Impacts of fire on soil organic carbon stocks in a grazed semi-arid tropical Australian savanna: accounting for landscape variabilityD. E. Allen A E , P. M. Bloesch A , R. A. Cowley B , T. G. Orton A C , J. E. Payne A D and R. C. Dalal A
A Landscape Sciences (ESP), Department of Science, Information Technology, Innovation and the Arts, Queensland State Government, GPO Box 5078, Brisbane, Qld 4001, Australia.
B Northern Territory Department of Primary Industry and Fisheries, PO Box 1346, Katherine, NT 0851, Australia.
C Faculty of Agriculture and Environment, The University of Sydney, 1 Central Avenue, Australia Technology Park, Eveleigh, NSW 2015, Australia.
D Soils & Landscapes, Landcare Research, PO Box 69040, Lincoln 7640 New Zealand.
E Corresponding author. Email: Diane.Allen@dsitia.qld.gov.au
The Rangeland Journal 36(4) 359-369 https://doi.org/10.1071/RJ14044
Submitted: 1 April 2014 Accepted: 21 August 2014 Published: 24 September 2014
Fire and grazing are commonplace in Australian tropical savannas and the effects of these management practices on soil organic carbon stocks (SOC) is not well understood. A long-term (20 years) experiment studying the effects of fire on a grazed semi-arid tropical savanna was used to increase this understanding. Treatments, including frequency of fire (every 2, 4 and 6 years), season of fire [early (June) vs late (October) dry season] and unburnt control plots, were imposed on Vertosol grassland and Calcarosol woodland sites, which were grazed. Additionally long-term enclosures [unburnt (except the Calcarosol in 2001) and ungrazed since 1973] on each soil type adjacent to each site were sampled, although not included in statistical analyses. SOC stocks were measured to a soil depth of 0.3 m using a wet oxidation method (to avoid interference by carbonates) and compared on an equivalent soil mass basis. Significant treatment differences in SOC stocks were tested for, while accounting for spatial background variation within each site. SOC stocks (0–0.3 m soil depth) ranged between 10.1 and 28.9 t ha–1 (Vertosol site) and 20.7 and 54.9 t ha–1 (Calcarosol site). There were no consistent effects of frequency or season of fire on SOC stocks, possibly reflecting the limited statistical power of the study and inherent spatial variability observed. Differences in the response to frequency and season of fire observed between these soils may have been due to differences in clay type, plant species composition and/or preferential grazing activity associated with fire management. There may also have been differences in C input between treatments and sites due to differences in the herbage mass and post-fire grazing activity on both sites and changed pasture composition, higher herbage fuel load, and a reduction in woody cover on the Vertosol site. This study demonstrated the importance of accounting for background spatial variability and treatment replication (in the absence of baseline values) when assessing SOC stocks in relation to management practices. Given the absence of baseline SOC values and the potentially long period required to obtain changes in SOC in rangelands, modelling of turnover of SOC in relation to background spatial variability would enable management scenarios to be considered in relation to landscape variation that may be unrelated to management. These considerations are important for reducing uncertainty in C-flux accounting and to provide accurate and cost-effective methods for land managers considering participation in the C economy.
Additional keywords: bulk density, calcareous, fire, grazing, organic matter, soil organic carbon.
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