Carbon is an essential building block of life on Earth, forming abundant organic compounds present in all living things. Managing the flow of carbon between soil, plants and the atmosphere can improve soil productivity and mitigate climate change (see Carbon capital assets).
Carbon cycle and budgets
The land and the atmosphere are linked through the carbon cycle (e.g. Kirschbaum et al. 2001) (see the Climate chapter). Carbon constantly moves between the atmosphere and living organisms in water and on land, and is either temporarily held or more permanently sequestered in biomass, rocks, oceans, soils and sediments.
On land, the carbon cycle is dominated by plants absorbing carbon dioxide (CO2) from the atmosphere, which is stored in the biomass of vegetation. It then enters the soil as organic matter through plant exudates, litter and root decay, and some is released to the atmosphere through respiration of living organisms. In terrestrial environments, soils are the largest reservoir of carbon.
Carbon cycles are fundamental to land productivity. Depending on the use and management of the land, carbon can be absorbed from the atmosphere (e.g. via revegetation and soil rehabilitation) or released (e.g. via removal of vegetation through clearing or bushfires). Sources and sinks of CO2 are summarised in the Australian carbon budget (Figure 15). Management of terrestrial sources and sinks helps balance the release of carbon by industry and due to fires.
Between 2010 and 2019, land-use change in Australia reversed to become a ‘sink’, with 15 million tonnes of CO2 sequestered on average per year (Canadell 2021). This sequestration is small compared with net ecosystem productivity (above and below ground), which sequestered 746 million tonnes of CO2 per year over the same period. However, the sinks from the land sector are still not enough to balance carbon emissions from coal, oil, gas and cement, as well as major losses due to fire (398 million tonnes of CO2 released annually to the atmosphere). A net 23 million tonnes of CO2 were released to the atmosphere on average each year over 2010–19 (Figure 15), which further contributes to global warming (see Climate change) (see the Climate chapter). The 2019–20 bushfires across eastern Australia released between 670 (Bowman et al. 2021) and 830 (DISER 2020b) million tonnes of CO2 to the atmosphere, more than doubling in one season the annual average of the previous decade.
Carbon above the ground in vegetation biomass
Primary production – where plants use the energy from sunlight to grow and produce biomass through photosynthesis – drives the global carbon budget. The amount of biomass that can potentially be supported on land depends on the availability of sunlight, the climate, and the availability of water and nutrients from the soil and supporting substrate. Terrestrial primary production fluctuates over time with seasons and over longer periods coupled to climate cycles, such as El Niño/La Niña in Australia (see the Climate chapter). In 2011, for example, the global terrestrial carbon sink increased by 20% during the strongest sustained La Niña since 1917 (Cleverly et al. 2016a). In addition, evidence is emerging that increased CO2 concentration in the atmosphere (now 50% higher - Betts 2021) is potentially the dominant driver of the positive terrestrial carbon sink since pre-industrial times (1700s) (Haverd et al. 2020, Walker et al. 2021).
Above-ground biomass in forests and other terrestrial vegetation is critical to the global carbon cycle because it stores and sequesters carbon from the atmosphere. From a census of the global biomass (living and nonliving forms) distributed among all the kingdoms of life, Bar-On et al. (2018) estimated there were 550 gigatonnes of carbon (Gt C), of which plants make up around 450 Gt C (more than 80%) (Figure 16a). They also showed that 99% of total global biomass is terrestrial, with a much smaller marine biomass (6 Gt C).
In revised baseline estimates for the pre-1750 era of maximum above-ground woody biomass for Australia’s soils and climates, Roxburgh et al. (2019) found that, under optimal conditions, woodlands could yield, on average, around 49 tonnes of dry matter per hectare, and forests around 230. The total potential above-ground woody biomass stock for Australia is 34.2 gigatonnes (Gt) of dry matter (DISER 2017, Roxburgh et al. 2019). The areas with highest potential biomass are the well-known forested regions of eastern and southern Australia (Figure 17). This revised maximum potential biomass map (pre-1750) was developed as an input to the Full Carbon Accounting Model (FullCAM), which is used in Australia’s National Greenhous Gas Inventory to estimate carbon stock changes and emissions in response to deforestation and afforestation or reforestation (Roxburgh et al. 2019).
