Building soil carbon for productivity and implications for carbon accounting

| Date: 13 Jun 2008

Kris Broos and Jeff Baldock

CSIRO Land and Water, Adelaide, SA

 

Take home messages

 

Composition of soil organic carbon

·       Soil organic carbon is composed of a wide range of different materials with different chemical and physical properties and different extents of decomposition.

 

Roles of organic carbon/organic matter in soil

·       Soil organic matter contributes to a variety of biological, chemical and physical properties of soils.
·       Chemical – cation exchange, pH buffering, reduces effects of sodicity.
·       Physical – water retention, soil structural stability, soil temperature.
·       Biological – energy for microbes, provision of nutrients and resiliency.
·       Each fraction of soil organic carbon contributes differently to various soil properties.

 

Calculating changes in soil organic carbon content

·         Soil carbon content represents the balance between inputs and outputs.
·         Values are required for the depth, bulk density and carbon content of the soil layer you are interested in to determine how much carbon is present.
·         If management changes induce changes in bulk density – these must be accounted for in the sampling/calculations.
·         Corrections for inorganic C are required.
·         Changes in soil carbon content are slow and typically require at least five years to be detectable.
·         Simulation models can be used to predict the likely outcomes of management practices on soil carbon content.

 

$$ from sequestration – fact or fiction?

  • Optimising crop productivity will maximise carbon inputs and soil organic carbon content.
  • At current prices, it is hard to justify modifying management practices for the sole purpose of selling carbon credits.
 
 

Soil Organic Carbon: what is it?

Soil organic carbon is a complex and heterogeneous mixture of materials. These materials vary in their physical size, chemical composition, degree of interaction with soil minerals and extent of decomposition. Although determining the impact of management practices on soil organic carbon contents is important, it does not tell us anything about the type of organic carbon present. For example, is the organic carbon dominated by pieces of plant residue or more recalcitrant charcoal? It is therefore important to determine the composition of soil organic carbon to gain an appreciation for the implications of management practices and changes in organic carbon content on soil productivity.

We now recognise four different types of soil organic carbon:

  • Crop residues – shoot and root residues >2 mm residing on and in soil.
  • Particulate organic carbon – individual pieces of plant debris that are smaller than 2 mm but larger than 0.053 mm.
  • Humus – decomposed materials less than 0.053 mm that are dominated by molecules stuck to soil minerals.
  • Recalcitrant organic carbon – dominated by pieces of charcoal.

 

Functions of organic carbon/organic matter in soil

 

Organic carbon/organic matter contributes to a variety of functions in soils. These functions can be broadly classified into three types: biological, chemical and physical (Figure 1). Strong interactions (represented by the grey arrows) often exist between these different functions. For example, the biological function of providing energy that drives microbial activity also results in improved structural stability and creates organic materials that can contribute to cation exchange and pH buffering. 

 
Figure 1. Functions performed by organic matter present in soils.
 
 
What determines soil organic carbon content?
 
The amount of carbon in a soil results from the balance between inputs (plant residues) and losses (microbial decomposition and associated mineralisation). In Figure 2 the bucket represents the amount of carbon a soil could potentially hold. This amount will vary with factors such as soil clay content, soil depth, and bulk density and is not influenced by management. The bucket will be smaller for a sand than a clay soil.
 
Inputs are controlled by the type and amount of plant residue added to the soil. Any practice that enhances productivity and the return of plant residues (shoots and roots) to the soil opens the input tap. For example, appropriate use of fertilisers to maximise productivity also maximise returns of organic residues to the soil. However, an upper limit of input exists in Australian dryland agriculture because of the limitation that the availability of water places on potential plant productivity.
 
Losses of carbon from soil result from decomposition and conversion of carbon in plant residues and soil organic materials into carbon dioxide. Processes that accelerate decomposition open the losses tap further. The content of organic carbon in a soil therefore results from the balance between carbon inputs and losses over many years.
 
Figure 2. Inputs and losses define soil organic carbon content. 
 
 
 
How much organic matter can be retained in soil?
 
