Anthony Doesburg: Where's the carbon? You're standing on it

An increase in the soil's carbon content could earn New Zealand billions in emissions credits. Photo / Rotorua Daily Post

The country's week-old Emissions Trading Scheme may or may not cut our greenhouse gas production. But it has certainly got people unearthing carbon in all kinds of unexpected places.

Researchers at a Science Media Centre briefing in Wellington have revealed that there is more carbon in the soil than in the atmosphere and plants combined.

The implication of that, now we have an ETS, is that a tiny increase in the amount of soil carbon could produce carbon credits worth billions of dollars to the economy. Equally though, a tiny decrease could incur a costly deficit.

However the scientists, all of whom work with partner organisations of the New Zealand Agricultural Greenhouse Gas Research Centre, admit they don't yet have the means to accurately measure soil carbon. Although full carbon accounting - including totting up what's in the ground - isn't yet mandated as part of international efforts to curb greenhouse gas emissions, that day is expected to come.

Hence, says David Whitehead, the greenhouse gas centre's principal soil carbon investigator, more than two dozen scientists are engaged in about 40 projects related to measuring soil carbon and understanding how land can be managed to increase, rather than reduce, the amount.

There's a lot at stake. Carbon's concentration in New Zealand ranges from 50-500 tonnes/ha, with most of the country in the 100-200 tonnes range.

It's been estimated that if soil carbon increased by 0.2 per cent on the country's 14 million ha of grazing land, that would represent a carbon trading value of $5 billion at $20 a tonne.

Effectively, the researchers are trying to come up with scientific formulae for what traditional farmers know from centuries of experience.

Soil testing on adjoining 13-year-old organic and non-organic orchards, for instance, shows that the former, fertilised with compost, stores almost 50 per cent more carbon. There's no mystery in that since about half of the organic matter found in soil is carbon.

So traditional horticulturalists, who compost waste organic material and dig it into the ground for the sake of soil health and promoting plant growth, have been practising carbon sequestration for centuries.

Other testing brings to light what could be worrisome results for the booming dairy industry. Analysis of soil samples taken about 25 years apart on dairy land shows carbon being lost at an average yearly rate of about 700kg/ha. But Whitehead is cautious about what can be read into the figures, collected mainly on Waikato farms.

"The problem is we don't know why [the reduction has occurred]. The sampling is limited in terms of scope so there's just not a lot of data around.

"The question is, is this a new level of soil carbon that has been reached after 100 years of farming, or is it still on the way down? And if it's still on the way down, we have reason for concern."

What part might technology play in measuring and managing soil carbon? Whitehead says a concerted effort is being made to develop computer models to explain the biological processes of carbon in soils.

"You cannot go around the whole country and measure soil carbon. What you can do is some critical experiments, take some measurements ... then you've got to say, how does this change in relation to land use? How does it change in relation to climate, how does it change in relation to management?

"And that's what the computer models are doing - they are simulating what is going to happen if any of those variables change."

The biological - or chemical-free - farming movement advocates a no-tech approach to increasing soil carbon, and Whitehead concedes anecdotal accounts of its effects are encouraging.

"But there isn't the evidence that that can be used on a wide scale across all our pastoral estate."

Te Awamutu dairy farmer Dave Harris isn't waiting around for the scientists to give him the answers. By switching to lime fertiliser, and leaving paddocks twice as long to recover between herd grazing so the grass can put down deeper roots, he has increased his farm's soil carbon by 9 tonnes/ha in a year.

Harris, who hopes to supplement his milk income by selling credits for the additional carbon his farm is storing, urges other farmers to follow suit.

"If they got on with it we'd be a lot better off," he says.

Whitehead thinks biochar, carbon produced by pyrolysis, or burning plant waste in the absence of oxygen, could be the great white - black, actually - hope. Added to soil, it stays put while helping nutrient and water retention.

It will be years, however, before it's known for sure whether there's a net carbon gain from biochar's production and application. In the meantime, get shovelling the compost.

Carbon content

Where the Earth's carbon is stored (estimated, billions of tonnes)

* Marine sediments, rocks: 66-100 million.
* Ocean: 38,000 to 40,000.
* Fossil fuels: 4000.
* Soil: 1500-1600.
* Atmosphere: 766 (as of 1999).
* Land Plants: 540-610.

Source: Encyclopedia of Earth

Anthony Doesburg is an Auckland technology journalist