Soil: Carbon dynamics

2016

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

Management                                     Option        

SOC

benefita

Confidenceb

Justification

Shifts within an existing cropping or mixed system

Maximising efficiencies

  • Water use
  • Nutrient use

0/+

L

Yield and efficiency increases do not necessarily translate to increased SOC return to soil

Increased productivity

  • Irrigation
  • Fertilisation

0/+

L

Potential trade-off between increased SOC return to soil and increased organic matter decomposition rates

Irrigation can increase the rate of soil carbonate precipitation, but, depending on the source of calcium and bicarbonate, the net reaction can be an atmospheric carbon sink, a carbon source or carbon neutrality

Stubble management

  • Elimination of burning and grazing

+

M

Greater carbon return to soil should increase SOC stocks

Tillage

  • Reduce tillage
  • Direct drilling

0

0/+

M

M

Greater organic matter return to soil should increase SOC stocks

Reduced till has shown little SOC benefit

Direct drilling reduces erosion and destruction of soil structure, thus slowing decomposition rates; however, surface residues decompose with only minor contribution to SOC pool

Rotation

  • Elimination of fallow with cover crop
  • Increased ratio of fallow to crops
  • Pasture cropping

+

+/++

++

M

H

M

Losses continue during fallow without any new SOC inputs; cover crops mitigate this

Pastures generally return more SOC to soil than crops

Pasture cropping increases SOC return with the benefits of perennial grasses, such as water use throughout the year and increased below-ground allocation, but studies are lacking

Organic matter and other offsite additions

++/+++

H

Direct input of SOC (often in a more stable form) into soil; additional stimulation of plant productivity

Shifts within an existing pastoral system

Increased productivity

  • Irrigation
  • Fertilisation

0/+

L

Potential trade-off between increased SOC return to soil and increased organic matter decomposition rates

Irrigation can increase the risk of soil carbonate precipitation, but, depending on the source of calcium and bicarbonate, the net reaction can be an atmospheric carbon sink, a carbon source or carbon neutrality

Rotational Grazing

+

L

Increased productivity, including root turnover and incorporation of residues by trampling, but field experience is lacking

Shift to perennial species

++

M

Plants can use water throughout the year; increased below-ground allocation, but few studies to date

Shift to a different system

Conventional to organic farming system

0/+/++

L

Likely highly variable, depending on the specifics of the organic system (e.g. manuring, cover crops)

Cropping to pasture system

+/++

H

Annual production minus natural loss is now returned to soil; active management to replant native species often results in large carbon gains

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).

Box LAN10 Soil Carbon Research Program results

Queensland

  • There has been no increase in soil organic carbon (SOC) stocks in response to trash retention and no-till management at 4 sugar cane sites in Queensland, over the top 0.1 or 0.3 metres of the soil profile. Such practices are thus unlikely to lead to significant carbon sequestration for the purpose of greenhouse gas abatement (Page et al. 2014a).
  • No-till management, stubble retention and nitrogen fertiliser addition were not able to increase SOC stocks under the climatic conditions found throughout Queensland. To increase SOC stocks in this region, a period of carbon input in the form of a pasture ley is likely to be required (Page et al. 2014b).
  • In tropical and subtropical grazing lands, SOC stocks are strongly influenced by temperature, vapour pressure deficit, standing pasture dry matter, soil type and dominant grass species; the effect of grazing management is less clear (Allen et al. 2014).

New South Wales

  • Total pre-clearing SOC stocks amounted to 4.21 petagrams (Pg) in the top 30 centimetres, which, compared with a current stock estimate of 3.68 Pg, suggests a total SOC loss of 12.6 per cent over the entire state. The extent of SOC decline in both absolute and relative terms was found to be highly dependent on the climate, parent material and land-use regime, reaching a maximum of 50 per cent relative loss in cooler (moist) conditions over mafic parent materials under regular cropping use (Gray et al. 2016).
  • SOC levels in the surface 0.1 metres are 17–28 per cent higher under minimum tillage than under conventional tillage (McLeod et al. 2013).
  • No differences in total SOC stock or soil carbon fractions were observed between cropped sites treated with organic amendments and those treated with chemical fertiliser. Relative abundance and microbial community structure, measured on a subset of the cropping sites, showed a higher bacteria:fungi ratio in chemically fertilised sites and suggested enhanced mineralisation of organic matter under conventional management.
  • There was some evidence of increased soil carbon stock under rotational compared with continuous grazing, but the difference was not statistically significant (Cowie et al. 2014).
  • Sowing perennial tropical grasses improved soil organic matter (including carbon) in the surface 0.1 metre for both cropping and grazing systems (Schwenke et al. 2014).

Victoria

  • Across Victoria, SOC content exhibits an extremely wide range (2–239 tonnes of carbon per hectare in the top 30 centimetres). Most of the variation is attributable to differences in climate, annual rainfall or vapour pressure deficit (i.e. humidity). Texture-related soil properties accounted for a small, additional amount of variation in SOC.
  • After accounting for climate, differences in SOC between management classes (continuous cropping, crop–pasture rotation, sheep or beef pasture, and dairy pasture) were small and often not significant. Management practices such as stubble retention, minimum cultivation, perennial pasture species, rotational grazing and fertiliser inputs were not significantly related to soil organic carbon stock. Across Victoria, there is a general hierarchy of influence on SOC stock: climate > soil properties > management class > management practices (Robertson et al. 2016).

Tasmania

  • Clay-rich soils contained the largest carbon stocks. Cropping sites had 29–35 per cent less SOC in surface soils (0–0.1 metres) than pasture sites, as well as greater bulk densities.
  • Rainfall, Australian Soil Classification order and land use were all strong explanatory variables for differences in SOC, soil carbon stock, total nitrogen and bulk density (Cotching et al. 2014).

South Australia

  • Differences in SOC between broadscale cropping and crop–pasture systems were limited. In the mid-north, variability in SOC stocks and fractions was high, and could not be explained by environmental or management variables. In the Eyre Peninsula, higher SOC concentrations were observed in the surface 0.1 metre of soils under cropping than under crop–pasture; the particulate organic carbon fraction accounted for most of this SOC and is unlikely to represent a long-term stable pool (Macdonald et al. 2014).

Western Australia

  • Although historical losses of soil organic matter associated with agricultural production are significant, soil type, climate and land use influence the potential for SOC storage in Western Australia. Modelling indicates that the greatest storage capacity is below the soil surface (i.e. below 0.1 metres) (Hoyle et al. 2014).
Metcalfe D, Bui E (2016). Land: Soil: Carbon dynamics. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/land/topic/soil-carbon-dynamics, DOI 10.4226/94/58b6585f94911