Mulch improves our soils, but it’s not the best nor the easiest thing to do.

I don’t know about you, but I have spent too many hours mulching. Getting hot and dusty rolling out round bales to cover bare ground. I’m sure it’s not good for my health. Breathing in all that dust …

And the mulch disappears so quickly.

Makes you wonder why you bother.

Mulch does have benefits. It provides food for the decomposers in our soil food web, reduces soil erosion and water evaporation, and buffers soil temperatures.

But research shows that plants growing mulch right where you need it, together with living roots do a superior job.

I’ll explain.

Soils are complex. Soil science is complicated. It’s important, though, to tease out what scientists have discovered. We can then make the best use of our time.

 

This picture illustrates the ‘old-fashioned’ view of organic matter recycling in our soils. We believed that the above-ground inputs from mulch together with leaves and branches from plants growing on the site provided the majority of organic inputs, with some extra organic material from dead roots (smaller arrow). How long the organic matter lasted in the soil depended on the chemical complexity of the organic materials. For example, lignin lasted longer than grass.

 

Let’s have a look at how our understanding of recycling organic matter and carbon has changed -particularly in relation to how we cover our ground.

Firstly, using living mulch we get the benefits of Root Exudates

Root exudates are ‘energy drinks’ plants secrete from their roots. When scientists first discovered these exudates, they thought that the plants had something wrong with them, and called the phenomenon ‘Leaky Root Syndrome’.

Now we know better. We know plants deliberately secrete these simple and complex sugars for a variety of purposes. These include:

  • changing the chemical and physical properties of soil
  • regulating the soil microbial community around their roots
  • encouraging beneficial connections with mycorrhizal fungi.

We’re not talking about small amounts of carbon.

Plants transfer 5% to 21% of all the carbon they remove from the atmosphere as C02 to the soil as root exudates4.

Why do plants do this?

Because somewhere between 85 to 90% of the nutrients plants require for healthy growth are acquired via this exchange of sugars with soil microbes. The sugars supply microbes with their energy needs. In return, bacteria and fungi in the Rhizosphere around the plant’s roots, and mycorrhizal fungi  – help plants obtain the nutrients they need.

How do root exudates from living mulch plants benefit soil

First, the carbon in the exudates is good for our soil. Carbon improves aggregate structure, water-holding capacity, and nutrient availability.

Second, because the root exudates provide an ‘Energy Drink vending machine’ for the microbial construction workers.

In return for the energy drinks, the microbes amass a workforce to construct soil aggregates and Humus – the long-term store-house for carbon.

Creating Humus is a construction rather than deconstruction process, as was previously thought. The microbes use simple carbon compounds as the bricks and mortar creating these larger, complex and stable carbon compounds in our soil.

Living Mulch is an easier way to increase carbon levels in our soil

It now seems that most of the carbon from our above-ground inputs, like mulch, is released back into the atmosphere with each out-breath of soil organisms feasting on the plant and animal remains. Long-term studies on mulch suggest that it has limited effect on soil carbon levels compared with inputs from root exudates1, and usually don’t lead to increased levels of stable soil carbon2.

Even tougher plant materials like lignin, don’t necessarily hang around for long.

The type of organic material you put in your soil does not necessarily determine its stability, and therefore how long the carbon it contains remains in your soil.  What’s more important is your soil’s physical and biological environment5.

For example, long-term carbon storage happens better deep down in our soils. Soil chemistry, reactive mineral surfaces, climate, water availability and the presence of potential biological ‘degraders’ all play a role.

In general, most of the carbon in our soils is from root exudates and roots1.

The amount of carbon in our soils from roots represent a substantial portion of the total carbon input to our soils, and, carbon from plant roots is retained in soils much more efficiently than are above-ground inputs5.

The average time root-derived carbon stays in soil is 2.4 times that of carbon from above-ground supplies1.

The amount of carbon added to our soils from roots obviously depends on the volume and turnover of roots.

But one thing’s for sure.  Mulch doesn’t have any roots!

So next time you’re working hard spreading dead plant material, think instead about growing living mulch to provide energy drinks and carbon sandwiches…  and all the time and money you’d save.

Bibliography

  1. Daniel P. Rasse, C. R. &. M.-F. D., 2005. Is soil carbon mostly root carbon? Mechanisms for a specific stabilisation. Plant and Soil 269, 341–356.
  2. Jones, C., 2008. Liquid carbon pathway. Australian Farm Journal 338.
  3. Kong, A. Y. Y., 2010. Tracing Root vs. Residue Carbon into Soils from Conventional and Alternative Cropping Systems. Soil Biology & Biochemistry Vol74: Number 4, 1201 – 1210.
  4. Marschner, H., 1995. Mineral Nutrition of Higher Plants. Academic.
  5. Michael W. I. et. al, 2011. Persistence of soil organic matter as an ecosystem property. Nature Vol 478, 49 – 56.
Carbon pathway illustration adapted from Michael W. I. et. al (2011).
Featured image is a Scanning electron micrograph of a rye-grass root with root hair penetrating through soil aggregates (courtesy of Claire Chenu).