The structural role of flowing carbon

Everybody knows the presence of carbon is important for soil structure, however, it also has a structural role. This comes back to carbon being the main building block of pastures.

By structural role, I mean the way paddocks are more water efficient and have increased capacity to capture resources when plants are physically present.


Root response to different grazing pressure

The wick effect

Carbon has a structural role as part of plant the roots. Roots, which are 45% carbon, act as “wicks” to take water down through the soil profile, especially important with harder soils.

This is achieved by water travelling down beside roots. Better managed plants, with more extensive root systems, distribute water faster through the soil and to greater depth.

Resting pastures for a short period after rain increases carbon flows for root construction, which in turn improves the infiltration of future rain.

With perennial grasses, the roots grow and die back. This results in cavities where roots have previously been. These cavities are very effective for water infiltration.

Roots help secure nitrogen

Where pastures are not fertilised, the nitrogen present in the soil originates from the earth’s atmosphere, where it makes up about 78% of the air. Each hectare of the earth is covered by about 84,000 tonnes of nitrogen in the atmosphere.

Nitrogen comes in with rain, so infiltration determines plant available nitrogen from this source. In the tall grass savannahs of the Northern Territory of Australia, each year rain brings in 2 kg of nitrogen per hectare. To put this into perspective in this area, at the end of the growing season there is 10-15 kg of nitrogen in standing grass, which is low compared with more fertile areas.  

There seems to be a trend towards more intense rainfall events, which is why improving water infiltration is even more important. 

Lifting wind & shading

Have you ever given any thought to why clothes on the line dry quicker on a windy day than on a still one? Going back to basics, moisture is released from the wet clothes into the drier air around them. When the immediate air beside the clothes is saturated, no more moisture is released from the clothes. The wind dries the clothes because it blows away the moist air and replaces it with drier air, which then absorbs more moisture from the clothes.

The clothes line principle is central to another structural role of carbon. Standing plants lift wind off the soil surface which slows the drying of soil.   

Shading by plants is an important component of water use efficiency, as it slows evaporation and makes more water available to plants.

Maintaining the population of soil microbes over time

A lot of soil microbes are like perennial grasses, in that they have to go into dormancy when conditions are not favourable.

When the soil dries quicker than it should, this does not give soil microbes time to go through their normal process of going into dormancy and so they die.

The soil remaining dry for a long time during droughts is not an issue for dormant microbes. Microbes found in the tombs of Egypt have been reactivated.

Collecting dew

Another structural role of carbon is collecting dew in some environments. This relies on the surface area of grass being adequate.

German scientist, Dr Wihelm Ripl explains why plants are responsible for greater availability of water to the landscape, “Water vapour in the lower atmosphere close to vegetation, even without a single rain event, can be precipitated by leaves, needles and other structures with high surface to volume ratios. The result is a lowering of surface energy and a warming of these structures.”

In the case of frosts, for moisture to accumulate on the soil surface, there has to be enough moisture frozen then thawed as the sun rises. In an area suffering the driest winter in years, I observed that there was regularly moisture on the ground at the base of plants after the sun had risen. This phenomenon was only occurring where the grass was thick and tall. It was actually maintaining some green leaf at the base where it was protected from frost.

Conclusion

The structural role of carbon relies on the faster moving carbon residing in plants, which in turn reflects the management of carbon flows from what rain does arrive.

Management that leads to better water infiltration via increased roots and then slower drying of the soil surface due to wind being lifted, increases germination.

Roots play a very important role in Reef protection. Water has volume and if it gets together and gathers speed, then that is when it does damage by eroding soil and carrying nutrients into waterways. Getting more rainfall into the soil reduces this problem. Also, more standing pasture helps keep water apart and from gathering speed.

Next week’s discussion: “Financial analogy”

Alan Lauder

WHY CARBON FLOWS?,