Managing stubble to increase soil carbon

A project from the CSIRO, along with the GRDC, Kalyx and Delta Ag, is trialing different methods to increase the amount of soil carbon.

CSIRO researcher John Kirkegaard, said the work followed on from a project in Harden, NSW, which ran from 1990 to 2020.

Mr Kirkegaard said that trial looked at ways of treating stubble.

"Despite having treatments where we're burning the stubble, where we're cultivating stubble, where we're leaving the stubble, we weren't really seeing a big difference in soil carbon," he said.

Mr Kirkegaard said another researcher Clive Kirkby had suggested the need to focus on organic matter.

"You only find carbon in the soil in the form of different types of organic matter," he said.

"The stable organic matter that the bit that that is creating the fertility for you contains carbon, nitrogen, phosphorus and sulphur in predictable ratios.

"Clive tested 500 soils from around the world and they always had the same ratio of those elements."

Mr Kirkegaard said based on the ratios for every tonne of carbon that was in soil organic matter there was also 85 kilograms of N, 25kg of P and 12kg of S.

"That is why if you have a soil high organic matter it's really fertile - it provides nutrients to your crop and that's where the nutrients are coming from," he said.

"So it's reasonable to expect if you've got to put the soil organic matter back you're going to need the nutrients - you're not going to just put the carbon back."

Mr Kirkegaard said, in the trials at Harden in 2007, they started putting nutrients on one side of the plots when stubble was incorporated.

"After five years where we had not been putting nutrients on our soil, carbon went down and this was measured down to 1.6 metres," he said

"Where we had been putting nutrients on the stubble it went up five and a half tonnes in five years and so the net difference was 8.7 tonnes of carbon."

The project was continued with nutrients added until the soil carbon amount was levelling on, and Mr Kirkegaard said they then wanted to see what happened if they stopped.

"For five years we stopped adding the nutrients and we kept adding the stubble, and the difference did decline," he said.

"The differences at depths were first to go. The important point is if you stopped doing any of these things you will just start using up the organic matter that you've built up.

"And that is what we've always done. We had pasture phases and then we had cropping phases but in the context of a carbon project you may be penalised for this, then you need to remember that it's important to whatever process you're doing that you maintain it."

Mr Kirkegaard said the impact on yield was variable.

"However, almost every year, we saw increasing grain protein," he said.

"So that tends to suggest that that shift in in organic matter is creating a process as you'd expect that does feed crops late with with extra mineralisation. So very consistent improvement in grain protein."

Mr Kirkegaard said the new project was taking a step forward on these trials.

"By looking at whether it works on other soils, do we have to incorporate and how much because we really don't want to do more than you have to," he said.

"Can we do better with different types of fertilisers, can we tailor things if our soil's already high and P and S, can we just just put the N on?

"And then what about the timing will not just all this just be easier if we put more on the crop and keep a positive nutrient balance."

The site at Rennie, near Corowa, NSW, involves small plot replicated trials with 10 treatments, which are a combination of putting extra nutrients on stubble and the crop, using granular or liquid fertiliser and incorporation using a rotary hoe or a speed tiller.

Mr Kirkeegard said the plots have been baseline sampled with soil cores taken down to 30 centimetres, as well as deeper.

"We'll monitor the soil carbon at its fraction," he said.

"We'll measure it in year three and five. We might go in an take some samples and look for more dynamic microbial changes annually in the surface."

Mr Kirkeegard said the project would provide an option for growers.

"Well managed pastures we know can sequester the carbon very effectively and if you want to shift soil carbon really quickly, then put in a good pasture and manage it well, and you can get up to 500 kilos a year," he said.

"In your cropping system the first thing you can do is try and maintain a positive nutrient balance.

"If you're doing that and you're not mining it, then you should be maintaining your soil carbon but this is an option to supply supplementary nutrients on heavy cereals stubbles or canola stubbles.

"That's what this project is looking into and this project is really about seeing if we can make that work commercially."

The trial will run for the next five years and has eight sites at Narrabri, Young and Rennie in NSW; Horsham, Vic; Keith and Tarlee in SA; Cuballing and Moora in WA.