Soil carbon – a climate change silver bullet?

Thanks to Bev Trowbridge for her contribution to this article.

Organic matter performs wonders in the soil. It stores water but facilitates drainage, binds soil particles, but enables easy root penetration and so much more. With concerns about climate change getting more organic matter in the soil is a great way to sequester carbon.

According to this Food and Agriculture Organisation (FAO) video, despite the way we have depleted the soils, an estimated 1417 gigatonnes of carbon remain in the first metre of soil across the globe, almost double the amount of CO2 in the atmosphere. The answer, or at least a part of the answer, lies in the soil, and increasing its carbon content.

In 2016 the FAO published the Voluntary Guidelines for Sustainable Soil ManagementThe document identifies the problem of soil degradation and the compelling reasons to implement solutions.

A loss of soil organic carbon (SOC) due to inappropriate land use or the use of poor soil management or cropping practices can cause a decline in soil quality and soil structure, and increase soil erosion, potentially leading to emissions of carbon into the atmosphere. On the other hand, appropriate land use and soil management can lead to increased SOC and improved soil quality that can partially mitigate the rise of atmospheric CO2. (page 8)

So the answer, or at least a part of the answer, lies in the soil, and increasing its carbon content. The document outlines the characteristics of sustainable soil management (page 3):

  1. Minimal rates of soil erosion by water and wind;
  2. The soil structure is not degraded (e.g. soil compaction) and provides a stable physical context for movement of air, water, and heat, as well as root growth;
  3. Sufficient surface cover (e.g. from growing plants, plant residues, etc.) is present to protect the soil;
  4. The store of soil organic matter is stable or increasing and ideally close to the optimal level for the local environment;
  5. Availability and flows of nutrients are appropriate to maintain or improve soil fertility and productivity, and to reduce their losses to the environment;
  6. Soil salinization, sodification and alkalinization are minimal;
  7. Water (e.g. from precipitation and supplementary water sources such as irrigation) is efficiently infiltrated and stored to meet the requirements of plants and ensure the drainage of any excess;
  8. Contaminants are below toxic levels, i.e. those which would cause harm to plants, animals, humans and the environment;
  9. Soil biodiversity provides a full range of biological functions;
  10. The soil management systems for producing food, feed, fuel, timber, and fibre rely on optimized and safe use of inputs; and
  11. Soil sealing is minimized through responsible land use planning.

More on soil carbon

Organic matter in the soil is transformed into humus, which is the magic/essential ingredient upon which the health of the soil depends.
However, it isn’t just the humus that determines the carbon content of the soil. It was long thought to be by degradation of organic matter that added carbon to the soil, through the process of decomposition from the surface. However, Australian soil scientist, Christine Jones’ research has demonstrated that the main pathway for carbon entering the soil is through the much more dynamic process called the Liquid Carbon Pathway, whereby liquid carbon in the form of sugars are passed from the phototsynthetic activity of the green plants via their roots to the symbiotic fungi called Mychorrizal fungi that in a healthy soil live in close species-specific association to all plant roots. The liquid carbon is passed down into the soil via the fungal hyphae in exhange for minerals that the plant needs from the subsoil, and is used in the growth & decay of many of the underground organisms in the soil ecosystem. In this way a healthy fully-functioning soil is constantly adding carbon to the soil in a net-gain process. This is why regenerative/biological/organic farms which restore the soil microbiology to full functioning can GROW TOPSOIL, rather than losing it to erosion as on most NZ farms.
Here is Christine Jones talking about carbon pathways.


Applying this to Northland

Our climate oscillates dramatically each year between super-saturated soils and then very high soil moisture deficit soils. This is a very tough environment for the microbiology of soils and they need management practices designed not to be damaging in these two climate extremes. Land-uses and practices must be designed not to cause biological or structural damage, or to leave soil without sufficient plant cover to combat these weather events and maintain soil integrity and function.
Northland is the beef capital of New Zealand on land that is often too steep to support this land-use. However, even on the land that is suitable for this farming type, its suitability varies greatly throught this annual climate cycle. The benefits of high annual rainfall  and high temperatures for winter grass growth enabling winter grazing are actually threats to the Northland soil, as most of this rain falls in the winter months and the abundance of grass means that heavy stock are left outdoors to graze in conditions that are highly unsuitable for soil health. Northland has some of the most compacted and poorly functioning soils in NZ due to this combination of factors and the economic imperatives of grazing large numbers of heavy cattle through the winter. Pugging compaction caused by cattle feet is the major cause of loss of soil function, loss of soil carbon, loss of productivity, increased GHC emissions, and soil erosion in this region. Added to this are the further damaging practices of adding nitrogen in the form of urea, and also superphosphates, as well as blanket spraying of thistles by air with herbicides.

A good starting point to use a document with the authority of the FAO behind it. The challenge is then to apply it to local conditions. Many factors combine to determine how best to apply soil management strategies, the nature and texture of the soil, its degree of degradation, crops grown, topography and socio-cultural factors to name a few. The best strategies will build on local successes.

These links are to videos of people achieving success in diverse locations.

Other resources

A virtuous circle

There are plenty of vicious circles in climate dynamics – where actions interact and compound to increase climate extremes. For example, as ice melts, its reflective ability is lost and the darker ocean water is exposed to more heat. As temperatures rise, fire becomes more  common, incinerating biomass and carbon, and exposing the soil, leading to erosion and further loss of organic matter.

While increasing soil organic matter sequesters carbon, it also exponentially improves soil quality to create greater biomass, and store more water and nutrients. Resilient soils support our resilience.

Here is Albert Bates talking about biochar, #72 Drawdown solution.