The Carbon Storehouse
Soil organic matter is beneficial for nutrient retention and availability, soil structure improvement, erosion resistance, water infiltration and retention, and drainage characteristics. As such it is an important indicator of soil health.
With its key role in both carbon sequestration and in improving soil condition, increasing the organic content of soils is beneficial for both climate change adaptation and mitigation. The potential store of carbon in soils is very large relative to anthropogenic greenhouse gas emissions and represents a potential long-term store of carbon, drawn down from the atmosphere via photosynthesis. Therefore, soils and soil management will undoubtedly play a significant role in future carbon offset or credit mechanisms. Natural resource management policies need to recognise the coupled benefits of soil carbon and invest strategically to maximise these benefits.
There is potential for confusion in the meaning of the various definitions of soil organic matter and soil carbon. The terms requiring definition include soil organic matter, soil organic carbon, soil inorganic carbon, soil organic carbon concentration and soil organic carbon stocks.
Soil Organic Matter
Soil organic matter is organic material produced by the decomposition of plant and other forms of organic material (largely particulate) by the action of soil biota and microbes.
It encompasses the complete mass of materials in the organic matter, including carbon, oxygen, hydrogen and the nutrients such as nitrogen, phosphorus, sulfur, potassium, calcium. and magnesium.
Soil Organic Carbon
Soil organic carbon is the carbon component of soil organic matter. In fact, most measurements of soil organic matter are actually derived from a value for soil organic carbon, because it is possible to measure organic carbon in soils relatively easily and effectively. Soil organic matter is about 58% carbon, so it is possible to estimate soil organic matter from soil organic carbon simply by multiplying by 1.72. However, to be precise about the percentage of C in soil organic matter, it can range from 40 to 58%, with a conversion factor ranging from 1.72 to 2.5, depending on the source and age of the OM. For general purposes, the value of 1.72 is used in most situations. However, it is essential to be clear about which value, whether soil organic carbon or soil organic matter, is being used in any report or discussion.
An important practical aspect of making reliable, repeatable measurements of soil organic carbon is that soil organic carbon is defined as organic carbon associated with the less than 2.0 mm mineral fraction of the soil. This component of the total soil organic matter is intimately associated with the functioning of soils. This means that organic materials in the soil, such as large pieces of root, leaves and stubble, are NOT included in the measurement of soil organic carbon. In fact, these need to be removed prior to measurement of soil organic carbon and should be accounted separately.
Soil Organic Carbon Concentration and Soil Organic Carbon Stocks
A clear distinction needs to be made between soil organic carbon concentrations or percentages in laboratory results and soil carbon stocks or soil carbon stores, which are used for assessing carbon credits.
Soil Organic Carbon Concentration
The soil organic carbon concentration is given in grams of organic carbon per 100 grams of oven dry soil. This is readily measured by taking a sample in the field at a specified depth range. A bulked sample is frequently used, as is a depth of 0 to 10 cm. Note that it is essential to have a consistent and defined depth for the results to be useful or meaningful. Generally, this measurement does not possess the statistical rigour or sampling requirements that are necessary for the measurement of soil organic carbon stocks. At the same time, it can be a useful indicator of soil condition.
Soil Organic Carbon Stocks
Soil carbon stocks are expressed as tonnes of organic carbon per hectare.
The soil carbon stocks are calculated from the soil organic carbon concentration, the soil bulk density and an area factor, to obtain the soil carbon stock to a specified depth, typically 0-30cm. As this measurement requires an estimate of the bulk density of the soil and in most cases a reliable estimate of the variability of the measurements, a more rigorous and complex sampling and analysis process is required. The estimate of soil organic carbon stocks for a field or paddock is a more costly and complex process than estimating the soil organic carbon concentration.
Changes in bulk density can complicate perceived changes in soil organic carbon stocks. To ensure reliable estimates of changes in soil organic carbon stocks it is essential to use the methodology of equivalent soil masses, as defined by the soil organic carbon methodology recommended by the Department of Environment (2014).
Other references to describe the need to use equivalent soil mass when estimating changes in soil carbon stocks over time are given below.
Dept of Env. (2014). Carbon Credits (Carbon Farming Initiative) (Sequestering Carbon in Soils in Grazing Systems Methodology Determination 2014. Methodology Determination under subsection 106(1) Carbon Credits (Carbon Farming Initiative Act 2011). Department of Environment. Australian Government.
Ellert, B. H. and Bettany, J. R. 1995. Calculation of organic matter and nutrients stored in soils under contrasting management regimes. Can. J. Soil Sci. 75: 529-538.
Murphy, B., Rawson, A., Ravenscroft, L., Rankin, M. & Millard, R. (2003). Paired site sampling for soil carbon estimation – New South Wales. Australian Greenhouse Office, Technical Report No 34, Canberra, Australia.
