By now it is pretty widely accepted that carbon plays an
important role in soil health. It improves the nutrient holding capacity,
nutrient cycling and water holding capacity of the soil. Although there are
other ways of increasing soil carbon, for example the application of organic
fertiliser and not tilling soils, the most effective way of building carbon
levels in the soil is through roots.
There are three main sources of carbon inputs to the soil
via plant roots:
- Root and shoot remains, which decompose and
break down after a plant dies.
- Organic substances, mostly root exudates, that
are released by roots during plant growth.
- Root hairs and fine roots which are shed during
The original source of carbon in the roots is
atmospheric carbon dioxide. Photosynthesis is a process which everyone has
heard of, and probably studied at school, but I don’t think we fully realise
and appreciate its uniqueness and value. Photosynthesis is a simple reaction
between carbon dioxide from the air and water from the soil, using sunlight as
an energy source and resulting in oxygen that is released back into the
atmosphere and carbohydrates being stored in the plant. Carbohydrates
predominantly consist of carbon, hydrogen and oxygen. In this way
photosynthesis transforms a gaseous form of carbon (carbon dioxide) into a
solid form (carbohydrates). Pasture plants have been found to move about 30-50%
of the carbons acquired through photosynthesis into the soil. Of this amount,
half can be found in plant roots, a third is released from root and microbial
respiration and the rest is incorporated into the soil microorganisms or
becomes soil carbon. The result of this whole process is that, on average,
pasture plants are able to move around 2200 kilograms of carbon per hectare per
year into the soil1.
The amount of photosynthesis taking place in pastures is
directly linked to the amount of growth. This is especially true when there is
a large amount of leaf surface area. Large leaf area equals greater
photosynthesis, which results in greater growth. The key in this process is
that the initial growth of a plant after grazing uses energy, nutrients and
carbohydrates from the roots, as there is no leaf surface area for
photosynthesis to take place. That means that for the initial period of growth
after grazing, there is not actually a transfer of carbohydrates into the soil,
it is actually opposite. This is the reason why grazing pastures at the correct
leaf stage is so important. Pasture grass must be left for a sufficient period
of time between grazing to ensure that more carbohydrates (carbon) are
transferred to the soil through photosynthesis, than were transferred from the
root during the initial growth phase.
Farm management practices should be geared towards
facilitating this process of transferring carbon from the atmosphere to the
soil. One of the most important mechanisms to doing this is good grazing
management, but this will be covered in a following blog.
1. Kuzyakov Y & Domanski G. 2000. Carbon input
by plants into the soil. Review. Journal of Plant Nutrition and Soil Science 163(4):421-431.