Recent vegetation changes in central Queensland, Australia:: Evidence from δ13C and 14C analyses of soil organic matter

This study examines whether changes from gassy to woody vegetation coincide with the initiation of European fanning practices (last 50-150 years) in central Queensland, using a site where historical records suggest invasion of woody vegetation (Acacia sp.). Soil samples were taken along a transect, spanning the transition between C4-dominated grassland and C3-dominated woodland. We utilize delta(13) C and C-14 data of bulk and size-separated soil organic matter (SOM) to infer the time course of changes in vegetation and soil carbon stocks. delta C-13 values of bulk SOM indicate a shift from grass (C4)- to tree (C3)derived carbon in the woodland compared with the grassland. The delta C-13 analyses of the size-separates showed that most of the labile, C3-derived carbon was present in the particulate organic carbon (POC) fraction (> 53 mu m) (down to 30 cm), whereas delta C-13 values in the < 53 mu m fraction showed greater C4 contributions. The delta C-13 values of the POC and the < 53 mu m fractions in the grassland and transition zone were significantly different, inferring a relatively recent change from a C4- to a C3-dominated system (< 100 years). This interpretation is supported by C-14 data as the C-14 activity of both the > 200 and 53-200 mu m fraction was greater than 108% modem (pMC), indicating that most of the C3-derived POC fractions were formed during the past similar to 45 years. This bomb-derived' carbon (> 100 pMC) was present to a depth of 30 cm. Thus, C-14 and delta C-13 data from the size fractions indicate that much of the vegetation change at this site occurred over the last 50 years. We also found that the thickened site had greater C storage in above-ground biomass and soil carbon stocks compared with the grassy site. Preliminary modelling of changes in carbon stocks, using the Roth-C model, support that the change from grassland to woodland probably occurred < 100 years ago. (c) 2004 Elsevier B.V. All rights reserved.