Soil carbon and soil nitrogen changes after clearing of mulga vegetation Ben Harms, Ram Dalal and Weijin Wang Department of Natural Resources and Mines, 80 Meiers Road, Indooroopilly Queensland 4068, Australia. www.nrm.qld.gov.au CRC for Greenhouse Accounting, Australia. www.greenhouse.crc.org.au Email harmsb@nrm.qld.gov.au Abstract Mulga (Acacia aneura) dominated vegetation originally occupied 11.2 million hectares in Queensland, of which 12% has been cleared. Clearing of mulga vegetation, and altered land use for a period of 20 years has caused a significant decline in soil carbon (C) and nitrogen (N) at the study site in southern Queensland. Soil C in the top 0.05 m of soil declined by 31% and 35% under buffel pasture and cropping respectively, while in the top 0.30 m depths soil C stocks declined by 2.4 and 4.7 t/ha respectively. Light fraction carbon, a reactive (labile) component of organic carbon, comprises about 19% of the total soil C under the mulga vegetation. After 20 years of changed land use (both pasture and cropping) more than half of the carbon present in this ‘pool’ has disappeared. Losses of soil N exceeded those of soil C for both cropping and pasture land use, resulting in higher C:N ratios in soil under pasture and cropping compared to soil under mulga. These results confirm a decline in soil fertility in mulga soils after clearing and have implications for the long-term sustainability of the cleared lands. Based on these results and given current tree clearing rates, loss of soil C due to clearing in the Mulga Lands of Queensland results in emissions to the atmosphere of approximately 1.4 Mt CO 2 -equivalents per year. Key Words Soil C, soil N, light fraction carbon, organic matter quality, soil fertility, greenhouse gas emissions. Introduction Mulga (Acacia aneura) is a significant Australian vegetation community, although estimates of its areal extent are complicated by its diversity in structural form – from sparse shrubland to open-forest – and its wide range of associated ground flora. Johnson and Burrows (1994) estimated the area of mulga to be 150 million hectares (Mha) or 20% of the Australian continent. The Mulga Lands Bioregion (Thackaway and Cresswell 1995) includes only a proportion of the mulga communities. In Figure 1, the distribution of approximately 100Mha of mulga vegetation is illustrated. In Queensland, about 12% of the original 11.2 Mha of mulga vegetation had been cleared by 1999 (Wilson et al. 2002). Between 1997 and 1999, remnant mulga vegetation in Queensland was cleared at an annual rate of about 35,000 ha (Wilson et al. 2002), while the clearing rate in the Mulga Lands Bioregion (Queensland only) was about 85,000 ha per year (Department of Natural Resources and Mines 2000). By 2001, the clearing rate for the Bioregion had increased to 157,950 ha per year (Department of Natural Resources- and Mines 2003). For some time, there has been concern about the sustainability of cleared mulga lands because of their occurrence in arid to semi-arid environments (250-500 mm rainfall, often exceeding 30% annual rainfall variability) and their fragile soils that are comparably low in soil organic matter and plant available phosphorus (Condon et al. 1969). Soil organic matter has a variety of important functions in soils; for example it acts as a reservoir of nutrients (principally N, P and S) and improves soil structure, infiltration and water holding capacity. Because of the complex nature and diverse composition of soil organic matter, organic carbon is used as its analytical measure. In the absence of inorganic carbon components such as carbonate, soil organic C may be referred to simply as ‘soil C’. Of the diagnostic tests that may be used to determine the N status of a soil, total N provides an indication of the soils long-term N-supplying capacity (Strong and Mason 1999). Several studies report losses in soil C and soil N following land clearing in Queensland (Dalal and Mayer 1986a, Harms and Dalal 2003); especially where the land use has been changed to cropping. In a literature review on land use change from of forest to pasture, Murty et al. (2002) found no significant overall change in either soil C or N, although changes in soil C at individual sites ranged from -50% to +160%. These findings showed a high variability in soil C stocks both within soil landscapes and following land use change. Hence, ecosystems may lose or gain C, depending on soil type, pasture © 2004. SuperSoil 2004: 3rd Australian New Zealand Soils Conference, 5 – 9 December 2004, University of Sydney, Australia. Published on CDROM. Website www.regional.org.au/au/asssi/ 1