Soil organic carbon (SOC) is an increasingly used term within discussions about the benefit SOC can have on soil, its productivity and also offsetting carbon emissions via increasing SOC with the potential of receiving carbon credits. For the agricultural industry, both these areas are relevant to increasing efficiency and sustainability which makes it worth understanding further.
SOC, as its name suggests, is the organic carbon within the soil and excludes carbon sources such as carbonates or carbon dioxide. SOC includes obvious material such as fresh residues, the carbon in soil microbes (whether dead or alive), the sugars and amino acids secreted by plant roots, the plant roots and the long-term permanent sources of carbon such as humic substances (Figure 1).
Some of these sources of carbon are transient, (meaning that they don’t stay in the soil for extended periods of time and turn over at a very rapid rate), but they still provide a key benefit in their role as the building blocks of more stable carbon. Stable carbon are those materials that become permanent sources of SOC, i.e. humic substances and organic carbon bound to soil particles. These permanent carbon sources represent the factors which will eventually count towards carbon credits.
Increasing SOC is not a short-term project and takes a number of years simply due to the fact that while SOC can be increased via the addition of organic inputs, it is simultaneously decreased during the breakdown of the organic material through leaching and the release of carbon dioxide.
So how do we increase SOC, in light of the fact that the carbon within the soil is in continual flux?
In very basic terms, in order to increase SOC the amount of carbon entering the soil has to be greater than the amount leaving the soil. However, due to the continual turnover of carbon and the degradation of organic material it is not just a matter of adding organic material to the soil. For every tonne of organic material added approximately 80% of this will be lost over a period of 3 to 4 years in carbon dioxide alone (not including any leaching) (Figure 2).
This is because microbes use the organic material as a source of energy to grow and multiply, and subsequently release carbon dioxide, resulting in high levels of carbon loss from the initial inputs. In essence, only about 20% of the organic material applied will end up as a more permanent source of carbon.
While this may seem like a great start to increasing the underlying SOC levels, the huge weight of bulk soil makes for some interesting calculations of how much SOC actually increases by this method. A fairly simple calculation shows that; 1 hectare of soil to a depth of 10cm weighs approximately 1200 tonnes, by adding 10 tonnes of organic matter of which 60% is carbon this will only increase the SOC by 0.36%. Again this seems like a good approach until you consider that after a few years the action of soil microbes will have whittled this down to ~0.07% of residual permanent SOC. This would indicate that there is limited practical scope to directly increase SOC by applying large amounts of low-quality organic matter. Furthermore, many of these low-grade organic inputs have their own issues (discussed later). This calculation leads to the importance of a distinction between high-quality and low-quality organic material.
High quality organic material will contribute carbon to the soil at the same time as stimulating the soils biological activity. And while it may seem counter intuitive to stimulate soil microbes which will only increase and speed up the breakdown process of organic inputs, what bacteria and fungi do is convert some of the transient SOC into more permanent stable SOC.
This direct action of soil microbes is not the only way in which they influence the accumulation of SOC. The number and diversity of soil bacteria and fungi in the soil are significantly increased by the addition of organic matter. This results in increased nutrient cycling and nutrient availability for plants and microbes, also helping to protect plants from pests and disease and directly stimulating plant growth by the production of plant growth hormones.
It is the above properties of soil microbes which creates an increase in plant and root growth, allowing plants to secrete more organic compounds into the soil. This results in further microbial activity which will have a positive long-term impact on SOC. The soil microbes turn some of the root material of dead plants into stable SOC and as bacteria and fungi die their cellular structures are degraded by still other microbes.
Once the material cannot degrade any further it essentially becomes part of the permanent SOC pool and can be considered as humic substance or resistant organic matter. Until recently permeant SOC such as humic substances were considered to be of solely of plant origin, however, more recently it has been shown that a significant amount of humic substances are in fact residual microbial components (Figure 3).
This knowledge further supports the theory that the choice of organic material added to soil can significantly affect the rate and deposition of long-term SOC. There are several critical nutrients that need to be present for degradation to occur, the most obvious being nitrogen. Manganese is a less well-known nutrient required for degradation, but it is critical for a range of enzymes which degrade cellulose and without sufficient levels of manganese, degradation is slowed significantly.
Organic material inputs can be assessed by their structural complexity, a factor that impacts the time it takes them to degrade. Wood wastes, stubble or straw which are an example of a structurally complex combination of carbon, oxygen and hydrogen. These materials will take a long time to degrade but should result in more humate material in the soil. Greenwaste contains elements such as nitrogen, potassium, phosphorous and some micronutrients and is less complex, therefore break down more quickly, but leave less residual humates.
The key is the combination of the organic material input, with nutrients such as those in organic based fertilisers that will expedite degradation.
A commercially viable solution is to apply a biological based fertiliser, such as Bounce Back from Neutrog Australia. Bounce Back has high enough nitrogen levels to avoid nitrogen drawdown and sufficient manganese to accelerate the breakdown of material, resulting in a quicker accumulation of permanent SOC.
The solution needs to form part of a multifaceted long-term soil improvement strategy to increase SOC levels including farming practices such as reduced tilling, leaving stubble (and not burning it) and utilising cover crops. The expectation of these practices should be that the impact will be an incremental increase in SOC over an extended period.
Increasing the SOC by just 1%, will trap approximately 20 tonnes of carbon per hectare in the top 10 cm of soil. The additional benefit of increasing the SOC is significantly improving the water holding capacity of the soil, increasing the presence of humic substances acting as soil pH buffers and retaining micronutrients which are normally leached through the soil profile. All elements that combine to improve overall production efficiencies.