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

On-going, high-profile public debate about climate change has focussed attention on how to monitor the soil organic carbon stock (Cs) of rangelands (savannas). Unfortunately, optimal sampling of the rangelands for baseline Cs – 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 Cs 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 Cs and the stable carbon isotope ratio of soil organic carbon (δ13C) (a measure of the relative contributions of C3 and C4 vegetation to Cs); (ii) examine the spatial covariation of Cs and δ13C; and, (iii) explore the amount of soil sampling required to adequately determine baseline Cs. 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 Cs to a depth of 0.3 m in the profile. At a depth of 0.5 m there was no effect of grazing on Cs, but the soil type effect on Cs was significant. Soil type influenced δ13C 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 Cs with δ13C, particularly to a depth of 0.1 m in the soil profile. This suggested that increased Cs at the study site was associated with increased input of C from C3 trees and shrubs relative to the C4 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 Cs 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 Cs 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.