Carbon dynamics

In light of international agreements such as the Paris Agreement—which emerged from the 21st Conference of the Parties of the United Nations Framework Convention on Climate Change (UNFCC) in 2015, and governs greenhouse gas emissions mitigation, adaptation and finance from 2020—the management and monitoring of soil carbon is a matter of national and international importance. Putting back an additional 0.4 per cent of carbon into the soil every year could neutralise the impact of greenhouse gases released into the atmosphere. Soil carbon can be a significant source or sink for greenhouse gases, depending on how land is used and managed, and whether the soil carbon is organic or inorganic (Sanderman 2012, Monger et al. 2015). Management of soil carbon is also central to maintaining soil health and ensuring global food security.

The organic carbon content of soil is a key indicator of its health. It is a variable that indicates the functioning of many ecosystem processes (e.g. nutrient and waste cycling, water storage, biodiversity). The carbon comes primarily from plant materials that are created through the capture of atmospheric carbon dioxide via the process of photosynthesis. These organic materials are cycled through the soil, and used by organisms as a source of energy and nutrients. A significant amount of carbon dioxide is returned to the atmosphere as a result of respiration. Increasing soil organic carbon (SOC) leads to an increase in:

• energy supply for microbes, macrofauna and earthworms
• direct nutrient supply to plants (particularly nitrogen, phosphorus and sulfur)
• the capacity of the soil to retain and exchange nutrients
• aggregation of soil particles and stability of soil structure
• water storage and water availability to plants
• beneficial thermal properties
• pH buffering (helping to maintain acidity at a constant level).

The maximum equilibrium carbon content for a soil at a given location is determined by environmental factors such as rainfall, evaporation, solar radiation and temperature. SOC content is generally higher in cool, wet environments, whereas inorganic carbon, in the form of carbonate minerals, is higher in semi-arid environments. A lack of nutrients, and a limited capacity of the soil to store and supply water can reduce this potential maximum, as can other constraints to plant growth (e.g. toxicities). Within these constraints, the actual amount of organic carbon contained in a soil will be determined by the balance between carbon inputs and losses, which are strongly influenced by land management and soil type. Agricultural practices that alter rates of carbon input (e.g. plant residues, compost, mulch) or loss (e.g. removal of crops, cultivation) change the stock of SOC.

Soil carbon stocks in Australia

At the global scale, the amount of carbon contained in terrestrial vegetation (550 ± 100 petagrams—Pg; 1 Pg is 1 billion tonnes) is of a similar order to that in the atmosphere (800 Pg). However, the organic matter in soils is 2–3 times this amount. Approximately 1500–2000 Pg of carbon is in the top metre of soil, and as much as 2300 Pg is in the top 3 metres. In the Australian continent, the estimated total stock of organic carbon in 2010 in the top 0–30 centimetre layer of soil is 24.97 Pg, with 95 per cent confidence limits of 19.04 and 31.83 Pg (Viscarra Rossel et al. 2014). This represents approximately 3.5 per cent of the total stock in the upper 30 centimetres of soil worldwide. Given that Australia occupies 5.2 per cent of the global land area, the total organic carbon stock of Australian soil is relatively less than in other parts of the world.

SOC stocks are low in many Australian agricultural systems. Conversion from native vegetation to agriculture typically reduces SOC by 20–70 per cent (Luo et al. 2010, Sanderman et al. 2010), and results in declining soil health and significant emissions of greenhouse gases. Conservative forms of land management, such as reduced tillage, stubble retention, green manuring and application of organic amendments, can restore SOC stocks, and have a significant impact on national and global emissions. This opportunity is the motivation behind the ‘4 per 1000’ initiative to increase SOC stocks, which was launched at the UNFCCC Paris meeting, and the Australian Government’s Emissions Reduction Fund.

Carbon resilience and land management

There are different types of organic carbon in soils. It is useful to recognise 3 primary fractions (Merry & Janik 2004):

• particulate organic carbon (POC) -  organic carbon associated with particles larger than 0.05 millimetres (excluding charcoal carbon)
• humus organic carbon (HUM) - organic carbon associated with particles smaller than 0.05 millimetres (excluding charcoal carbon)
• resistant organic carbon (ROC) - organic carbon found in soil particles smaller than 2 millimetres, having a chemical structure similar to charcoal.

The 3 primary fractions have contrasting dynamics. POC can be readily increased in a soil, but also breaks down quickly. In contrast, ROC takes much longer to increase unless it is added via an amendment such as biochar (charcoal produced from plant matter), which is produced in bushfires.

A review of replicated Australian field trials with timeseries data (Sanderman & Baldock 2010) provided an important insight into carbon dynamics in agricultural systems. It concluded that, although the implementation of more conservative land management practices may lead to a reduced rate of loss of, or indeed a relative gain in, SOC, absolute SOC stocks may still be slowly declining.

Analysis of major management options for sequestering carbon in agricultural soils highlights the trade-off between agricultural production (i.e. carbon exports in the form of crops, fibre and livestock) and carbon sequestration (capture and storage) in soils (Sanderman et al. 2010; Table LAN4).

Table LAN4 Summary of major management options for sequestering carbon in agricultural soils

0 = nil; + = low; ++ = moderate; +++ = high; H = high; L = low; M = medium; SOC = soil organic carbon

a Qualitative assessment of the SOC sequestration potential of a given management practice

b Qualitative assessment of the confidence in the estimate of sequestration potential, based on both theoretical and evidentiary lines

Source: Sanderman (2012), Sanderman et al. (2010)

Assessment of state and trends in carbon across Australia

A group of experts in soil carbon and land resource assessment was convened to provide an assessment of the state and trends in SOC across Australia in 2011. Their assessment summary has been updated, where possible, with more recent state and territory SoE reports, to provide ratings for regions where the most significant issues are apparent (Figure LAN17). The ratings for all physiographic regions are available on the Digital SoE website.

The following conclusions can be drawn from the currently available evidence:

• The time since clearing is a key factor determining current trends. For example, large parts of Queensland are on a declining trend because widespread clearing for agriculture was still occurring in the 1990s.
• Few regions have increasing SOC stores.
• Regions with intensifying systems of land use (e.g. northern Tasmania) have decreasing stores.
• Most regions with a projected drying climate have declining trends.
• The savanna landscapes of northern Australia have significant potential for increasing SOC stores, but this requires changes in grazing pressures and fire regimes (see also Box LAN8).

Some of the extensive cropping lands in southern Australia with weathered and naturally infertile soils are rated as good (i.e. 30–70 per cent loss) or very good (i.e. less than 30 per cent loss) because they had small carbon stores at the time of European occupation and have not changed substantially (although soil biodiversity has undoubtedly changed). Many of these soils have also benefited from the addition of fertiliser and the correction of trace element deficiencies.

In 2009, the Australian Government, with additional investment from the Grains Research and Development Corporation, established the Soil Carbon Research Program, which aimed to:

• assess rapid and cost-effective methodologies for deriving the data required to quantify SOC stocks and composition (allocation to particulate, humus and resistant forms of carbon), and to measure soil bulk density
• develop and implement a nationally consistent approach to quantifying SOC stocks under combinations of major land-use and management regimes, climate, and soil types used for agricultural production in Australia
• quantify the input and subsequent fate of carbon added to soil by agricultural systems based on subtropical perennial pasture species.

Results from the program were published in 2013 in a special issue of the scientific journal Soil Research (Box LAN10).