The modelled density of above-ground carbon stocks in woody vegetation based on land cover in 2016 is shown in Figure 18. This analysis identified above-ground living biomass carbon stocks in forests and woodlands of approximately 5.6 Gt C in 2016, which translates to approximately 11.2 Gt of dry matter (DISER 2021d); with nonforest vegetation including cropping lands, above-ground living biomass carbon stocks were estimated as 5.9 Gt C (DISER 2021c:6). In an independent study, which additionally included sparse woody vegetation (Liao et al. (2020) predicted the woody biomass of Australia to be around 6.6 Gt C in 2018, and found that woody vegetation change is primarily driven by water availability and its effect on bushfire and plant mortality, particularly in the drier interior of the continent and during periods of drought. This finding is consistent with that of other researchers (e.g. Cleverly et al. 2016a, Bennett et al. 2020, Tarin et al. 2020).
Sustained forest regrowth is one mechanism for increasing carbon sequestration (see Table 6.L.6 in DISER 2021h). In addition, fire can affect biomass, over timeframes from years to decades. For example, large areas of savanna (wet or dry tropical forest and grasslands) across northern and central Australia burn each year (Figures 19–20). The tropical savannas of Australia extend over 23% of the continent (DPIRD 2021b). Temperate forests generally burn much less frequently, accumulate more biomass during fire intervals and then burn with much greater intensity (Tran et al. 2020). Although bushfires release significant amounts of CO2 from burning vegetation, in most instances, that carbon is sequestered again into biomass. In 2019, for example, the net emissions due to bushfires on forested lands, which is the main driver of variability in emissions from those areas, was ↓0.4 megatonnes of CO2 equivalent (Mt CO2-e) (DISER 2021d:6). There is evidence that the extent of high-severity burnt temperate forests in southern Australia has increased through time, and this is having a homogenising influence on those ecosystems (Tran et al. 2020), which reduces their biodiversity carrying capacity (see Vegetation condition).
Following an extended period of drought, the extent and severity of the 2019–20 bushfires across eastern Australia (reported in DAWE 2020j) resulted in an unprecedented level of CO2 emissions. Shiraishi & Hirata (2021) estimated the annual CO2 emissions from March 2019 to February 2020 to be 806 ± 69.7 Mt CO2 per year, which is equivalent to 1.5 times Australia’s greenhouse gas emissions (in CO2-e) in 2017. They concluded that the coincidence of lower-than-average precipitation and fires in high-biomass-density areas were major contributing factors. Nearly 7 million hectares of Australia’s temperate forests were burned in 2020 (see Figure 6.4 in DISER 2021h).
Carbon below the ground in soils
Soil stores 3 times more carbon than either the atmosphere or terrestrial vegetation (Delgado-Baquerizo et al. 2017) and makes up two-thirds of the terrestrial carbon pool (Jackson et al. 2017). This carbon storage is critical for stabilising the level of CO2 in the atmosphere and limiting climate change (Jackson et al. 2017). Australian soil carbon stocks were estimated to be about 28 megatonnes (Mt) of carbon in 2016 (Table A6.5c in DISER 2021c).
Carbon below ground in soils is in 2 forms – organic and inorganic. Organic carbon derives from decaying plant matter, soil organisms and microbes, and its volume is strongly influenced by land management practices over relatively short periods of time (Gougoulias et al. 2014). Inorganic carbon, such as relatively stable carbonate minerals, derives from weathering of parent material or reaction with the atmosphere, particularly in desert environments, over geological timescales (Monger 2014).
Sequestering soil carbon in its organic forms can be influenced by changed agricultural practices (e.g. reduced tillage, erosion control, groundcover), and is known to improve soil health with benefits for food production and human wellbeing (Paustian et al. 2019). For example, based on a study across a range of sites in New South Wales, Rabbi et al. (2014) found that local climates and soil type explained more than 40% of the variation in carbon stock fractions, whereas land use and management accounted for less than 10%, which limits the potential positive effects of land-use reversion on carbon storage (Rabbi et al. 2015). Future climatic shifts in temperature and rainfall are also expected to have a major influence on the amount of soil organic carbon that can be stored. For example, under a high-emissions scenario, Grace et al. (2006) estimated that by 2021 the Australian continent would become a source of atmospheric CO2, resulting in a net reduction of 6.4% (518 Mt) in topsoil carbon compared with no climate change, which would be partially balanced by an increase in net primary productivity.
Human activities associated with intensified land use over the past 250 years have substantially depleted soil organic carbon (Figure 21) and contributed to global climate change. Australia is listed as number 3 (behind China and the United States) among countries with the highest loss of soil organic carbon (Sanderman et al. 2017a). Land use accounts for around 10% of soil organic carbon loss to a depth of 200 centimetres (Table 2). This loss accords with the level of influence of land use and management over soil carbon stores (Rabbi et al. 2014). These depleted lands (mainly used for cropping and grazing) present opportunities for regenerative agriculture to sequester carbon in soil (see Land use).
