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

Abstract

This study examines whether changes from grassy to woody vegetation coincide with the initiation of European farming 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 δ13C and 14C data of bulk and size-separated soil organic matter (SOM) to infer the time course of changes in vegetation and soil carbon stocks. δ13C values of bulk SOM indicate a shift from grass (C4)- to tree (C3)-derived carbon in the woodland compared with the grassland. The δ13C analyses of the size-separates showed that most of the labile, C3-derived carbon was present in the particulate organic carbon (POC) fraction (>53 μm) (down to 30 cm), whereas δ13C values in the <53 μm fraction showed greater C4 contributions. The δ13C values of the POC and the <53 μm 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 14C data as the 14C activity of both the >200 and 53-200 μm fraction was greater than 108% modern (pMC), indicating that most of the C3-derived POC fractions were formed during the past ∼45 years. This "bomb- derived" carbon (>100 pMC) was present to a depth of 30 cm. Thus, 14C and δ13C 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. © 2004 Elsevier B.V. All rights reserved.