Soil organic carbon

My career as a Plant Pathologist has primarily required me to work on soil-borne (or stubble-borne) pathogens, such as Verticillium dahliae, Pyrenophora tritici-repentis, Blackspot disease complex (Mycosphaerella pinodes, Phoma medicaginis var. pinodella, Phoma koolunga and Ascochyta pisi) and a number of Fusarium species. In my research on soil microbiology and systems, I have become quite interested in the role of 'Soil Organic Matter' (SOM) and 'Soil organic carbon' (SOC) in Microbial ecology and nutrient cycling.

What are SOM and SOC?

'Soil Organic Matter' (SOM) refers to the fraction of soil which is organic in origin and usually refers to larger particulates such as, leaves, bark, branches, dead or decaying insects, animals and microbes. 'Soil organic carbon' (SOC) refers to the carbon fraction of the SOM and generally refers to the smallest fraction of carbon-based molecules. Soil organic carbon can also be decomposed organic matter, such as humus, which has not leached from the soil via microbial respiration (CO2), erosion or other processes.

Why is carbon important?

Carbon can readily form covalent bonds with positive (cation) and negatively (anion) charged elements and molecules. This makes Carbon-based molecular structures great at binding and filtering labile nutrients, toxins and metals.

Cation exchange Capacity

Carbon (or SOC) is correlated to the Cation Exchange Capacity (CEC) in soils. CEC is the ability for the soil (under a neutral pH) to bind cations such as:

• Potassium (K⁺)
• Ammonium (NH₄⁺)
• Sodium (Na⁺)
• Zinc (Zn⁺⁺)
• Manganese (Mn⁺⁺)
• Calcium (Ca⁺⁺)
• Magnesium (Mg⁺⁺)
• Iron (Fe⁺)

Anions are not able to be bound as easily in soils and are readily leached through water runoff.

ie.

• Nitrate (NO₃^-)
• Phosphate (HPO₄^--)
• Boron (BO₃^----)
• Molybdenum (MoO₄^--)
• Chlorine (Cl^-)
• Sulphate (SO₄^-)

Increasing SOC with SOM 

Mulching is the most obvious and easiest method to introduce SOC into the soil. But what is the best type of mulch to add?

Sugarcane mulch/baggase

Sugar cane mulch is a popular and cheap mulch which is most often used in veggie-patches and home gardens. It is light in density and colour, thus provides a great insulative layer to prevent moisture loss. However, it has a low lignin content of around ~10%. Other grass or monocot mulches, such as wheat straw, will have around the same lignin contents 9 - 20%.

Woodchip

Woodchip mulches like Cypress or Hoop pine mulch have a greater density and also contain much higher lignin contents and thus take longer to decompose. Cypress mulch contains a lignin content of 29%; Teak, 26-28%. Woodchips, from pine trees or Gymnosperms, have higher lignin contents 25 - 35% when compared to Angiosperms 15-28% (Flowering plants: eg. Eucalypts, Oaks and Elms).

While Woodchip minimises moisture loss to an extent, it is probably less effective than the same volume of sugarcane mulch.

Bark fines

Bark fines are another type of mulch similar to Woodchips, but contains subtle difference where lignin contents can be as high as 43%. They also tend to be less dense than Woodchips thus providing greater insulation and soil drainage. However, they tend to be more hydrophobic which may counteract it's potential to hold water.

Why is lignin content important in mulch?

Lignins are a class of molecules with a complex molecular structure which is highly resistant to microbial decomposition. When lignins are able to be broken down, they form smaller molecules called polyphenol.

So what are polyphenols? Polyphenols are molecules resistant to decomposition due to their antimicrobial properties and therefore increase carbon sequestration. One example is Melanin, a molecule produced in fungi and human skin. In humans, melanin is an important sleep regulator and protects the skin from UV radiation. In fungi and soils, it is one of the numerous variations of polyphenols produced by organic decay. Polyphenols have been associated with increased carbon sequestration and building soil humus.

Prepping my veggie patch with soil organic matter.

In 2017, when I joined my local community garden and was able to break ground on my own garden patch, I was keen to inject as much SOC as possible to condition the soil.

The soil profile was 20cm of topsoil with 20cm of compacted clay, and alluvial clay under the compaction. I was keen to increase drainage through the addition of soil organic matter (SOM).


It must have seemed as if I was burying a body as in the heat of summer I dug a hole almost a meter deep so I could fill it with free mulch I found through Gumtree. Drainage has improved, however, SOM applications are still needed 3-6 times a year through mulching.

In hindsight this was probably not the best way to improve my soil; but more on that another day.

Regenerative agriculture is anchored in the goal of sequestering more soil carbon.

Mulching is a great way to increase soil carbon and is the main method for sequestering carbon in regenerative agriculture. However, mulching methods in agriculture are substantially different from the home garden. Retaining stubble and growing cover-crops to be mown or rolled, aim to increase soil carbon, reduce moisture loss and minimise soil disturbance.

Ultimately this aims to recover previous losses in soil carbon to the soil type capacity. This process is long-term, as you can imagine most carbon from this process is lost via respiration before it forms the stabile humic fraction of soils. ~10% of a compost carbon fraction (not 10% of the overall weight), which has already undergone a significant amount of microbial respiration, is estimated to become humus.

There is a need for soil organic carbon to be preserved and actively maintained to reap the benefits of soil fertility, lower soil carbon dioxide emissions and the range of important biotic interactions in the soil. Soil health and the carbon cycle within the soil is important for everyone; be it gardener, farmer and other.