Dynamics of Acacia aneura - Triodia boundaries using carbon (14C and δ13C) and nitrogen (δ15N) signatures in soil organic matter in central Australia

Abstract: Soil organic matter (SOM) was sampled from soil profiles on a near level sandsheet at the southern limit of the Tanami Desert in central Australia to determine if boundaries of Triodia hummock grassland– Acacia aneura shrublands had changed in the Holocene. Accelerator Mass Spectrometry (AMS) 14C dating of 16 soil profiles showed that SOM that had accumulated at 100 to 140 cm depth, (near the base of most profiles) had ages between 1175 and 2630 14C years, averaging 1906 14C years. The stable carbon isotopic (ä13C) composition of SOM from the upper 50 cm soil profiles in the A. aneura shrubland (inhabited by plants with predominantly C3 photosynthetic pathway) was significantly more 13C-depleted than the comparable soil interval beneath a Triodia grassland (predominantly C4 photosynthetic pathway). Mean age of SOM at 50 cm depth was 830 14C years, suggesting the vegetation has been stable for about 1000 years. However, soil profiles in Triodia grassland adjacent to the shrubland boundary had slightly more depleted ä13C relative to sites >0.5 km from the boundary. With respect to stable nitrogen isotopic values, only surface soils in the Acacia shrublands were found to be 15N-enriched relative to all other soil depths. Although there were no obvious environmental discontinuities, such as change in soil type or slope angle, associated with the ecosystem boundaries, the Acacia shrublands were found to occur on more clayrich soils with higher concentrations of total phosphorus, nitrogen and potassium compared with the surrounding grasslands, and these trends became more pronounced with increasing distance from the ecotone: it is unclear if these differences are a cause or an effect of the vegetation mosaic. The concordance of the vegetation boundaries with the ä13C and ä15N and of soil nutrients are consistent with only minor attrition of the A. aneura shrublands in the late Holocene at this site.