Friday, March 29, 2024

Scientists paint carbon picture

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Efforts to get a better handle on New Zealand’s soil carbon stocks have gained momentum with money from the Agricultural Greenhouse Gas Research Centre and Primary Industries Ministry as NZ ramps up understanding of carbon stores.
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While soil carbon plays a critical role in soil health and function it is also a valuable reservoir for holding carbon dioxide with more carbon stored below ground than the combined amount in plants and the atmosphere.

Landcare Research study leader Dr Paul Mudge said NZ already has a system for quantifying the carbon invegetation and soils in native and exotic forests.

The new study will sample 500 sites covering all agricultural land broken into the broad land-use classes of  dairy pasture, flat and rolling drystock pasture, hill-country drystock pasture, horticulture and short rotation cropland.

Sampling soil carbon is not simple. It requires physical effort from the three teams of scientists that will be sent from Waikato, Palmerston North and Lincoln in next year.

The most difficult sites are the stony soils in eastern parts of both islands. Two people are likely to do only two sites a day digging 60cm into the soil, separating rocks and soil, carefully recording their weights then calculating the volume of soil by filling the pits with plastic beads with a known volume-mass ratio. 

Mudge said there is a good reason those soils are underrepresented in sampling.

“We have already sampled 60 hill country sites and  will be doing another 110 sites across the country every year from those land use types, rather than doing it region by region. 

“That means we could provide a snapshot of NZ’s soil carbon levels after year one and an increasingly exact picture for every year of sampling. By the time we get to year five we will be resampling and be able to give an indication if soil carbon is increasing or decreasing.”

Centre director Dr Harry Clark said evidence suggests soil carbon content in flat, grazed pasture has not changed greatly in the past 20-30 years. The exception is drained peat soil where carbon continues to be lost, largely as carbon dioxide through oxidation as water is removed.

“There is some evidence hill-country grassland soils gained soil carbon between about 1980 and 2010 but it isn’t clear how widely spread these gains are and if they are ongoing.”

Mudge said if the gains in the hill country soil have continued they represent a very positive story for the agri-sector. 

Practices that remove even a small percentage of carbon dioxide from the atmosphere and lock it up in soil could prove highly beneficial to the sector as it works to reduce its emissions in the next decade.

More specific experimental work led by Waikato University environmental biochemist Professor Louis Schipper has found lighter-stocked Waikato dairy units gain considerably more carbon over a year than more heavily stocked, more intensive operations. 

Schipper’s team has also found pasture renewal leads to net carbon losses during the renewal period when soils are bare and losses are higher when the soil is wet.

The longer-term impacts of pasture renewal and maize cropping in a pastoral system are ongoing areas of research.

Typically, NZ soils are well manged with little continuous cropping and therefore carbon stocks are already relatively high. That is augmented by a temperate climate that results in continuous carbon input.

Other countries have done varying levels of soil carbon stocktakes. 

Mudge said France and Denmark have some of the most comprehensive methods in the world, having national grid monitoring systems that have been running for decades.

“Australia and the United Kingdom also have soil carbon monitoring systems in place,” Mudge said.

The researchers hope they can, over time, build up the number of sampling sites, potentially by including links with industry or even individual farmer initiatives provided consistent methods are used. 

In time that could help identify specific management practices that help maintain or increase soil carbon stocks.

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