In Figure 3 the size of the bucket in Figure 2 is represented by the bar labelled “potential sequestration” which gives the maximum organic carbon content that could be attained for a soil where no limitation on inputs exist (SOCpotential). The potential amount of plant material that can be produced at any given location is limited by environmental conditions (limiting factors) that may be beyond the control of a farmer. In Australian dryland agriculture, the availability of water provides such a limitation. Because this places an upper limit on plant production and thus inputs, it also restricts soil organic carbon to a level indicated by the bar labelled “attainable sequestration” (SOCattainable). The value of SOCattainable is the realistically best case scenario for any production system. To achieve SOCattainable, no constraints to productivity can exist. However, reductions in productivity due to a series of reducing factors (e.g. low nutrient availability, weed growth, disease, subsoil constraints, etc.) can reduce the amount of plant residue returned to the soil to values lower than optimum. This further reduces soil organic carbon content to the level indicated by the bar labelled “existing sequestration” (SOCactual).
 
 
Figure 3. Levels of soil organic carbon (SOC) for in a particular soil.
 
Optimising agricultural management will allow SOC contents to move from SOCactual values towards SOCattainable but it is not possible to move beyond this point due to the restrictions on plant inputs induced by water availability or what ever other factor may be placing an upper limit on plant productivity. The only way to further increase soil organic carbon is to add an external source of carbon (compost, waste residues, etc.) on a regular basis.
 
 
Calculating changes in soil organic carbon content
 
The amount of organic carbon found in a soil can be calculated using values for the depth (cm) of the soil layer of interest, the soil bulk density (g/cm3) and the soil carbon content (%) (Equation ). Using Equation indicates that a 20 cm layer of soil having a bulk density of 1.2 g/cm3 and a carbon content of 1.2% contains approximately 29 Tonnes of C/ha.
 
       
 
Suggestions have been put forward that altering management practices can increase soil organic carbon content from 2% to 4% in five years. Is this really possible?
 
If we use the same bulk density as above (1.2 g/cm3) and restrict our calculations to the top 10 cm of soil where organic carbon is most easily increased, at 2% carbon the soil would contain 24 tonnes C/ha. At 4% carbon the same soil layer would contain 48 tonnes C/ha. This indicates that 24 tonnes of C/ha would have to be added to the soil. Since plant residues contain approximately 45%C this would equate roughly to 50 tonnes/ha of dry matter (DM). If this increase was to occur over five years, then an additional 10 tonnes DM/ha above that currently being added would be required if no decomposition occurs. Since we know that at least 50% of the added plant residues will decompose, annual additions of approximately 20 tonnes DM/ha above that currently being added would be required to achieve an increase in soil organic carbon content from 2% to 4% in five years. Under dryland conditions typical of the Australian cereal belt, increases in returns of dry matter of this magnitude are unlikely and thus it is hard to substantiate such changes in C content. However, in specific locations where rainfall may not be used efficiently to produce agricultural crops/pastures (particularly regions with significant amounts of summer rainfall and where annual crops are being produced) significant increases in crop production and residue returns are possible by modifying existing management practices. Conversion of annual to perennial pastures and altering grazing practices from set stocking to rotational grazing may enhance plant dry matter production and increase soil carbon content.
 
 
Predicting the amount of organic carbon that can be present in a soil
 
Soil organic carbon content changes very slowly. When this fact is considered along with the annual variability in rainfall normally experienced at any given location, measurements of soil organic carbon over several decades may be required to accurately define the effects of particular management treatments on soil organic carbon contents. We have used a soil carbon model (RothC) to predict the likely SOCactual values that would be obtained under wheat production using average climatic conditions and retaining all crop stubble. At each location the water limited grain yield was calculated using the French-Schultz approach. To define the potential long term soil carbon content (equilibrium soil carbon content), 85% of this water limited grain yield was used along with a harvest index of 0.45 and a root:shoot ratio of 0.5 to calculate the crop residue addition rate including roots. The equilibrium soil C contents (tC/ha) predicted for the 0-30 cm layer at each location are presented in Table 1. Estimates of the associated carbon content in the 0-10cm soil layer are also presented in Table 1. It should be noted that in these modelling analyses a constant clay content of 15% was used at all sites. If actual clay contents are lower, the equilibrium C content will decrease and if actual clay contents are higher, the equilibrium C content will increase (Table 2) because clay can protect organic carbon from decomposition. 
 
