How Much Carbon can Soil Store

Recent interest in carbon sequestration has raised questions about how much organic carbon (OC) can be stored in soil. Increasing the amount of OC stored in soil may be one option for decreasing the atmospheric
concentration of carbon dioxide, a greenhouse gas.

Increasing the amount of OC stored in soil may also improve soil quality as OC contributes to many beneficial physical, chemical and biological processes in the soil ecosystem.

Making Sense of Biological Indicators

Biological indicators give information on living organisms in soil. Biological indicators of soil quality therefore measure dynamic soil properties, i.e. properties that change over time and/or with management. It is important to monitor biological indicators as they respond more quickly to changes in management or environment than physical and chemical indicators.

For most biological indicators, there is little evidence currently available which directly links the value of the indicators to productivity or, in some cases, the risk of adverse environmental impact. However, there is good evidence from field trials carried out on a range of soils in Australia of links between biological indicators and soil processes. These have been used to create guideline ranges for the biological indicators, similar to those used for the dynamic physical and chemical indicators.

Total Organic Carbon

The term organic is used to describe materials relating to or derived from living organisms. The amount of organic matter in a soil is often used as an indicator of the potential sustainability of a system. Soil organic matter plays a key role in nutrient cycling and can help improve soil structure.

Carbon makes up approximately 50% and nitrogen 0.5 to 10% (dependent on residue type) of the molecules in organic matter; some of which turns over rapidly (labile fraction) and is available to plants, whilst other more
recalcitrant forms contribute to the stable (passive, slow turnover fractions) organic pools. Soil micro-organisms mineralise organic matter to obtain carbon, nitrogen and other nutrients for their own metabolism and growth.

Organic Carbon Pools

Soil organic matter is made up of four major pools?plant residues, particulate organic carbon, humus carbon and recalcitrant organic carbon. These pools vary in their chemical composition, stage of decomposition and role in soil functioning and health. Management is capable of altering not only total organic carbon stocks, but the proportion of carbon present in these different pools. Knowledge of how carbon pools change in response to management provides valuable information on likely soil functioning and health.

Labile Carbon

Although significant amounts of organic carbon are present in soils, some of this is relatively inert. Soil organic matter is made up of different pools which vary in their turnover time or rate of decomposition. The labile pool which turns over relatively rapidly (< 5 years), results from the addition of fresh residues such as plant roots and living organisms, whilst resistant residues which are physically or chemically protected are slower to turn over (20-40 years). The protected humus and charcoal components make up the stable soil organic matter pool which can take hundreds to thousands of years to turnover

Microbial Biomass

Microbial biomass is an important indicator of soil health because it is closely related to nutrient release from crop residues. The microbial biomass consists mostly of bacteria and fungi, which decompose crop residues and organic matter in soil. This process releases nutrients, such as nitrogen (N), into the soil for plant uptake. About half the microbial biomass is located in the surface 10 cm of soil and most of the nutrient release also occurs here.

Microbial Biomass - Qld

The soil microbial biomass consists mostly of bacteria and fungi, which decompose crop residues and organic matter in soil. The microbial biomass typically makes up less than 5% of total soil organic matter, but it plays a very large role in a number of key soil functions, including nutrient release, the maintenance of good soil structure and the suppression of plant pathogens. Changes to the microbial biomass can also be an early indicator of changes in total soil organic carbon. Unlike total organic carbon, microbial biomass carbon responds quickly to management changes, and can often be measured before changes in total organic carbon are detected.
Biological activity in Queensland cropping soils is generally low compared to other land uses, such as pasture or native vegetation, due to the low amount of labile carbon present. Adopting management practices that increase soil carbon and thus soil microbial biomass increases nutrient cycling important for crop growth.

Benefits of Retaining Stubble

Historically, stubble has been burnt because it improves weed control and creates easier passage for seeding equipment. However, the practice of burning stubble has recently declined due to concerns about soil erosion and loss of soil organic matter.

Benefits of Retaining Stubble in Tasmania

Historically, stubble has been burnt because it improves weed control and creates easier passage for seeding equipment. However, the practice of burning stubble has recently declined due to concerns about soil erosion and loss of soil organic matter. Instead of being burnt, stubble is increasingly being retained, which has several advantages for soil fertility and productivity.

Benefits of Retaining Stubble in Qld

Historically, stubble has been burnt because it improves weed control and creates easier passage for seeding equipment. However, the practice of burning stubble has declined throughout Queensland cropping regions due to concerns about soil erosion and loss of soil organic matter. Instead of being burnt, stubble is now more commonly retained, which has several advantages for soil fertility and productivity. Some of these include a reduction in erosion risk, increased soil water content and improved biological functioning of the soil.

Biochar for Agronomic Improvement

CSIRO is leading collaborative research across Australia to analyse the properties and potential of a variety of biochars to improve soil health and sequester carbon. Two major projects, including one funded by the Grains Research and Development Corporation (GRDC) and another by the Australian Department of Agriculture, Fisheries and Forestry (DAFF), are helping to answer some of the key questions about the potential of biochar.