Wendt, JW and Hauser, S. (2013). An equivalent soil mass procedure for monitoring soil organic carbon in multiple soil layers. European Journal of Soil Science 64, 58 – 65.
Soil Inorganic Carbon
Significant amounts of inorganic carbon can occur in soils, especially in more arid areas and can include calcium carbonate as concretions, nodules or as diffuse carbonate, which is very common in some soils. They can also occur as dolomite or magnesium carbonate, in association with igneous parent materials such as basalts or with limestone. The inorganic carbon is not included in the soil organic carbon content and measures are required to ensure it is not included in any determination of the soil organic carbon levels.
Functions of Soil Organic Matter
Soil organic matter is critical for soil function through its influence on water holding capacity, structural stability, nutrient cycling, cation exchange capacity and soil buffering capacity. Improved management to increase soil organic matter will improve the long-term resilience and buffering capacity of the soil in the face of issues such as climate change and increased pressure from agriculture.
The conversion of plant material by microbes releases carbon dioxide, encourages microbial growth and diversity, and forms organic compounds that will form the humus which is the most resilient and most important fraction of the soil organic matter. An active and well-balanced microbial population and an adequate supply of nutrients are essential for the efficient conversion of plant and other organic materials into humus.
On the other hand, soil microbes decompose soil organic matter to release essential plant nutrients, including nitrogen (N) and phosphorus (P). This process, combined with the exposure of soil organic matter (particularly the humus fraction) by soil cultivation (disturbance) will reduce the soil organic matter level. This dilemma of building soil carbon, as opposed to organically deriving plant nutrition from organic matter decomposition, can be addressed with good land management practices. These include conservation cropping techniques and active plant growth, particularly by the inclusion of pasture rotations and cover cropping practices.
Generally, soil organic matter (the humus fraction) contains elements in the ratio of carbon (C) : nitrogen (N) : phosphorus (P) : sulfur (S) of 100:8:1:1. Hence, to form the humus fraction of soil organic matter, it is essential to have adequate quantities of N, P and S. The C:N ratio of humus is 10 to 12, and that of the microbial biomass 8 to 12, with fungi having a slightly higher ratio than bacteria. Some plant materials have a C:N ratio of 20 to 25 (especially legumes); when decomposed they are converted into compounds with a C:N ratio of 10 to 12, with the remaining carbon released as carbon dioxide in energy-releasing respiration reactions. If the C:N ratio of plant material is greater than 25, (wheat and other cereal straw have C:N ratios of about 80), the microbial population must utilise N from the soil to grow. This ties up nutrients, which are then unavailable for plants. However, over time these will be released as organic nutrients from the breakdown of organic matter by microbial activity.
Managing Soil Organic Carbon
Key management actions for improving and/or maintaining soil organic matter include maintaining and maximising biomass input by: ensuring adequate yields of crops and pastures and maintaining groundcover; by minimising soil disturbance and soil organic matter loss; and by maintaining a nutrient balance in the soil and a favourable chemical and physical environment essential for good biological activity.
Soil Organic Carbon
- The NSW Soil Knowledge Network recognises the critical importance of soil organic matter and soil carbon.
- It is important to understand the differences between the definitions of soil organic matter, soil carbon and soil carbon stocks.
- Increasing soil organic matter improves soil productivity and economic returns (e.g. reduced artificial fertiliser use) and reduces land degradation.
- There are potential benefits from carbon sequestration and trading but there may be physical or economic limitations which restrict potential carbon sequestration rates.
- It is easier to lose soil organic matter than it is to increase it.
- Over the long term, plant organic matter must be continually added in order to maintain or increase soil carbon. This is best achieved by photosynthesis of actively growing plants. At the same time, to reduce soil organic matter loss, soil disturbance should be minimised.
- Projects that aim to measure the effects of land management practices on soil organic matter and soil carbon must allow for differences in climate, soil type and landform, all of which influence soil organic carbon levels.
- Methods for measuring soil organic matter must be appropriate for the purpose. For example, measuring soil carbon to obtain payment for carbon credits requires a much more rigorous, appropriate and expensive process compared with measuring organic matter to assess soil condition or benchmarking to assess if there has been a general increase (eg. soil carbon concentration alone, Weil method, calico strips). Methods used must be economical and practical, to encourage adoption by land managers.
- While calcium (Ca) and magnesium (Mg) are essential for the growth of plant and microbial populations and are necessary for some biological reactions and possibly the stability of cell walls, the amounts required are small and readily supplied either from the plant material which the microbes decompose or from the soil itself. Most soils have sufficient Ca and Mg except for some coarse sands and strongly bleached A2 horizons.
Some attention has been given to the Ca:Mg ratio as an indicator of soil health and microbial activity, but evidence suggests most soil biological systems, both plants and microbial operate satisfactorily provided the ratio is in the range of 1 to 20 and maybe 1 to 30. Most soils are well within this range.
Further information: soil carbon sequestration