Soil carbon levels are ultimately the result of a balance between processes and inputs that add carbon to soil and those that remove it. To increase soil carbon you need to reduce processes that lead to loss of soil carbon carbon loss and increase processes that lead to carbon gain. This sounds simple and in some respects it is. If you want to increase soil carbon stop doing things that decrease soil carbon and start doing things that increase soil carbon. But what are the good and bad things?
The conventional view is that soil carbon is all derived from the surface and matches the surface yield. This top down thinking says that vegetation and litter at the surface is progressively incorporated into the soil matrix where it is gradually transformed by biological processes into more stable forms such as humus. Along the way most of the carbon is consumed by bacteria and readmitted as CO2.
The top down view of soil carbon completely misses the importance root exudates in the formation and stabilisation of soil carbon. In grasslands, a large portion of the living biomass is in the roots. The above ground parts of the plants produce carbohydrates by photosynthesis. Some of this is used to grow the plants and roots but a significant portion of the production is directed below ground and is exuded by the roots to feed both bacteria and fungi that in turn provide minerals and nutrients back to the roots and plants. By feeding the soil biology a plant greatly expands the resources it can access by its roots alone.
Roots and root exudates play a vital role in soil carbon formation. Grasses continuously lose or slough off roots as part of normal growth and in response to grazing. This is particularly the case for finer root hairs. Understandably, roots are well adapted to being in the soil and are resistant to biological decay so tend to remain in the soil long after being separated from the plant. Exudates also play an important role in helping to promote stable aggregates in the soil structure. These aggregates can range from the microscopic scale to many mm. In the right conditions these aggregates will bind to and protect root and other organic matter isolating them from further biological decay. The aggregates also contribute to overall soil structure and water infiltration and retention.
It is also important to understand what destroys soil carbon. Soil carbon can be lost when there is a lack of carbon input, soil temperature increases, soil is ploughed and or compacted or when substances are added that disrupt soil biology including causing an increase in biological activity that results in a breakdown of previously stable soil carbon. Over grazing, bare ground, ploughing, bare fallowing, some forms and methods of fertiliser, pesticides and herbicide use, excess concentrations of nitrogen from manure and urine can all contribute to a loss of soil carbon.
It is not at all surprising that grasses and grazers have co-evolved over millions of years. Many grasses have evolved to respond to grazing leading to the term “Obligate Grazophils” meaning that many grasses need to be grazed in order to be more productive. Research has shown that there is a sweet spot between no grazing and overgrazing that maximizes net primary production.
It is not just the amount but the timing and intensity of grazing that matters. When a plant is grazed it rapidly adjusts its physiology in response. Resources are diverted to regrowing new tillers and roots, growth stops and the plant can even shed roots to balance the overall needs of the plant. It is only after the plant has regenerated that resources are put back into the roots.
The plants response also depends on the season and the availability of water and the intensity of grazing. If excess plant material is removed the plant will struggle to recover. If the plant is repeatedly cropped before it recovers, it never gets to the stage that the roots start to regrow and without strong roots the plant cannot access enough water and nutrients to reach its potential.
Timing is also important. When a plant is dormant grazing has less of a direct impact because the plant is not actively growing. The extent of grazing will however affect the plants capacity to respond to rain and the growing season. Conversely if a plant is repeatedly cropped during the growing season, it never has the opportunity to develop strong roots.
Finally it is also important to consider what happens in the absence of grazing. Here the above ground leaves ages and die or become less efficient. The dead matter restricts sunlight from reaching new growth. The plant stops growing and becomes unpalatable and difficult to digest.
The impact of different grazing approaches is clearly demonstrated in the following picture.
With this knowledge comes a simple formula for improving soil carbon in a grazing system. Good grazing results in good grass, which results in good roots, which builds soil carbon.
Resource Consulting Services (RCS) is the leading provider of knowledge and grazing systems. Courses and consulting on grazing and business management can accessed through the following contact details:
1800 356 004