A RESEARCH project looking at the potential of sequestering carbon in North Australian soils has found some modest opportunities with more long-term field studies needed.
Soil carbon has created plenty of interest in recent years, with Microsoft purchasing $500,000 worth of soil credits from an Australian cattle company in 2021 and the Australian Government making its first large scale issuance of credits last month.
An organisation called Downforce Technologies, which is headed up by former UN chief scientist Jacqueline McGlade recently completed a study which determined that an average increase in Soil Organic Carbon (SOC) of just 1pc globally on agricultural land has the potential to sequester 84.9 gigatonnes of carbon (GtC) or 311 gigatonnes of CO2 equivalent (GtCO2e). This amount represents a figure close to the 2030 global target reduction in global greenhouse gas emissions.
Commissioned by the Queensland Government, the project looked at more than 200 papers relevant to northern grazing lands – mostly from Qld and the Northern Territory.
While the main finding was that more comprehensive research was needed to give realistic modelling on the long-term viability of soil carbon sequestration, there were some interesting trends.
- Sowing more productive grasses or nitrogen-fixing forage legumes generally increases sequestration but the rate of increase declines over time and may be close to zero after periods as short as a decade
- High grazing pressure is generally associated with lower soil carbon than conservative stocking
- Destocking may result in a small increase in soil carbon sequestration, especially in degraded grasslands, but can be economically undesirable in productive landscapes
- Overall, rotational grazing studies showed no significant impact on carbon sequestration in soil
- Converting cultivated cropping land to permanent pasture consistently increased soil carbon sequestration. However, there are limited opportunities for this strategy in northern Australia
- Converting from tree cover to grassland did not significantly change soil carbon
- Converting well-managed grassland to forest cover did not significantly change soil carbon. However, where degraded grassland was converted to forest cover, there was a small increase in soil carbon.
Queensland University of Technology adjunct associate professor Beverley Henry told a recent FutureBeef and Beef Central webinar that the increase in soil carbon with the sowing of legumes was easy to understand.
“Most of the soils in Northern Australia are low in nitrogen, so when legumes were able to introduce more nitrogen they build more carbon,” Dr Henry said.
“The other property that legumes have is that they are deep rooted, which provides better drought resilient and longer input of organic matter. These practices are becoming more popular in the north and they do offer opportunities.”
Dr Henry said the studies had assessed areas with a good land condition score and compared it with areas that had a poor land condition score – with a preliminary conclusion that land condition was not a reliable indicator of soil carbon.
“This is a bit counterintuitive, and we need more research to properly understand the risks and benefits of using land condition scores,” she said.
“But using data down to 10cm or 30cm did not indicate a clear correlation between soil organic carbon and land condition score.”
Issues in measuring carbon
Measuring the impact of management on soil carbon was another challenge Dr Henry laid out in her research. She said the main influence of soil carbon was rainfall.
“If you look at the management factors, the impact on soil carbon is much lower,” she said.
“Measuring soil carbon has specific requirements, it is costly and uncertain because of the natural variability over time.
Dr Henry said most of the studies done in soil carbon looked at measuring the topsoil down to 10cm, which was a great indicator for agronomy but not sufficient for measuring carbon.
“Under internationally accepted reported rules in quantifying carbon credits, the measurements have to be made to a depth of at least30cm,” she said.
“There are some advantages in going deeper, but most of the change normally happens in the 30cm.”
- To read Dr Henry’s full report click here