Roots, not shoots, build soil carbon so nudge your thinking to below the ground

Canadian soil science expert Joel Williams, Integrated Soils, spoke at the Wilmot Cattle Company field day near Armidale last week.

The main driver of building soil carbon is roots, not shoots, he said.

"We are obsessed about what happens above ground - that's what is important for productivity," he said.

"But all the stubble, litter and residue on the surface makes a very minor contribution to building soil carbon. It's really about what is happening below ground."

Mr Williams explained what has happened to roots in agriculture over the past 50 years has not served the soil sequestration purpose well.

"We have gone down the annuals root and annuals produce a whole lot less roots than perennials," he said.

"That shift has led to a decline overall in root production.

"Couple that with plant breeding for high yield. That process of selecting for above-ground biomass has come at a trade-off for below-ground biomass.

"Modern high-yielding varieties put carbon above ground not below."

The third factor contributing to the decline in roots has been nitrogen fertiliser.

"We all know nitrogen does not have equal impacts on root biomass as it does above-ground biomass," Mr Williams said.

"The productivity increases we've made via high-yielding pastures is increasing the wrong fraction of carbon we need to build soil carbon.

"We need to nudge our thinking to below ground."

One of the best ways to promote the root exudation process is better grazing management, Mr Williams said.

"When we give plants a rest period, we give them the chance to fully express their potential both above and below ground," he said.

"This is a key way we are going to drive more meaningful advances in soil carbon."

Meanwhile, the research is showing that grazing exclusion is not the answer.

Mr Williams presented the findings from a Florida study where animals were taken out of the equation. It led to less below-ground carbon allocation and root biomass, less root exudate production and less food for microbes.

"There is clearly a beneficial relationship. Whether it is the hoof or the saliva, something triggers more below-ground root recovery where livestock are present," he said.

The aim is not just to get more carbon into the system because it can cycle out. It has to be stabilised to be sequestered, Mr Williams said.

There are two ways to do this.

"Chemically stabalising refers to all the mineral surfaces in the soil - the sands, the silks and the clays that hold onto more moisture and nutrients," he said.

"In the same way, they hold onto more carbon. The carbon is literally adhering to those mineral surfaces."

There is also a physical process, which is when soil particles are clumped together and the carbon is trapped inside and protected.

This is why it is important to minimise soil disturbance, which breaks up those aggregates.