Latest research by Plant and Food Research scientists will help farmers ultimately identify where on their farms carbon levels are highest and where they may be most vulnerable.
In a State of Science webinar hosted by NZ Agricultural Greenhouse Gas Research Centre, Plant and Food scientist Dr Sam McNally and Dr Mike Beare said the challenge put out to scientists has been to try and determine and upper limit on carbon in NZ soils, and then apply that theory in a practical manner on-farm.
“Soil holds large amounts of carbon globally; it is 3 times the amont of carbon in soil than the atmosphere,” McNally said.
“Our focus is on the large pool that is in the farm environment. It can be both a source and a sink, and can form part of the climate change mitigation process.”
Internationally, NZ soils are regarded as relatively high in their carbon levels, raising conjecture about how easy it may be to alter that level and sequester more carbon within them.
However, not all soils are created equal when it comes to carbon capture.
Landcare work back in 2012 revealed a range of carbon stock in the country’s soils.
“It was quite regionally specific, depending upon soil type. Soils from volcanic activity tended to have a high level of carbon compared to other soils,” he said.
Those high carbon soils included ones in Waikato and Taranaki.
McNally says taking the theory on where carbon lay to a practical on-farm level raised three key questions.
“If we know the maximum amount of carbon that can be held, we can identify where it can be increased, a targeted approach of mitigation practice to soils that respond the best,” he said.
“Secondly, if you know what soils already have a high carbon level, you can reduce the risk of carbon loss from those specific soils.”
Thirdly, the knowledge would make research more relevant and applicable.
“If we know soils with the greatest carbon capacity we can test those practices (to lift carbon) on those soils,” he said.
A key property determining soil’s carbon holding capacity was its surface area.
“So, if you know across your farm what soil has the higher surface area, they are likely to respond to carbon maintenance and protection the best,” he said.
The scientists’ latest research also indicates soils with greater deficits look like they can retain more carbon, and early work indicates it may be possible to develop an indicator for what a soil’s upper carbon sequestration capacity is.
Taking the work to Lincoln University’s 160ha dairy farm, which is extensively soil mapped in 200 sample spots, the scientists have been able to develop a map highlighting low and high carbon zones within the farm boundary. They can also detect areas where there is potential to lift carbon levels.
“There is a wide range of surface areas of different soils across the farm. Carbon stored varied from 60 to 100 tonne per hectare, corresponding to the surface area of the soil types,” he said.
They were also able to identify specific soil zones that had potential to lift carbon, and those that had the least, and ultimately determine the farm’s entire carbon stock.
“All up it was 1200 tonne of carbon. It does not seem a lot, and is equivalent to 5.5 ha of exotic trees,” he said.
Efforts to map the farm’s vulnerable carbon stocks could help influence future management when it comes to activities like winter grazing or cultivation in those zones.
Dr Beare acknowledged the work is presently confined to the top 30cm of the soil zone, and that deeper rooting plants on farms may have a beneficial effect on carbon sequestration.
“We do not know yet how much carbon those sorts of plants can contribute,” Beare said.
The scientists also say despite support for biochar as a means of adding carbon to soils, it remained an expensive option and probably not cost effective.
At present MPI is advising that due to uncertainty about how NZ’s soil carbon stocks are changing within a land use and how much management may influence those stocks – current green-house gas inventory does not account for soil carbon stock change in a land use type.
This is instead limited to accounting only for soil carbon stock change when the land use itself is changed, for example from pasture to forest.
