A new Government report has challenged the findings of a study released last year showing New Zealand sheep and beef farms are already offsetting the bulk of their agricultural emissions.
The Ministry for the Environment (MfE) report found rather than meat farms coming close to being completely carbon neutral, as the previous research found, the true figure was much lower.
Last year's peer-reviewed study was led by Dr Bradley Case at Auckland University of Technology (AUT) and commissioned by Beef + Lamb New Zealand (B+LNZ). It concluded the woody vegetation on sheep and beef farms across the country was offsetting on-farm agricultural emissions by between 63 and 118 percent.
However, the MfE report - Net Emissions and Removals from Vegetation and Soils on Sheep and Beef Farmland - found a number of failures in Dr Case's report and concluded the actual figure of emissions being offset was 33 percent.
The vastly different estimate - 63 percent lower than the midpoint of the previous study - was due to the fact the initial study did not account for the amount of forestation harvested or chopped down each year, the MfE report stated.
"The exclusion of carbon losses arising from forest harvesting, deforestation and scrub clearance has a significant impact on the overall estimate of net emissions and removals," the study said.
It added that although exotic planted forests had the greatest estimated removals of emissions, "there was no assessment of the carbon losses that would be occurring from the harvesting or deforesting of these forests".
And while the absence of harvesting emissions was acknowledged as a limitation in the first study and the authors recommended including it in future estimates, the MfE study said this did not go far enough.
"As harvesting is such a significant driver of net emissions in planted forests, emissions from this activity must be included when assessing the overall net removals from this land use."
The Government study also called into question the initial study's estimate of sequestration rates for given vegetation types, saying the sequestration rates used in the study "may give an indication of the potential sequestration some of these vegetation types could reach under specific conditions, but do not accurately represent the total vegetation area present on farmland."
The ministry's report said the first study also overestimated how much additional carbon dioxide scrub and shrub vegetation could sequester after reaching maturity.
The new report also took into account emissions rates of drained organic soils (e.g. previously drained wetlands), whereas the first one did not.
"Given that these soils are a known source of emissions, for completeness they should be included in an assessment of net emissions and removals on sheep and beef farmland," the report said.
In a statement on Monday, Sam McIvor, chief executive B+LNZ said he stood by the AUT research, noting the two studies used different methodologies.
"We absolutely stand by Dr Case's independently reviewed robust and credible research. While there are differences in some of the methodologies MfE used in their report – particularly their netting-off of all harvested forest that doesn't take into account the replanting and additional new planting we know is happening – it reinforces the importance of on-farm sequestration."
As well as the issue of replanting, McIvor said the MfE study used different mapping approaches to estimate what forest was present on sheep and beef farms. It also discounted small blocks of planting throughout sheep and beef farmland, including shelter belts and riparian planting, which he said B+LNZ disagreed with because "our understanding is smaller blocks of vegetation within a farm are, across the sector, significant".
Despite the difference in findings, McIvor said the latest study was an "important contribution to conversation" about the subject.
"What is encouraging is that MfE's report recognises there is significant sequestration happening on sheep and beef farmland. Even using a highly conservative approach, they've arrived at a figure of a 33 percent offset of on-farm emissions by vegetation, which shows farmers are well on the journey no matter who is crunching the numbers. This sequestration is on top of the 30 percent reduction in absolute emissions that sheep and beef farmers have made since 1990."
He also said Dr Case's report was a "relatively new area of research" and was important because it was the "first time anyone had attempted to measure the sequestration happening on farm".
"We'll keep advocating for a farm-scale view of emissions and sequestration, as well as encouraging our farmers to keep up their great work protecting and enhancing their landscapes.
"This isn't just about carbon offsetting – native vegetation, in particular, provides other important ecosystem services such as enhanced biodiversity, soil conservation, and improved water health outcomes.