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Dynamics of Acacia aneura—Triodia boundaries using carbon (14C and 13C) and nitrogen (15N) signatures in soil organic matter in central Australia

School for Environmental Research, Charles Darwin University, Darwin 0909, Australia, david.bowman{at}utas.edu.au

Guy S. Boggs

School of Science and Primary Industries, Charles Darwin University, Darwin 0909, Australia

Lynda D. Prior

School for Environmental Research, Charles Darwin University, Darwin 0909, Australia

Evelyn S. Krull

CSIRO Land & Water and CRC for Greenhouse Accounting, PMB 2, Glen Osmond, South Australia 5064, Australia

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 (¹³C) 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 ¹³C 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 clay-rich 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 ¹³C and ¹⁵N and of soil nutrients are consistent with only minor attrition of the A. aneura shrublands in the late Holocene at this site.

Key Words: Aboriginal fire management • AMS dating • Acacia shrubland • carbon stable isotopes • desert environments • grassland • nitrogen stable isotopes • soil organic matter • vegetation boundary dynamics • late Holocene.

The Holocene, Vol. 17, No. 3, 311-318 (2007)
DOI: 10.1177/0959683607076442


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