The cycle of soil organic carbon

Explaining how to build soil carbon, while at the same time promoting soil biology that respires carbon back into the atmosphere, is not a simple task.

However, it was made simple by consultant David Hardwick from Soil Land Food who presented at the recent Soils Alive event in the Lower Blackwood area.

David is an incredibly engaging presenter who employs fun activities like soil judging and role-playing. But within this simple role-playing was the more complex theme of cycling nutrients found in soil organic matter and the consequences for soil carbon.

Soil organic matter (SOM) is any living or dead plant or animal material in the soil. It is composed of soil organic carbon, nitrogen, phosphorus and typically all the other essential plant nutrients. SOM comes in different forms, with some more easily and quickly decomposed than others.

David explained how SOM is decomposed by fungi and bacteria that are then eaten by bigger organisms such as protozoa and nematodes, which are in turn eaten by mites and springtails for example.

These interactions along the food chain results in nutrients like nitrogen being mineralised and released into plant available forms. But what happens to the carbon?

For easily decomposed forms of SOM (e.g. particulate organic matter), most of the carbon is respired back into the atmosphere as carbon dioxide. However, some of the carbon is converted into more complex organic molecules (e.g. from castings for example) that make humus, a form of organic matter that is more resistant to degradation and therefore persists in the soil for longer (only 1-3% of the humus fraction is mineralised each year).

Humus has many beneficial traits including water holding capacity, cation exchange capacity and moderating changes in soil pH.

A key factor affecting the decomposition of SOM is the rate of soil respiration, which could indicate that the microbial community is very active, stressed, or the size of the microbial community is large.

A very high level of respiration and microbial activity is not necessarily desirable because it may mean we are losing soil organic matter faster than we can replace with crop residues and the like.

Some key factors that increase respiration are high soil temperature (when moist), soil disturbance, soil acidity (<5.5 in CaCl2) and an excess of soluble nutrients.

On the other hand, if microbial activity and soil respiration is low, which can occur in waterlogged or highly acidic soils (e.g. <4.5 in CacCl2), nutrient cycling may be reduced, inhibiting plant growth and potentially resulting in a build-up of SOM.

So, to build soil carbon, we could try to limit losses by reducing soil disturbance and excessive nutrient application and maintaining soil pH above 5.5. Limiting soil erosion by maintaining groundcover can also reduce losses.

On the other side of the coin to losses from respiration and erosion are gains from organic inputs such as from crop or pasture roots, above-ground biomass and manure. Low rates of organic inputs make it harder to keep up with losses. So, we need to grow and allocate biomass back into the soil rather than exporting it all as produce. This means promoting pasture and root growth by addressing constraints to growth, not grazing too hard too often and perhaps leaving more stubble behind.

 

In conclusion, we want organic matter decomposition in our soils because it provides plants with nutrients and feeds soil biology. But we must accept that this function will result in a significant loss of carbon as carbon dioxide. We may be able to balance these competing aims by maintaining a higher rate of organic inputs compared to losses of SOM, and where possible, influence the rate of loss by understanding how our management may help to moderate respiration rates.

For more information, see:

• Soil organic matter – frequently asked questions. https://www.agric.wa.gov.au/soil-carbon/soil-organic-matter-frequently-asked-questions-faqs
• Soil respiration. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_051573.pdf
• The soil carbon dilemma: Shall we hoard it or lose it? https://www.sciencedirect.com/science/article/abs/pii/S0038071705003731

The workshop was jointly funded through the Australian Government’s Future Drought Fund, the Western Australian Government’s State NRM Program, the Australian Government’s National Landcare Program, in partnership with South West Catchments Council, and by the Lower Blackwood Land Conservation District Committee.