A combination of size fractionation and chemical analysis was used to differentiate three forms of soil organic carbon: particulate, humus and resistant. The fractionation procedure was applied to 312 samples collected from 0-30 cm layer of a diverse range of Australian soils. Substantial variations in the content of each fraction were noted with average contributions to soil organic carbon of 19, 56 and 26% for the particulate, humus and resistant forms, respectively.

Relative to traditional laboratory analysis, more rapid and cost effective estimates of the content and composition of soil organic carbon were possible using mid-infrared spectroscopy. More reliable estimates were obtained when the analysis was completed separately on a regional basis compared to those obtained using a national approach. When combined with soil bulk density and gravel content data, the derived soil carbon content and composition data can provide the soil C stock data required for C accounting and soil C models.

Changes to crop management practices may have the potential to increase soil carbon storage and decrease the emission of greenhouse gasses. This study found that management practices such as no-till, stubble retention and nitrogen fertiliser addition were not capable of increasing soil carbon stocks under the climatic conditions found throughout Queensland, Australia. To increase soil carbon stocks in this region a period of carbon input in the form of a pasture ley is likely to be required.

Recent changes to sugarcane management practices may have the potential to increase soil organic carbon and help reduce greenhouse gas emissions.  We examined changes to soil organic carbon in response to trash retention and no-till management at 4 sites throughout Queensland, but observed no increase in organic carbon stocks over the top 0.1 m or 0.3 m of the soil profile.  The use of such practices is thus unlikely to lead to significant carbon sequestration for the purposes of greenhouse gas abatement.

Quantifying the different effects of environmental parameters, land-use history, and management practices on soil organic carbon nitrogen, and bulk density is required to better inform management of agricultural soils. Cropping sites had one third less carbon than pasture sites and clay-rich soils contained the greatest carbon stocks. Farmers can influence SOC more by their choice of land use than their day-to-day soil management but they are able to select practices for their ability to retain more SOC.

There has been much speculation about the ability of agricultural soils to sequester carbon and for farmers to participate in carbon trading schemes. This paper explores variations in soil organic carbon of some common land uses of the Murray Catchment and investigates those climatic, soil physico-chemical and land use factors which are correlating with carbon stocks. Understanding which factors correlate with soil carbon contents will allow land managers to be better informed about the proposition of sequestering carbon.

To estimate the potential carbon storage in the soil it is important to first understand the influence of environmental variables before land management can be assessed. A survey of 354 sites across the Lachlan and Macquarie catchments of New South Wales, Australia examined soil organic carbon (SOC), associated soil properties and land management. At 0-10 cm average annual rainfall had the strongest positive influence on SOC stock, while at 20–30 cm, the proportion of silica (SiO₂) had a negative relationship.

We examined two land-use systems (“onative” and “oimproved” pastures) on a single soil type (Chromosol) across the northern Tablelands of NSW for their capacity to store organic carbon. We could detect no significant difference between these land-uses in carbon concentration (%) nor in total carbon stock (Mgha^–1). Within land-use systems however, larger total carbon stock was associated with a larger proportion of “osub-surface” carbon suggesting that historical rather than contemporary management has influenced the overall carbon storage at any given location).

Cultivation of soil leads to carbon stock loss, some of which is emitted to the atmosphere. Soil carbon level of 147 properties surveyed in the NSW North-West Slopes and Plains region indicated that on both black and red cropping soils, carbon level in the surface 0.1 m depth is 17-28% higher under minimum tillage compared to conventional tillage. This demonstrates the potential of minimum tillage to reduce soil carbon emission from cropping soil. Soil carbon level was also affected by environmental and management factors, suggesting the need for long term research on permanent sites to confirm these findings.

Quantifying soil organic carbon (SOC) stocks in Australian grazing landscapes is important to assist management decisions, including carbon-trading initiatives and landscape sustainability.  We sampled four types of grazing management undertaken in Queensland tropical and subtropical grazing lands and found SOC stocks strongly influenced by temperature, vapour pressure deficit, standing pasture dry matter, soil type and dominant grass species, although the effect of grazing management was less clear.  Key challenges for the future include describing and relating grazing management information to SOC stocks with confidence, while ensuring background response(s) to climate and soil properties is taken into account.

Modified management of cropping and grazing lands has the potential to enhance soil carbon stocks, and thus contribute to greenhouse gas mitigation. A survey of properties treated with chemical fertiliser or organic amendments did not detect a difference in soil carbon stocks between these treatments.  Though not statistically significant, there was an indication of higher soil carbon under rotational grazing compared with continuous grazing. Further research is required to definitively assess the potential for change in land management to increase soil carbon stocks.

Rapid and accurate analysis of soil organic carbon is essential for carbon accounting purposes. This paper examined the ability of mid infra-red spectroscopy to detect changes in total organic carbon stocks and carbon fractions due to different cropping management, and found that this technique did not accurately detect management effects. Our results indicate that this technique would need to be further refined before it was suitable for detecting relatively small changes in organic carbon stocks.

Many decades of grain-growing in northwest NSW has depleted cropping soils of soil organic matter – the source of many beneficial physical, chemical and biological soil properties. Through a sampling survey of 145 farm soils, we found that sowing perennial tropical grasses improved soil organic matter (including carbon) in the surface soil. Grain growers may need to diversify into pasture production to rebuild the health of degraded cropping soils.

Soil organic carbon (OC) stocks in the Mid-North (38 Mg ha^–1) and the Eyre Peninsula (29.1 Mg ha^–1) both varied ~20 Mg ha^–1 and it was difficult to determine differences between cropping and crop–pasture soils. We demonstrate that the use of a management-based index approach was useful in improving understanding of regional level variability in management and in soil OC.

Perennial grass pastures are promoted as potential carbon sinks. Radioactive carbon was used to quantify the amount of carbon dioxide removed from the atmosphere and stored belowground under Kikuyu and Rhodes grass. 20-24% of the carbon dioxide fixed by these grasses was found in soil organic matter after six months (Rhodes) and 12 months (Kikuyu). For Kikuyu, about 1 t/ha of carbon accumulated in soil organic matter over one year, confirming the potential to off-set greenhouse gas emissions from livestock.

We measured changes in the carbon isotope composition of carbon fractions following the introduction of subtropical perennial grasses to study the turnover of carbon in these pasture systems. In sandy soils, carbon was only accumulating in a coarse size fraction, whereas in finer textured soils, carbon was accumulating in both the coarse and fine size fractions. These findings indicate that not all newly sequestered carbon is equally stable and soil physiochemical properties will play an important role in determining the stability of this new carbon.