The carbon: nitrogen ratio

The carbon : nitrogen ratio (C:N ratio) of living things and their residues is essential for determining the outcomes of many processes. This ratio is always discussed in papers looking at landscape function and animal performance, yet it is rarely discussed in literature available to rural producers.

The first thing to understand about plants is that the carbon is fairly constant in the break-up, it is the nitrogen that varies. Most trees are about 50% carbon while grasses are about 45% carbon. Like all figures used in plant analysis, this is based on a dry weight.

As the plants are the start of the two food chains, above and below the ground, the C:N ratio of the plants will determine how efficient these other consumption processes are.

In plants, nitrogen is necessary for chlorophyll synthesis, and as part of the chlorophyll molecule, is involved in photosynthesis. It is chlorophyll that gives the plant its green colour. Lack of nitrogen and chlorophyll means that the plant will not utilise sunlight as an energy source to carry out its essential functions such as nutrient uptake. A badly managed pasture with low soil carbon levels, and hence lower nitrogen levels, struggles with carbon introduction following rain, due to a lack of nitrogen. Dysfunctional landscapes become increasingly infertile, because they have a low potential for capturing new resources.

Discussions often focus on how different plants have different nitrogen levels. However, it is also important to appreciate that the nitrogen levels vary within a plant. This is why animals, especially sheep, will select certain plant parts over others.

In the future, more attention will be placed on the C:N ratio of the diets of ruminant animals, as it influences the production of methane per kg of production.

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