Summer crops help build carbon stocks in Wheatbelt

Growing pastures during the summer months could be one of the keys in helping increase soil organic carbon on farms.
But the trade off could be the use of precious soil moisture stocks for next season’s crop.
That’s one of the findings from a six-year long study involving soil testing on more than 100 farms in the WA Wheatbelt.
The study was the first of its kind to measure soil carbon change in farming soils in WA, and was led by natural resource management group Wheatbelt NRM and the University of Western Australia, with funding from the Australian Government’s Caring for our Country program.
The group’s Dr Guy Boggs said the study was trying to find what impact different farming practices were having on soils.
One of the key findings was an overall decline in topsoil carbon levels.
“Soil organic carbon is the fraction of soil that is made up of organic material including material like charcoal or decomposed plant matter,” Dr Boggs said.
“Soil organic carbon is important for nutrient cycling, retaining soil moisture and helping to stabilise soil from wind erosion.
“We thought the information could also be used to help farmers increase carbon stocks as the market for storing carbon progresses.”
The biggest reason for the drop in soil organic carbon levels over the past six years appeared to be increased summer rainfall and a greater reliance on summer weed spraying.
Responsible for collecting and analysing the soil samples was Dr Andrew Wherrett, a soil scientist previously with UWA and now working for independent York-based agricultural research company Living Farm.
“We wanted to see if there had been a change in organic carbon from sites we visited between four and six years ago as part of a previous soil health project,” Dr Wherrett said.
“What we found was that soil organic carbon levels on average have decreased slightly.”
As part of the research, 10 soil cores were bulked together at each of 100 individual sample sites at three depths between zero and 30 centimetres.
Each sample was tested for soil organic carbon using both the Walkley-Black and LECO methods.
Samples were also sent to CSBP for standard soil testing including pH and electrical conductivity (salinity).
Predicta-B disease testing, water repellence, microbial biomass and soil nitrogen supply were done separately.
“On the whole it appears that soil organic carbon has decreased, particularly in soils that had a high value at the previous sampling time,” Dr Wherrett said.
“We found soil organic carbon ranged from 0.4 per cent to 2.5 per cent in the top soil, which equated to anything from 14 tonnes per hectare to 60 tonnes per hectare of carbon in the top 30 centimetres.”
He said from a science perspective it was not as simple as measuring soil carbon at set intervals, because the conditions preceding soil sampling appear to have such a great impact.
“If you look at 2011, most places in the Wheatbelt received between 100 and 250 millimetres of rainfall between October and February,” Andrew Wherrett said.
“That compared with between 50 and 70 millimetres of rainfall when testing took place at the same sites six years earlier.
“The hotter, wetter summer in 2011 promoted more microbial activity and organic matter breakdown but the lack of growing plants meant no new soil carbon was being created.
“So much of the Wheatbelt has destocked and farmers are now using chemicals to control weeds during summer to conserve soil moisture.
“Without an active summer crop or pasture, the elevated moisture levels and increased microbial activity chews up what organic carbon is available, while no fresh carbon can be made.
“It seems soil carbon levels can change more quickly than people realise.”
Beverley farmer Deane Aynsley is one of the many Wheatbelt farmers who have destocked and now rely solely on chemicals to control weeds during summer.
While he’s always been keen to find out how to increase soil organic carbon, preserving soil moisture during the summer was more important.
“This raises the question, do you control your weeds in summer and preserve your moisture or do you grow summer crops and retain soil carbon?” Deane Aynsley said.
“For me, I’m going to preserve my soil moisture every time, as well as nitrogen and nutrient stocks and kill my melons which block up the air seeder during seeding.”
Deane Aynsley crops canola, wheat, oats and barley and said there needed to be a market before he would farm for carbon.
“For me it’s about cost, if you are not going to make dollars from carbon, I’m not going to farm for it,” he said.
“The alternative is growing a summer crop and that’s risky, plus your paddocks don’t get that break from pests and disease.”
Deane Aynsley said more research was needed to give farmers alternatives to spraying weeds during the summer.
“Summer weed spraying is the worst job on the farm,” he said.
“You generally have to do it in the middle of the night when its cooler, and you’ve already spent most of the year on the sprayer, so you don’t get a break.”
Living Farm’s Dr Wherrett said while the results from the samples taken don’t point to one particular farming practice that would help increase carbon stocks, active removal of live ground cover does seem to have a negative impact.
“The trade off is you might increase your carbon levels if you have an active summer crop or pasture, but you will use more moisture, ” he said.
“In the Wheatbelt there is such a focus on the growing season, we need to look at what we are doing over the summer months, because livestock are no longer a key management tool on many farms.
“With climate predictions indicating wetter, hotter summers, this is an area that needs more work and more options for farmers.”