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 (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 (δ¹³C) (a measure of the relative contributions of C₃ and C₄ vegetation to C_s); (ii) examine the spatial covariation of C_s and δ¹³C; 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 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 C_s with δ¹³C, 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 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 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.