Radical technique buries carbon in soil
Researchers have found deep soil holds potential to off-set greenhouse gas emissions and improve production for farmers.
Dr Mike Beare and his colleagues at Plant and Food Research have been studying how soils differ in their potential to store carbon, and the risk for carbon loss.
Beare said many of New Zealand’s long-term pasture top soils are approaching saturation and don’t have the potential to store carbon near the surface.
Many continuous pasture soils in New Zealand are stratified, with carbon levels declining rapidly with depth. ‘‘Where there is much greater potential to store additional carbon is below the surface soil,’’ Beare said.
The potential lies beneath the top 15 centimetres.
Plant roots are an important source of the carbon that is constantly being fed into soils and help form the organic matter that improves soil health.
The problem is that even with plants that do send roots below that depth, most of the roots still tend to be concentrated near the surface.
‘‘The challenge is to find productive and profitable plants that produce enough roots below ground,’’ Beare says.
The organic matter that roots add to soils is constantly turning over.
Roots die and decompose, which contributes carbon but the microbes break that carbon down and most of it is lost in a relatively short period of time.
‘‘There are some species that we know do have large deep-root systems but we don’t have enough evidence yet that they actually produce enough roots, and deposit enough carbon below ground, to significantly increase our soil carbon stock.
‘‘I wouldn’t say that there’s a silver bullet there.’’
Beare said researchers have been working on a practice called full inversion tillage to bury carbon during pasture renewal.
Farmers re-seed pastures every 10-to-15 years, to improve the pastures’ production. During this pasture renewal, Beare said farmers could create a deeper topsoil.
Full-inversion tillage buries the carbon-rich top soil below 15cm and brings up the sub-soil material that is under-saturated in carbon.
The new highly productive grass is then sown and as it grows it builds carbon on the new surface, with the buried carbon decomposing slowly underneath.
‘‘The net effect will be that we’ll actually accumulate more total carbon in the whole soil profile.’’
Beare said researchers had done modelling work which showed the practice has potential and could be applied across reasonably large land areas, although not on all soils.
Full-inversion tillage is a onetime event, Beare said, and there is a higher cost to do it but it can also lead to an improvement in pasture production.
Trials found the increased feed production paid for the additional costs of the tillage. ‘‘And It does it reasonably quickly,’’ Beare said, ‘‘within about one year the cost has been recovered.’’
Any gains beyond that were a net benefit. The research is being carried out at Lincoln and, in collaboration with Massey University, at two sites in the North Island, which cover a range of soil types.
However, there are many other soil types where the responses could be different, Beare said.
They want to collect more data to support their hypothesis, and expand the number of sites to look at a wider range of soils and pasture renewal practices over a longer timeframe.
There was also a need to confirm that there were no adverse environmental trade-offs with the practice, Beare said.
‘‘We also want to ensure this practice doesn’t result in any increase in nutrient losses, either through nitrate leaching, or nitrous oxide emissions to the atmosphere,’’ Beare said.