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Soil Organic Carbon

Soil Organic Carbon
A reward for good land stewardship

  • Soil organic carbon (SOC) enhances the chemical, physical and biological functioning of soils and is a critical component in the resilience of soils to stress.
  • Loss of SOC is a major threat to agriculture due to having a major influence on soil condition.
  • SOC levels are influenced by climate, land use, soil type and plant residue management.
  • Adoption of management practices that enhance plant productivity and residue return to the soil will contribute to increasing soil organic carbon levels.

The decline in fertility attributed to diminishing concentrations of soil organic carbon (SOC) is a major threat to the productivity and sustainability of our agro-ecosystems.  While conventional broad-acre land management practices tend to result in the loss of SOC, economically viable changes to land management may reverse this trend and allow some soils to store much larger quantities of carbon. For example  adoption of no-till technologies,  incorporating perennial pasture species and modifying grazing management to maintain pasture cover..

What is soil organic carbon?

SOC is a complex and heterogeneous mixture of materials that play a critical role in a variety of soil processes, thereby having a major impact on soil condition.  SOC is the measurable component of soil organic matter (DAFWA, 2015). The mixture that contains this carbon (organic matter) acts as a repository of nutrients, provides energy for soil organisms and helps to bind soil particles together.  These functions enhance numerous soil properties such as nutrient supply and retention, biological activity and diversity, soil structure, water-holding capacity, and resilience to erosion.

The amount of SOC at any given time results from the balance between carbon inputs and carbon losses and the potential maximum quantity of organic carbon a soil can hold. Carbon inputs are determined by the amount of plant residue added to the soil.  Any management practice that enhances plant productivity and residue return to the soil enhances carbon inputs.  This is why ameliorating constraints to productivity, such as soil acidity, compaction and infertility, are beneficial investments for both enhanced profits and soil condition. 

Conversely, SOC can be lost rapidly under intensive cropping systems due to the reduced biomass production of crops versus pastures, but also due to the removal of a large portion of biomass in grain and/or straw.  Ultimately it is economic considerations that will direct the choice of land use and while lengthy cropping phases may initially rapidly draw down SOC, a return to highly productive pasture phases can reverse this trend over time.

biological functions

Diagram 1. Functions of soil organic matter. The black arrows represent the various classes of functions and the grey arrows indicate the interactions which can occur between the classes (Baldock and Skjemstad 1999).

NOTE: For the purpose of this case study, diagrams describing soil organic matter have been used to represent the fractions of soil organic carbon. 

What influences it?

SOC levels are influenced by climate, land use, soil type and plant residue management and is  concentrated in the top few centremetres of soil. Generally speaking, of the organic materials returned to the soil in a given year, anywhere between 50 and 85 per cent of the mass of this material is lost to the atmosphere as carbon dioxide in the first year.  While land management affects how close a soil comes to its potential, the potential varies between soils due to their clay content and depth of soil.  The enhanced fertility, surface area and physical protection from clay particles ensure that the threshold will be smaller for a light textured soil than one with a heavy texture.

SOC is highly variable,  spatially and temporally and requires many  yearsto quantify the effects of management treatments.  . Furthermore SOC concentrations change slowly and are subject to seasonal variability due to differences in inputs and outputs of carbon to the soil.

What are the different types?

SOC exists in several distinct chemical and physical fractions and is generally divided into three compounds (NSW DPI, 2015):  

  1. Particulate Organic Carbon consists of fresh residues and living organisms, or material that is available to soil organisms for decomposition. Soil organisms break down particulate matter to create humus, which is the final product of the decaying process (it will break down no further).
  2. Humus is important for binding soil particles together. It improves the water and nutrient holding capacity of soils, and these are essential for plant growth. Humus stores or sequesters carbon for decades, or even centuries.
  3. Resistant Organic Carbon (Charcoal) is the result of incomplete burning of plant material or fossil fuels. It is believed to be biologically and chemically unreactive compared with other soil organic matter components. This means that the carbon stays locked in the charcoal in the soil and isn't readily released or taken up by soil organisms.

The particulate organic carbon is readily decomposed (months to years), and the resistant organic carbon accumulates very slowly (decades). The only way to enhance soil organic carbon over the long term is to increase the proportion of humus material. With current research focused on quantifying a labile organic matter pool such as the particulate organic carbon fraction described above, it is hoped that this more dynamic carbon pool can be used as an early indicator of future changes in SOC in response to changed landuses / management practices.

living organic matter

Diagram 3. The vast majority of soil organic matter is dead or decaying with living organisms making up less than 15 % of the soil organic matter pool (DAFWA, 2015)

How to improve it?

SOC enhances the chemical, physical and biological functioning of soils and is a critical component in the resilience of soils to stress.  Therefore, the adoption of management options to increase SOC such as: the amelioration of soil constraints, enhanced plant productivity through optimized fertilization, more productive perennial and annual pasture species and enhanced residue retention, should be promoted within the context of retaining both a profitable and viable farming systems.

Research

Perennial pastures are generally expected to store more soil carbon than annual pastures due to their extensive root system which persists all year round. As part of the Climate Action on Farms  project South Coast NRM, in conjunction with the University of Western Australia, conducted research on a south coast farm to determine if soil carbon increased in paddocks planted to kikuyu (perennial pasture) opposed to an annual pasture and also if cropping into the kikuyu affected soil carbon stores.

Below is a summary of the research trial undertaken and the results.

Trial Aim

To investigate:

  1. soil carbon stores under kikuyu grazing systems compared with an annual grazing system; and
  2. how cropping into a kikuyu pasture affects soil carbon.

What we did

Compared total soil carbon under different farming systems within the same farm:

  1. Annual grazing system
  2. Kikuyu grazing system (15 years)
  3. Crop converted from kikuyu paddock (15 years)

How we did it

Through soil testing consistent with the CSIRO’s Soil Carbon Research Program (SCRP) methodology. Randomly distributed 25 x 25m quadrats were used to sample the soil profile from 0 to 30 cm to measure total soil carbon.

What we found

annual pasture graph

Figure 1. Mean of total carbon (0-30 cm) in different farming systems relating to the different fractions of soil carbon.

  • Total soil carbon is higher in kikuyu grazing system compared to annual grazing systems.
  • Cropping for up to two years into a kikuyu grazing system doesn’t change the total soil carbon.

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