CARBON: FROM AIR TO THE SOIL - Dryland Pasture

vs 3 – 30 Oct 20

The basic question

How does Carbon get from a gas in the air (where too much is bad) into solid forms in the soil, where it does a lot of good? The secret lies in sunlight, green plants and the countless micro-organisms in the soil. The energy from the sun drives the process. Sunlight is highly reliable and predictable. It is also free.

Green plants are the only realistic way to extract carbon from the air.

2Plant leaves produce sugars by using the energy of sunlight to combine water from the soil with carbon dioxide (CO2) from the air. The plant itself uses some of these sugars. The remaining sugars are transported to the roots. These use some to function and grow and exude some, which feeds the soil microbiota. This consists of billions of organisms, from microscopic ones to earthworms, that improve soil structure. Microbiota is crucial for the health, growth and optimal functioning of the plant.

3.Roots can create compounds from the sugars that get desirable responses from the microbiota, a form of ‘communication’ between plants and microbiota.  The amount of root exudate varies during a plant’s life – the blue sections below.

4  Fig 1-Seedling stage Fig 2-Near full growth Fig 3-Early flowering Fig 4-Full life cycle

Microbiota

Much of the soil microbiota depends on plants. Some root exudate feeds bacteria and other life forms that recycle dead plant material back into food for the next generation of plants. They make that food available in forms that plants can take up easily e.g. as chelated forms

5  Some also fix nitrogen from the air, which will become available to plants. Some exudate is taken up by a wide range fungi which form carbon structures. Some carbon structures are quickly used by the next plant, while others can last longer. Some exudate is taken up by specific fungi growing in association with the root: mycorrhizal fungi. These spread their hyphae, the thin structures that are their main body, over long distances. They can free up minerals locked in the soil and dissolve minerals out of rocks. The minerals can be transported back to the roots and exchanged for exudates. Many other fungi convert dead plant material and form complex, long-term carbon-based molecules. These can last for centuries when left undisturbed by cultivation. They are part of humus, the lasting part of soil organic matter. Much of this has been lost due to human activity. Humus molecules can absorb and hold large quantities of water, which plant roots can extract easily when needed. All forms of humus add to the water-holding capacity.

1 Carbon can take many forms

• Carbon in the air is mainly in the form of a gas: Carbon dioxide (CO2).

• Carbon can combine with many other elements and then form the organic matter, the myriad of complex

molecules that are the basis of all life on earth (including us).

2 Carbon Capture and Storage at coal fired power-stations requires huge investments in machinery, storage

tanks, transport and energy. Green plants run on free solar energy and reproduce themselves.

3 Wikipedia - https://en.wikipedia.org/wiki/Microbiota

4 https://www.youtube.com/watch?v=o55RGuELglI

5 https://en.wikipedia.org/wiki/Chelation: bonding mineral ions to organic compounds that plants can take up.

More carbon indicates more microbiota and more organic matter and humus in a soil.

Photo 1 shows a hard, compacted soil with low carbon, low aeration and reduced soil biological activity.

Photo 2 shows a high carbon soil, well aerated and with soil biology sticking soil to roots. Photo 2 is for illustration but the principles equally apply to grazing land. (Even if the effects may be less dramatic.)

Photo 1    Photo 2

The better the soil structure, the better the water infiltration and the more water that soil can store after rain. An increase of 1% humus has been found to potentially add an extra 10-15 mm being held in the soil after rain instead of running off. Plus better retention of minerals and healthier microbiota, where ‘bad’ organisms are held in check by ‘good’ ones.

Getting optimal benefits from rain on grazing land

Grazing management affects the carbon content of soils and thus the retention of rain water, the rate of regrowth after grazing and ultimately the long-term financial returns.

Figure 5 shows a rough balance between leaves and roots throughout the plant’s lifecycle. The amount of sugars produced at the seedling stage is small because there are few leaves, see figure 1. Of this small amount, only a small part is available as exudate. Larger plants produce larger amounts of sugar, and a larger proportion of that is exuded, as shown in figures 2 and 3. Also, it takes much longer for a plant to re-grow from very small to mid-size than from mid-size to fully grown.

Fig 5 - Stages of growth

Figure 6 shows that a reduction of green leaves leads to a more or less corresponding reduction of active roots.

Combining this with figures 2 and 3 shows why it pays to keep the plants at least at mid-sized, if at all possible, and not graze them into the ground or burn the pasture. More roots are left to supply water to leaves and more leaves to produce more sugars. That means quicker regrowth after rain and more exudates available to feed the microbiota.

Fig 6 - Less green parts = fewer roots

Management 

Key management practices to build up the microbiota - a form of invisible capital - are:

• Managing grazing pressure by rotational grazing in some form.

• Avoiding chemicals and some fertilizers that kill or reduce the microbiota.

• Maintaining groundcover where possible to help water infiltration and prevent erosion.

• Avoiding exposing the microbiota to direct air and sunlight.

• Avoid burning pastures: it sets plants back to the seedling stage and leaves soil bare.

6 https://www.striptillfarmer.com/articles/1714-using-compost-to-increase-water-holding-capacity

Darling Downs Concerned Community groups: Safeguarding Food and Water Security 2/2

See Original Article here