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

Abstract

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 13C 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.Crown

Removing Grazing Pressure from a Native Pasture Decreases Soil Organic Carbon in Southern New South Wales, Australia

“…There are reports of decreases in soil C with increased grazing intensity (Derner et al, 1997;Frank et al, 1995;Golluscio et al, 2009;Ingram et al, 2008;McSherry & Ritchie, 2013), increases in soil C with increased grazing intensity particularly when light grazing was compared with no grazing (Ganjegunte et al, 2005;Liu et al, 2012;McSherry & Ritchie, 2013) and no difference in soil C regardless of grazing intensity (Kieft, 1994). In particular, seven Australian studies, which compared the impact of grazing management by rotation with continuous grazing on soil C to 0·30 m, detected no difference in soil C stocks (Allen et al, 2013;Chan et al, 2010;Orgill et al, 2014;Pringle et al, 2011;Pringle et al, 2014;Sanderman et al, 2015;Sanjari et al, 2008). Meanwhile, results from the USA showed soil C stocks increased by 8·4 Mg C ha À1 (to 0·50 m) after 25 years of rotational grazing compared with an adjacent continuously grazed pasture (Conant et al, 2003).…”

A geostatistical method to account for the number of aliquots in composite samples for normal and lognormal random variables

“…The samples from all 473 locations were split into three soil‐depth intervals (0–10, 10–20 and 20–30 cm), and bulk densities were calculated and screened for erroneous values. Each of these 1419 samples was mixed and subsampled, and carbon concentrations of the subsamples were analysed with an Isoprime isotope ratio mass spectrometer (IRMS) coupled to a Eurovector elemental analyser (Isoprime‐EuroEA 3000, Isoprime Ltd, Stockport, UK) with HCl pretreatment (concentration 10%) to remove carbonates (see Pringle et al , , for further detail). The SOC content for each site was calculated for the 0–30‐cm fixed soil depth (we refer to this variable as S 30 ).…”

“…For example, percentages of soil organic carbon (SOC) often require logarithmic transformation (e.g. Pringle et al, 2011), as do concentrations of heavy metals, trace elements and plant nutrients (e.g. Marchant et al, 2009;Lacarce et al, 2012).…”

Application of High Carbon:Nitrogen Material Enhanced the Formation of the Soil A Horizon and Nitrogen Fixation in a Tropical Agricultural Field

“…horizons 1 and 2. In general, agricultural activities influence carbon stock to a depth of 0.3 m in the profile in savanna systems [19]; we therefore focused on the top three horizons in the following discussion. At least 7 cm of soil A horizon was formed in 4.5 years.…”