Susan Orgill

The relationship between soil organic carbon and soil surface characteristics in the semi-arid rangelands of southern Australia

The potential carbon sequestration in rangelands is largely due to the extensive areas they occupy, even though levels of soil organic carbon (SOC) are low. There is considerable uncertainty in achieving this potential due to the inherent patchy spatial and temporal distribution of rangeland vegetation and resources. At a paddock scale, determining appropriate sampling scales is a critical first step in the accurate estimation of size and spatial distribution of stocks of SOC. This issue was addressed by examining the spatial distribution of SOC and determining the association of SOC with other site characteristics such as ground cover and vegetation. This was done in a pilot study conducted in a 136-ha paddock located on the Cobar Pediplain Bioregion in western New South Wales, Australia. Each of 104 sites was sampled using a 0.25-m2 quadrat to assess biomass and ground cover category (percentage of perennial plants, bare ground, cryptogams, annual plants and litter) of a soil core taken from the centre of each quadrat, and proximity to trees and shrubs. The soil core was used to determine total organic C (TOC), total N (TN) and the C : N ratio at four depths (0–5; 5–10; 10–20; 20–30 cm). From the quadrat and ground cover categories of the soil cores, six microsite categories were identified using cluster analysis: cryptogams; litter (≥25% litter); bare (≥60% bare ground); annual (≥40% annual plants); litter-P (≥15% litter and ≥10% perennial plants) and perennial (≥30% perennial plants). Microsite, depth in soil profile and the presence of trees and shrubs all had a significant (P < 0.001) effect on TOC concentration. The predicted means (s.e. of mean) of TOC at the soil surface (0–5 cm) were perennial 1.26 (0.04) %; litter-P 1.20 (0.05) %; annual 1.18 (0.06) %; litter 1.12 (0.05) %; bare 1.03 (0.05) % and cryptogams 0.88 (0.06) %. Higher concentrations of TOC were associated with the presence of trees and were almost 30% higher in close proximity (<1 m) to a tree. There was a consistent finding that higher concentrations of TOC, TN and the high values of C : N ratio were each associated with higher ground cover of perennial plants. The autocorrelation range for soil C stocks was ~30 m and for categories of ground cover which varied from 10 m to over 200 m. The spatial predictions for ground cover of perennial plants closely mirrored those for C stocks, which were 22.9 Mg C ha–1 in the top 30 cm of soil in this environment. As both tree proximity and ground cover had a significant effect on TOC, quantifying the levels of soil organic C at a paddock scale will require an understanding of the spatial patterns of vegetation (woody and ground cover), which provides a basis for within-paddock stratification before soil sampling.

Parent material and climate affect soil organic carbon fractions under pastures in south-eastern Australia

In the present field survey, 72 sites were sampled to assess the effect of climate (Monaro, Boorowa and Coleambally regions) and parent material (Monaro region only; basalt and granite) on soil organic carbon (OC) under perennial pastures. In the higher-rainfall zone (Monaro and Boorowa; >500mm mean annual rainfall), OC stocks under introduced and native perennial pastures were compared, whereas in the lower-rainfall zone (Coleambally; <500mm mean annual rainfall) OC stocks under crops and pastures were compared. Carbon fractions included total OC (TOC), particulate OC (POC), resistant OC (ROC) and humic OC (HUM). Higher OC stocks were associated with higher spring and summer rainfall and lower annual temperatures. Within a climatic zone, parent material affected the stock of OC fractions in the 0–30cm soil layer. Within a climatic zone, when grouped by parent material, there was no difference in OC stock with vegetation type. There were significant correlations between soil factors associated with parent material and OC concentration, including negative correlations between SiO2 and HUM (P<0.05) and positive correlations between cation exchange capacity and TOC, POC and ROC (P<0.01). TOC was also positively correlated with total nitrogen (N) and available sulfur (S; P<0.05), indicating organic matter in soil is important for N and S supply for plant production in the studied regions, and vice versa. Although ensuring adequate available S may increase OC stocks in south-eastern Australia, the large stock of OC in the soil under perennial pastures, and the dominating effect of climate and parent material on this stock, may mean that modest increases in soil OC due to management factors go undetected.

Sensitivity of soil organic carbon to grazing management in the semi-arid rangelands of south-eastern Australia

This study compared the effects of grazing management on soil organic carbon (OC) stocks in the semi-arid rangelands of New South Wales, Australia. A field survey was conducted at three locations (Brewarrina, Cobar–North and Cobar–South), with paired sites of long-term (>8 years) rotational grazing management and continuously grazed pastures (either set stocked or no stocking). At each location, soil OC, carbon (C) fractions, soil nitrogen (N) and microsite and site factors (including ground cover and woody vegetation) were measured. The control of total grazing pressure (TGP) through rotational grazing and exclusion fencing did not increase soil C stocks compared with continuous grazing for the majority of comparisons. However, in some parts of the landscape, higher soil C stock was found with TGP control, for example on the ridges (21.6 vs 13.3 t C ha–1 to 0.3 m). C stocks increased with litter and perennial ground cover and with close proximity to trees. At Brewarrina, C stocks were positively affected by perennial plant cover (P < 0.001) and litter (P < 0.05), whereas at Cobar–North and Cobar–South C stocks were positively affected by the presence of trees (P < 0.001), with higher C stocks in close proximity to trees, and with increasing litter cover (P < 0.01). The present study demonstrates that natural resource benefits, such as increased perennial cover, can be achieved through controlling TGP in the rangelands but increases in soil C may be limited in certain parts of the landscape. These findings also highlight that interactions between managed and unmanaged TGP and microsite factors, such as ground cover and proximity to woody vegetation, need to be considered when evaluating the role of changed grazing management on soil C.