Plants don't just sit there and take it

Plants may not be able to move around but that doesn’t mean they have no control over their destiny. They have an amazing array of strategies for surviving and improving their environment.

We need a feed and a drink and so do plants. Apart from the carbon in the atmosphere being a food source for them, plants get the other things they need from the soil.

Plants are the ultimate networkers. They send chemical instructions via root exudates to soil microbes, to get them to do what they need done. 

Root exudates: what they are & what they do

Not all the carbon from photosynthesis is used in the construction of leaves, stems and new roots. Some is released as organic substances by the roots of plants. The energy is released by plant roots in the form of root exudates. This energy is released to the organisms living on or near their roots while plants are growing (photosynthesising). Because of the direct energy contribution from plants, the population of microbes in the rhizosphere around root tips can be 5-50 times greater than in the rest of the soil.

Root exudates are a direct energy transfer from plants to soil microbes, while organic matter is an indirect transfer.

The complexity of plant interactions with soil microbes

Carolyn Ditchfield generously wrote this for me some years ago. This explanation by Carolyn explains everything much better than I could do.

“Perhaps not consciously, there is a pervasive impression that plants are at the mercy of their environment. This is reinforced by conventional agricultural practices that focus on feeding the plant and protecting them with various chemical concoctions.

It is true that plants are not mobile so cannot physically escape their location, but they have an amazing array of strategies for surviving and even manipulating their environment. 

Often overlooked is their ability to modify the root zone. Up to 30% of the photosynthetic energy accumulated by a plant is dumped into the root zone as sugars, proteins and carbohydrates. Apart from the fact that combined, all these exudates contain carbon, they also act as a food source for soil biology. 

Much like the food web above ground, different soil microbes respond to different food sources below ground, i.e. different exudates attract/stimulate different microbial populations; and these soil microbes are a remarkably powerful workforce with individual species able to solubilise minerals (plant nutrients), fight ‘disease’, fix nitrogen, decompose organic materials, restructure soils, hold water, etc. 

Maybe coincidently, different plants produce different exudates. But even more interesting, individual plants change the composition of their exudates with environment, season, climate or phase of growth.

Although the research is yet to formalise the link, the clues are accumulating. A plant’s ‘decision’ to release a particular type of exudate from its roots has an active effect on which microbes get ‘nurtured’ in the root zone, and hence an active influence over its own maintenance and survival.”

Some figures put on plant decision making

Research has shown that P-stressed lupin plants secrete about 20- fold more acid phosphatases from roots compared to P-sufficient plants.

The graph below shows the percentage of photosynthesis that cropping plants allocated to exudates (liquid carbon) depending on the abundance of labile carbon (short term carbon) in the soil. Perennial grasses display similar behaviour.


Source: Ken Sharpe

Mis-manage carbon flows and livestock get less of future incoming carbon

 

It is well known that plants allocate more carbon from photosynthesis below ground when soils are less fertile. This means less of the production from photosynthesis becomes ground cover for sheep and cattle to eat. All else being equal, soil fertility reduces if carbon flows into the paddock reduce.

Below is a paddock where plants are now allocating plenty of carbon above ground.

This is an area where plants have been allowed to improve their own environment. This photo was taken in the Traprock country in South East Queensland which is known for its low fertility. 

Conclusion

As soil organic carbon gets lower, plants must exude more liquid carbon from their roots to grow.

Root exudates from grasses are the fastest moving/flowing carbon. This carbon will be back up in the atmosphere within 24 hours of entering the plant.

If the top half of a plant is hardly photosynthesising because of poor animal management, then the roots will be exuding little energy to the soil microbes.

Shortage of plant available phosphorus is known to be a production issue in some pastures. The availability of phosphorus to plants is influenced by how much soluble carbon plants are able to release to soil microbes.

Alan Lauder

Next week’s discussion:   “The two different processes plants use to create carbon flows”

WHY CARBON FLOWS?,