Soil carbon stock in the tropical rangelands of Australia: Effects of soil type and grazing pressure, and determination of sampling requirement

• Published in: Geoderma Vol. 167; pp. 261 - 273
• Main Authors: Pringle, M.J., Allen, D.E., Dalal, R.C., Payne, J.E., Mayer, D.G., O'Reagain, P., Marchant, B.P.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.11.2011; Elsevier
• Subjects: Agronomy. Soil science and plant productions; Biological and medical sciences; Carbon; Carbon stock; Cattle; Earth sciences; Earth, ocean, space; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; Geostatistics; Grazing; Isotope geochemistry; Isotope geochemistry. Geochronology; Rangelands; Raw materials; Residual maximum likelihood (REML); Sampling; Soil (material); Soil sampling; Soils; Surficial geology; Queensland Australia; Australia; Carbon stock; Rangelands; Residual maximum likelihood (REML); Sampling; Geostatistics; Carbon isotope discrimination; Australasia; tropical zone; vegetation; Isotopic composition; Carbon content; soil profiles; savanna; soils; Ungulata; organic carbon; climate change; geostatistics; stable isotopes; Bovine; isotope ratios; C-13/C-12; Grazing; Edaphic factor; Vertebrata; Mammalia; Artiodactyla
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/j.geoderma.2011.09.001

On-going, high-profile public debate about climate change has focussed attention on how to monitor the soil organic carbon stock ( C s) of rangelands (savannas). Unfortunately, optimal sampling of the rangelands for baseline C s – the critical first step towards efficient monitoring – has received relatively little attention to date. Moreover, in the rangelands of tropical Australia relatively little is known about how C s is influenced by the practice of cattle grazing. To address these issues we used linear mixed models to: (i) unravel how grazing pressure (over a 12-year period) and soil type have affected C s and the stable carbon isotope ratio of soil organic carbon ( δ 13 C) (a measure of the relative contributions of C 3 and C 4 vegetation to C s); (ii) examine the spatial covariation of C s and δ 13 C; and, (iii) explore the amount of soil sampling required to adequately determine baseline C s. Modelling was done in the context of the material coordinate system for the soil profile, therefore the depths reported, while conventional, are only nominal. Linear mixed models revealed that soil type and grazing pressure interacted to influence C s to a depth of 0.3 m in the profile. At a depth of 0.5 m there was no effect of grazing on C s, but the soil type effect on C s was significant. Soil type influenced δ 13 C to a soil depth of 0.5 m but there was no effect of grazing at any depth examined. The linear mixed model also revealed the strong negative correlation of C s with δ 13 C, particularly to a depth of 0.1 m in the soil profile. This suggested that increased C s at the study site was associated with increased input of C from C 3 trees and shrubs relative to the C 4 perennial grasses; as the latter form the bulk of the cattle diet, we contend that C sequestration may be negatively correlated with forage production. Our baseline C s sampling recommendation for cattle-grazing properties of the tropical rangelands of Australia is to: (i) divide the property into units of apparently uniform soil type and grazing management; (ii) use stratified simple random sampling to spread at least 25 soil sampling locations about each unit, with at least two samples collected per stratum. This will be adequate to accurately estimate baseline mean C s to within 20% of the true mean, to a nominal depth of 0.3 m in the profile. ► Carbon stock in the tropical rangelands of Australia was studied. ► Soil type and grazing pressure influence carbon stock to 0.3 m in the profile. ► Carbon sequestration may be negatively correlated with forage production. ► Stratified simple random sampling is proposed to characterise baseline stock.