Soil organic carbon Q&A

This article aims to help producers understand how soil carbon is measured for carbon markets, what influences sequestration rates, and a general indication of how much and how quickly carbon could be added to soil.

What is soil organic carbon?

Soil organic carbon (SOC) is the carbon component (~58%) of soil organic matter. To be considered part of the SOC pool, the material must pass through a 2 mm sieve and thus does not include living roots or less decomposed, fresher residues not yet broken down to less than 2 mm in size.

 

What is soil organic matter?

Soil organic matter (SOM) is any living, dead or decaying plant or animal matter (including living roots and microorganisms). Organic matter is composed of carbon, hydrogen, oxygen and small amounts of essential plant nutrients.

What are the benefits of soil organic matter (SOM)?

Organic matter is critical to the biological function of soils, can help retain cations and water (particularly relevant for sandy soils), as well as improve soil structure (especially in finer textured clay soils). It is an essential energy source for soil biota that underpin key ecosystem services such as decomposition and nutrient cycling. SOM is a significant source of nitrogen that will decline as SOM declines. At sufficient levels, SOM can help buffer changes in soil pH and bind aluminium to limit toxicity.

 

Why do we talk about SOC and not SOM?

It is difficult to measure SOM directly. Instead, we measure the carbon component, which consistently makes up about 58% of SOM and is able to be measured accurately. Soil organic matter levels can then be estimated by multiplying SOC by 1.72.

 

How do carbon markets estimate SOC for a given area?

Carbon trading frameworks require that SOC (%) is reported in tonnes of carbon per hectare, to a depth of at least 30 cm. To do this we need to measure bulk density (grams of soil per cubic centimetre – g/cm³) for each required depth increment. The carbon stock per hectare is calculated using bulk density multiplied by SOC (%) for each depth layer.

For example:

1. If bulk density for 0-10 cm = 1.3 g/cm³, then this equates to 1300 tonnes of soil per hectare to a depth of 10 centimetres.
2. If the SOC is 1.5%, this equates to 19.5 tonnes of carbon per hectare in the top 10 centimetres [1300 tonnes x (1.5/100)].

SOC is converted to CO₂ equivalents (CO₂e) to trade in carbon markets by multiplying tonnes per hectare by 3.67 (in this example above, 19.5 tonnes of carbon equates to 71.6 CO₂e).

Note that soil carbon projects must be registered with the Clean Energy Regulator before sampling and measuring baseline soil carbon levels (see Figure 1).

 

What influences the rate of change in SOC?

In general, to build SOC, the rate of organic residues added to soil (primarily determined by net primary productivity) must exceed losses from microbial decomposition, erosion and product removal.

 

How does climate influence changes in SOC?

Climate influences both the amount of organic matter that can be grown and added to the soil and the rate at which it is lost. Rainfall is one of the primary determinants of annual plant production. Low-rainfall environments typically grow less plant biomass compared to higher rainfall areas, limiting the amount of organic matter that can be added to the soil. Temperature can also limit plant production.

Climate also affects microbial activity and therefore how quickly it is transformed into carbon dioxide or more stable forms of carbon. At lower temperatures, microbial activity and decomposition of SOM is lower and, where longer periods of plant growth are supported, it is possible to build SOM. At higher temperatures where soil moisture is present, rates of decomposition are relatively higher and it can be much harder to build SOC. Note, soil moisture is required for microbial activity.

Over the past decade, annual rainfall has declined, and in some situations, this may reduce plant growth and inputs of organic residues. Higher frequency of summer rainfall events also means that warmer temperatures are combining with moist soils more often, supporting higher microbial activity and potentially faster decomposition of SOM, making it harder to build SOC.

 

How does soil type influence SOC?

Soils higher in clay content that can more readily form aggregates generally retain more SOC compared to sandy soils because organic matter is better protected from microbial decomposition through either physical dislocation (i.e. within aggregates, coated by clay platelets), or in chemical complexes.

 

How much SOM is decomposed by microbes?

Microbes are soil microorganisms less than 0.2 mm in size and include bacteria, fungi, archaea and protozoa.  When new organic matter is added to soil, the majority is broken down and fragmented by larger soil biota and weathering processes, and is subsequently decomposed or mineralised by microorganisms. During decomposition, microbes release nutrients surplus to their requirements along with carbon dioxide. A relatively small percentage of the carbon used by microbes is transformed to the more stable form of SOM called humus.

In Western Australia, microorganisms commonly release up to 85% of the carbon in fresh organic residues as CO₂, with the remainder entering the soil. Balanced against this new input of carbon, microbes continuously break down older SOM (humus) at a rate of 2-3% every year. Mineralisation of humus by microbes is a significant source of plant nutrients.

Soil pH significantly affects how much carbon is released as CO₂. When soil pH is below 5.5 (in CaCl₂), a larger proportion of the carbon consumed by microbes is respired because they are more stressed in low pH environments. Above a soil pH of 5.5, microbes retain more carbon or are more carbon-use-efficient.  So, a greater amount of carbon enters the soil.

 

How can management influence SOM?

As described above, to increase SOM, the rate at which plant and animal residues are added to the soil must exceed the rate at which these are lost through microbial decomposition, erosion or removal. Therefore, management practices that generate adequate amounts of high-quality residues are critical to rebuilding and sustaining soil organic matter.

 

What practices might maintain or increase SOM?