Viscarra Rossel et al. (2019b) investigated the composition, environmental controls and vulnerability of Australia’s soil organic carbon in 3 fractions: particulate, humus and biologically resistant (Table 3). The largest carbon stock was found to occur in the humus fraction (20.8 Gt), followed by the resistant fraction (9.7 Gt) and the particulate fraction (5.4 Gt). The total carbon stock (assuming no land use) estimated for Australia is 35.9 Gt, for which the lower 95% confidence limit of 23.6 Gt (Table 3) is in broad agreement with that of Sanderman et al. (2017a) (Table 2).
Soils such as Dermosols and Kurosols, which are favoured for agricultural production, are among the most vulnerable to soil organic carbon loss (Table 4) because they have higher proportions of the readily decomposable particulate fraction. Regions of southern Australia, where both the more easily decomposable particulate fraction and total carbon stocks are higher, were found to be the most vulnerable to soil organic carbon loss (Viscarra Rossel et al. (2019b); Figure 22). These are the most productive regions of Australia where favourable climates, human settlements and agricultural land uses predominate (see Figure 26 in Changes at the state and territory level). Areas of higher vulnerability to soil organic carbon loss (mean vulnerability of 0.5 in Table 5) also correspond with locations of extensive clearing of native vegetation in the intensive land-use zone (see Land clearing).
In contrast, the relatively stable landscapes of central and north-western Australia and semi-arid parts of eastern Australia (where clay is more abundant) were found to be the least vulnerable to soil organic carbon loss due to higher humus fractions, likely held in stable organomineral complexes (Jones & Singh 2014). These include the extensive desert and xeric shrub biome, and the tropical and subtropical grasslands, savannas and shrublands biome (Viscarra Rossel et al. 2019b, Viscarra Rossel et al. 2019a), which predominantly occupy the relatively natural zone (Table 5).
Goldstein et al. (2020) estimated the amount of carbon in ecosystems that is vulnerable to release upon changes in land use and which, once lost, cannot be recovered to avoid dangerous climate change impacts. Ecosystems with high densities of irrecoverable carbon include peatlands, mangroves, old-growth forests and marshes. For example, Maxwell et al. (2019b) found that when forgone carbon sequestration, selective logging, edge effects and defaunation were included in a full carbon accounting, the impact from loss of intact tropical forest increased by a factor of 6. Following a change in land use (e.g. clearing followed by agricultural development), the immediate loss of biomass and soil carbon is typically 20–70% (Sanderman et al. 2009, Luo et al. 2014). These losses can be recovered to some extent, depending on the carbon sequestration rate, but a portion would be irrecoverable (for an explanation, see Figure 1 in Goldstein et al. 2020).
Regenerative agricultural practices, along with other measures to restore landscape function, can help accelerate recovery of soil carbon while maintaining the economic benefits of production (Griscom et al. 2017). These practices are among a wide range of agricultural management options for sequestering and retaining soil carbon – for example, see the 2016 state of the environment report (Metcalfe & Bui 2017), which was based on the earlier work of Sanderman et al. (2009), Sanderman (2012), and updated in McKenzie et al. (2017).
Overall, Australian above- and below-ground carbon stocks continue to decline in areas subjected to clearing and modification of the landscape, land-use intensification, and unsustainable agricultural practices. Below-ground carbon stocks are declining in areas of vulnerable soils with intensive use. There are local areas of recovery where some changed agricultural practices are demonstrating capacity to sequester carbon over time, or where forest regrowth is sustained.
Related to United Nations Sustainable Development Goal target 13.2
Many above-ground ecosystems have been extensively cleared or replaced, and a large proportion of above-ground carbon has been lost. Many soils are vulnerable to carbon loss and have varying levels of intensity of agricultural land use. There has been minimal recovery (e.g. through sustained regrowth, replanting, regenerative land use).
Many of the native ecosystems have reduced biomass leading to depletion of above-ground carbon stocks. Some soils are vulnerable due to lower levels of groundcover associated with agricultural land use. Land-use intensification and conversion, as well as increasingly frequent and intense fires, continue to reduce standing biomass.
Many of the native ecosystems remain intact, but are vulnerable to potential reductions in biomass from ongoing small-scale land-use change and extreme events attributed to climate change. Fewer vulnerable soils are found in relatively natural areas and generally have less intensive land use.