Can stable carbon isotopes (δ13C) in soil carbon be used to describe the dynamics of Eucalyptus savanna-rainforest boundaries in the Australian monsoon tropics?

The history of isolated patches of monsoon rainforest within large tracts of Eucalyptus savanna is poorly understood because of the scarcity of reliable palaeoecological records in the Australian monsoon tropics. Elsewhere in the world, the ratio of the stable isotopes (13)C to (12)C (delta(13)C) in soil organic matter has shed light on the dynamics of rainforest-savanna boundaries because tropical grasses with the C4 photosynthetic pathway have a distinct delta(13)C signature (-17 to -9parts per thousand) compared with that of woody plants with the C3 photosynthetic pathway (-32 to -22parts per thousand). In order to determine the magnitude of the variation in delta(13) C, unreplicated soil profiles were sampled beneath different vegetation types on three boundaries between Eucalyptus savanna and rainforest that were both growing on Tertiary age laterite parent material. Replicated (n = 3) soil profiles, which were also derived from Tertiary age laterite, were sampled from beneath: (i) dense stands of African grasses within a frequently burnt Eucalyptus savanna; and within the same long unburnt Eucalyptus savanna, (ii) patches of African and natives grasses and (iii) clumps of Acacia trees. The strongly negative delta(13) C values of soil organic matter derived from the frequently burnt and long unburnt grassy understoreys in the Eucalyptus savannas showed that a considerable amount of the soil carbon was derived from C3 (woody) species despite the presence of a ground layer dominated by C4 grasses. However, a feature of these data was the considerable variability among the three 'replicate' profiles. The surface soil samples from beneath three clumps of Acacia trees in the unburnt Eucalyptus savanna had much less variable delta(13)C values and were similar to two of the three monsoon rainforests sampled. The pattern of delta(13)C values from unreplicated soil profiles from different vegetation types across three rainforest boundaries was also very variable and not always obviously related the known disturbance history of the extant vegetation. Given the considerable variability within and between vegetation types with contrasting disturbance histories, it is concluded that the use of carbon stable isotopes to advance understanding of the dynamics of rainforest and Eucalyptus savanna boundaries will require further development, such as determination of the (14)C age and delta(13)C values of different soil carbon fractions.