Carbon Storage - Albany Sand Plain

Sustainable management of soil, in particular carbon, is essential for the continued viability of Australian agriculture. Increasing the carbon retained in soil (also known as sequestration) improves soil quality and can also help to reduce atmospheric carbon dioxide. The Soil Carbon Research Program is working to identify land uses and management practices that growers can use to increase soil carbon storage and improve production in a changing climate.

Actual carbon storage in the 0?30 cm layer was less than attainable carbon storage predicted by modelling (figure 3, example for deep sand). Within the lower rainfall cropping zone the actual carbon storage on the deep sand was 29 t C/ha under continuous cropping (40% of attainable carbon) and 39 t C/ha under mixed cropping (50% of attainable carbon). In the higher rainfall zone actual carbon storage on the deep sand was 61 t C/ha under annual pasture (55% of attainable carbon) compared to 93 t C/ha under perennial pasture (77% of attainable carbon).

Carbon Storage - Esperance Sand Plain

Soils on the Esperance sand plain in Western Australia range from deep sands to shallow duplexes. Close to the coast, beef production dominates and some producers use perennial pasture (often kikuyu) to combat the ?autumn feed gap?. Perennial pastures have longer survival, deeper rooting and greater plant biomass during summer months. As a result, it has been proposed that they may store more soil carbon than annual pastures.
Soil carbon storage under perennial pastures was not greater than under annual pastures. Changing from annual pasture to perennial pasture has not increased soil carbon storage on these soils, and is likely due to the sandy nature of the soils. Soils with coarse texture have lower potential to store carbon than soils with more clay.

Rhizoctonia

Rhizoctonia is a disease affecting a wide range of crops and has become more prevalent throughout Western Australia in recent years following the introduction of minimum tillage practices. The previous practice of tillage
prior to seeding encouraged the breakdown of the fungus (Rhizoctonia solani) in the soil prior to emergence. Minimum tillage practices decrease the rate of organic matter breakdown, thereby providing a habitat for Rhizoctonia over summer. The disease affects most major crops to varying degrees, with barley being most susceptible and oat crops are least susceptible. Bare patch and root rot of cereals, and damping off and hypocotyl rot of oilseed and legumes are all caused by differing strains of R. solani.

Root Lesion Nematode

Root lesion nematodes (RLN) are microscopic worm-like animals that use a syringe-like "stylet" to extract nutrients from the roots of plant. Pratylenchus neglectus and P. thornei are the most common RLN species in Australia, although populations of P. teres and P. penetrans are also found in Western Australian soils where they have been found to cause significant yield losses. Overall, RLN’s affect all cropping regions of southern Australia, and are an increased risk in areas where minimum tillage has been adopted.

Root Lesion Nematode in Queensland

Root lesion nematodes (RLN) are microscopic worm-like animals that use a syringe-like ?stylet? to extract nutrients from the roots of plants (figure 1). Plant roots are damaged as RLN feed and reproduce inside plant roots. Pratylenchus thornei and P. neglectus are the most common RLN species in Australia. In the northern grain region P. thornei is the predominant RLN but P. neglectus is also present. These nematodes can be found deep in the soil profile (to 90 cm depth) and are found in a broad range of soil types – from heavy clays to sandy soils.

Cereal Cyst Nematode

Cereal Cyst Nematode (CCN, Heterodera avenae) is a pest of graminaceous crops worldwide. This nematode is a significant problem across eastern Australia, and is detected in the Northern and Central regions of Western Australia. CCN becomes more problematic in areas where intensive cereal cropping occurs. CCN will only infect, feed and develop on cereals and other grasses (particularly wild oat). Non-cereal crops will not host the nematode, so are useful in rotations to limit damage caused to cereals.

Take All Disease

Take-all is a soil borne disease of cereal crops and is most severe on wheat crops throughout southern Australia. In Western Australia the disease is caused by two variations of the Gaeumannomyces graminis fungus; G. graminis var. tritici (Ggt) and G. graminis var. avenae (Gga) and is most severe in the high rainfall areas of the agricultural region (ie. southern cropping regions and areas closer to the coast). Control of take-all is predominantly cultural and relies on practices which minimise carry-over of the disease from one cereal crop to the next.

Arbuscular Mycorrhiza's - S.A.

Arbuscular mycorrhizas (AM) are generally beneficial associations (symbioses) between plant roots and specialised soil fungi. The chances are that all the AM fungi in any particular soil will be able to colonise all the plants that are grown there. Only a few plants do not form AM symbioses. These include canola and other brassicas (cabbage family), lupins (all other crop and pasture legumes are AM), beet and spinach, otherwise virtually all crop species form these associations. High AM colonisation is an important indication of good soil health, although this is not always recognised.
The symbiosis is based on exchange of nutrients: the fungus increases the ability of host roots to take up nutrients while the plant provides the fungus with sugars. Importantly, AM fungi grow in the soil as well as inside roots and their fine threads (hyphae) extend and branch in soil pores (figure 1), increasing the amount of soil from which the host plants can extract nutrients. These threads also act to spread the symbiosis from plant to plant and to stabilise soil structure.