Given the benefits of SOM, it is critical to at least maintain levels if not increase them. Suggested practices include: managing soil constraints that limit production such as soil compaction, non-wetting soils, soil acidity and nutrient deficiencies; managing pests, disease and weeds; managing grazing to optimise photosynthetic capacity and root growth; sowing productive varieties with high root vigour; maximising pasture growth and quality (mix of grass and legumes); increasing frequency of pasture phases in cropping rotations; irrigation; increasing the growing season with perennials or summer crops where appropriate; and applying off-site organic amendments such as compost (where economically feasible to do so). It is critical to note, that where the application of amendments or higher amounts of organic input are stopped, gains in SOM are likely to be lost.

A critical and sometimes forgotten aspect of maintaining SOM is to limit loss pathways. Erosion events and excessive soil disturbance (e.g. tillage) can rapidly decrease SOM, whereas recovery using the practices above is slow (decadal) – particularly for more stable fractions like humus. Minimising erosion by increasing/maintaining groundcover; reducing stubble burns; avoiding bare fallow and reducing tillage are critical.

 

Can soil constraints limit microbial activity and SOM breakdown?

Soil constraints including waterlogging (low oxygen availability), water repellent soils (low soil moisture) or soil acidity (aluminium and/or hydrogen ion toxicity) can inhibit biological activity and decomposition of organic matter, resulting in a slower rate of decomposition and over time it is possible to accumulate SOC. This indicates why SOC levels, if considered in isolation to other soil properties and the ability of a soil to support a given land use, are not always the best indicator of a healthy soil.

Soil constraints can also limit net primary productivity. The removal of these constraints to plant growth will enable the plant to maximise its photosynthetic potential and optimise net primary productivity.

 

What rates of SOM change should we expect in the South West Ag region?

A study by Sanderman et al. (2010) analysed published evidence of changes in SOC resulting from changes in management. They suggest that while improved cropping practices such as enhanced pasture rotations, no-till farming or stubble retention can add 0.2-0.3 tonnes of carbon per hectare per year compared to conventional systems, lower rates often occur. However, because conventional systems are often in decline, this comparative improvement still resulted in an overall decline over time in some situations. The authors also suggest pasture improvements such as fertilisation, liming, irrigation and improved pasture varieties resulted in relative gains of 0.1 to 0.3 tonnes of carbon per hectare per year, with gains of up to 0.6 tonnes when cultivated land is converted to permanent pasture. However, confidence in these figures was much lower.

In Western Australia, Hoyle et al. (2016) concluded that there is potential for further accumulation of SOC (5-45% capacity) in deeper soil layers (10-30 cm), with annual accumulation rates of between 0.07 and 0.27 tonnes of carbon per hectare per year to a depth of 30 cm achievable in high rainfall regions (Hoyle et al. 2013). In lower rainfall environments these rates are likely to be much lower. Broader data across Australia suggests modest gains, or at least the prevention of further loss in SOC are possible with adoption of best management practices.

Meyer et al. (2015) modelled a change in land use from cereal cropping to grazed and concluded that increases in SOC were typically faster when the baseline soil carbon level was relatively low for a particular soil type and rainfall zone. In their modelling, relatively low SOC soils increased at 0.3-0.48 tonnes of carbon per hectare per year to a depth of 30 cm, while soils with relatively high SOC increased at rates of 0.02 to 0.23 tonnes of carbon per hectare per year.

Rapid changes in SOC are unlikely in the south-west of Western Australia. While there are examples of systems able to accumulate carbon, quite often reports of large increases can be attributed to sampling error and/or both temporal and spatial variability in soil carbon. Soils with sufficient clay usually have the greatest potential to store carbon (Creamer et al. 2016).

Thanks to Frances Hoyle from SoilsWest who co-authored this article. For more information, see https://www.agric.wa.gov.au/soil-carbon/soil-organic-matter-frequently-asked-questions-faqs

References and information sources

Creamer C, Jones D, Baldock J, Rui Y, Murphy D, Hoyle F & Farrell M (2016). Edaphic properties and microbial biomass size outweigh importance of nutrient stoichiometry for transformation and fate of labile carbon. Soil Biology & Biochemistry 103, 201-212.

Hoyle F, O’Leary R, Murphy D (2016). Climate factors dominate the quantity and distribution of soil organic carbon in a dryland agricultural systems. Scientific Reports 6, doi:10.1038/srep31468 (Aug 2016).

Hoyle FC, D’Antuono M, Overheu T, Murphy DV (2013) Capacity for increasing soil organic carbon stocks in dryland agricultural systems. Soil Research, 51, 657-667.

Hoyle F, Murphy D (2018). Soil Quality: 3 Soil Organic Matter. UWA School of Agriculture and Environment. https://books.apple.com/au/book/soil-quality-3-soil-organic-matter/id1444338744

 

Hoyle, F (2013). Managing Soil Organic Matter: A Practical Guide. Grains Research and Development Corporation. https://grdc.com.au/resources-and-publications/all-publications/publications/2013/07/grdc-guide-managingsoilorganicmatter

 

Meyer R, Cullen B, Johnson I, Eckard R (2015). Process modelling to assess the sequestration and productivity benefits of soil carbon for pasture. Agriculture, Ecosystems and Environment, 213, pp. 272-280.

 

Sanderman J, Farquharson R, Baldock J (2010). Soil Carbon Sequestration Potential: A review for Australian agriculture; report prepared for Department of Climate Change and Energy Efficiency; CSIRO. https://publications.csiro.au/rpr/pub?pid=csiro:EP10121