon and nitrogen isotope compositions of 204 plants and 99 soils from South Africa's Fynbos and Succulent Karoo biomes, from which we assess variability in δ13C and δ15N within and between the biomes. Higher foliar δ13C in the Succulent Karoo (−22.4 ± 4.7‰) primarily reflects a greater abundance of CAM photosynthesis relative to Fynbos (−25.7 ± 3.5‰). For C3 plants within the Fynbos Biome, there is no statistically significant relationship between δ13C and climate, suggesting that within the present sampling framework, local-scale variability masks expected climatic controls on C3 plant δ13C. Soil and foliar δ15N are highly variable, but foliar δ15N is significantly higher in the Succulent Karoo (7.7 ± 3.9‰) compared to the Fynbos (4.4 ± 3.3‰). Soil and foliar δ15N correlate, with soils 3 ± 1‰ more enriched in 15N. Both soil and foliar δ15N show stronger correlations with climate than δ13C, and they accord with the previously observed tendency for higher δ15N with greater aridity. The relationship between δ15N and mean annual rainfall is, however, relatively steep (2.4 ± 0.6‰ per 100 mm rainfall), potentially implying additional controls on δ15N variability. These data provide a benchmarking of contemporary isotopic variability, anticipated to support future archaeological, palaeoenvironmental and C and N cycling research.