Table 1: Equilibrium soil organic carbon content (SOCactual) predicted using the RothC soil carbon cycling model for three regions in SA with a different climate type. All soils were assumed to have equal clay content (15%) and bulk density (1.4 g/cm3).
 
 
Clare
Roseworthy
Waikerie
Growing season rain (mm)
491
338
170
French-Shultz slope (kg grain/mm)
20
20
20
French-Shultz intercept (mm)
180
110
80
Water limited potential grain yield (tonnes/ha)
6.2
4.6
1.8
 
 
 
 
Average productivity (85%)
 
 
 
Grain yield (tonnes/ha)
5.3
3.9
1.5
Total shoot dry matter (tonnes/ha)
11.7
8.6
3.4
 
 
 
 
Equilibrium soil carbon content
 
 
 
Modelled amount of C in 0-30 cm soil layer (tC/ha)
98
78
41
Estimated %C in the 0-10 cm soil layer
3.5
2.8
1.5
 
 
Table 2: Equilibrium 0-30 cm soil organic carbon content (tC/ha) predicted using the RothC soil carbon cycling model for different soil clay contents under continuous wheat production at Waikerie, Roseworthy and Clare SA.
 
Soil Clay content (%)
Clare
Roseworthy
Waikerie
5
81
65
35
15
98
78
41
30
108
93.
46
 
 
In Figure 4 the estimated changes in soil organic carbon content (%) of the 0-10 cm layer that occur with different levels of wheat production (grain yield) are presented. Results indicate that a sustained productivity of about 4 tonnes/ha/year of wheat grain yield is required to maintain the equilibrium soil carbon content at Roseworthy (Figure 4a). To double soil carbon content from 3% to 6% would require a sustained production of wheat grain of 8 tonnes/ha/yr for approximately 200 years. For soils with a constant clay content (15%) but different climates (e.g. Clare, Roseworthy or Waikerie), the model predicts large differences in the equilibrium soil carbon content under a sustained production of 4 tonnes/ha/year of the wheat grain (Figure 4b). Whereas this yield will maintain the current soil carbon content in Roseworthy as stated above, the same yield will decrease soil carbon in Clare by almost 1% in the top 10 cm and increase soil carbon in Waikerie by about 2% in the top 10 cm over a 500 year period. The large difference in the behaviour of Waikerie and Clare soils results from the influence that climate exerts on the rate of decomposition of the added crop residues. The drier climatic conditions at Waikerie result in slower decomposition rates and allow carbon to accumulate; while at Clare, the wetter conditions result in higher decomposition rates and a gradual loss of carbon. It is important to consider that it is very unlikely to produce 4 t/ha/year of wheat grain at Waikerie and thus the model results presented in Figure 4b are not realistic and are presented only to show the influence that climate can have on the rates of decomposition. 
 
 
Figure 4. Changes in soil organic carbon content predicted using the RothC soil carbon cycling model for different levels of average wheat grain yield in Roseworthy (a). Part (b) shows the different behaviour of soils (Waikerie, Roseworthy and Clare) with a different climate at a constant wheat grain yield of 4 tonnes/ha and 15% clay. 
 
$$ from sequestration – fact or fiction?
 
There is no doubt that soils could potentially hold more carbon. The challenge is to be able to do this while still maintaining an economically viable farm enterprise. Some potential options include:
·         enhancing the proportion of perennial vegetation in pastures or conversion of paddocks that continually give negative returns to perennial vegetation,
·         increased retention of crop residues, reduced stocking rates and increased use of green manure crops to return more plant material to the soil,
·         optimise farm management inputs to maximise water use efficiency and thus maximise the return of crop residues to soil.
 
With current prices of <$20 per tonne of sequestered carbon and the slow potential rates of soil carbon change, it will be hard to economically justify modifying management practices for the purpose of selling carbon credits alone. At this stage, carbon credits should be considered as a secondary benefit that may be realised whilst attempting to enhance soil productivity by building soil carbon content. 
 
Contact:                      Kris Broos
Ph:                               08 8303 8664
Email:                          kris.broos@csiro.au
 
GRDC Project Codes             CSO00029, CSO00030