Farm Ireland

Saturday 16 December 2017

Ireland under-estimating area under cropland by 46pc claim scientists

Figures could have implications for greenhouse gas reduction targets

Ploughing ground for new crop releases up to one tonne of carbon into the atmosphere.
Ploughing ground for new crop releases up to one tonne of carbon into the atmosphere.

Darragh Mccullough

Ireland could be under- estimating the amount of land it is declaring as cropland by 46pc, something that could have serious implications for our greenhouse gas emissions targets.

New research from scientists at Trinity College, Dublin said that the inaccuracy is linked to the exclusion of field history.

It states that pasture ploughed even once during the last five years should be included as cropland rather than grassland.

"Even reseeded grassland should be considered cropland, if it was ploughed in the last five years," said Dr Jesko Zimmerman, author of the study.

Ploughing ground for a new crop releases up to one tonne of carbon into the atmosphere.

In contrast, a permanent pasture removes one tonne of carbon from the atmosphere annually.

"The ploughing breaks up the soil aggregates, which releases carbon, and exposes the organic matter to bacteria that release more carbon," said Mr Zimmerman.

However, Teagasc's climate change expert, Dr Gary Lanigan said that the extra carbon released into the atmosphere was effectively removed again by the grassland over the following two years.

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"A much bigger problem is the situation where a field is converted from grassland to tillage for a period of say 15-20 years. This has the capacity to reduce the carbon locked into the soil from 180t to 100t. The problem is that it takes nearly 50 years to build that carbon back up again," said Mr Lanigan.

The Teagasc researcher pointed out that grazed grassland is also a source of harmful greenhouse gases in the form of nitrous oxide (NO) released from the urine and faeces generated by the stock grazing it.

"Grazed pastures generate about three times more NO than cropland, but relatively speaking, it is not as important as the volumes of carbon being released by ploughing," he said.

Researchers hope to tap into more detailed data on changing land use, but are finding the science difficult to implement, despite access to the massive database generated by the Department of Agriculture's Land Parcel Identification System (LPIS).

"When we added up all the area reported by farmers, we came to a total 1.5 times the size of Ireland, because the commonage area was being reported by several farmers in each case.

"In addition, farmers may classify their land as rough grazing one year, and another type of grassland the next.

"We also don't know when a farmer is reseeding land or not, so it's not just as easy to pin down as you might think," said Mr Lanigan.

Scientists in this area have traditionally assumed that 90pc of Ireland's agricultural area is being used as pasture, and that there are relatively small changes in land use over time.

However, the Trinity research recommends that this view on Irish land-use should be re-evaluated.

"While the area annually reported as cropland was on average 375,200ha, this area has been shifting around the country.

"In the 12 years from 2000 to 2012 only about half of that area could be considered permanent cropland, while the area with arable history in the timeframe was 737,300ha," concluded Mr Zimmerman.

"We could show that relying on annual data and not including land-use history led to an 45.7pc underestimation of area reported as cropland."

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