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Miscanthus – a carbon negative crop


1 April 2019

Most annually harvested crops require a lot of activity to get them established, grown and harvested. They need cultivation of the soil, weed control, planting, fertiliser, harvesting, sometimes waste disposal, packing and loading on a truck. Most of them need all that every year. In many cases, there is further cultivation, planting and cutting of a cover crop during the off season as well. Again, every year!

Miscanthus on the other hand needs cultivation, planting and weed control – once in at least 15 years – perhaps 25 years – plus harvesting and loading on a truck every year from year 2 onwards. There is also no waste to be disposed of with Miscanthus. There is no need to cultivate the soil again, no need for ongoing weed control, no need to replant, no need for fertiliser in most cases. The soil benefits too because it is not disturbed again for years. Greenhouse gas (GHG) emissions from growing Miscanthus of therefore a lot lower than alternatives.

In the course of research into “Growing Giant Miscanthus (Miscanthus × giganteus) Biomass for Gasification to Biofuel”, Dr Rocky Renquist of Bioenergy Cropping Solutions Ltd completed a very detailed Life Cycle Analysis (LCA) for establishing and growing Miscanthus in New Zealand. He also assessed its global warming potential (GWP). His analysis started right back in the tissue culture laboratory so nothing was omitted and he did not use industry averages at all.

He concluded: “The LCA for Miscanthus found that GHG emissions from Miscanthus growing are very low and [there is] a strong basis to support its use as a biomass crop for biofuel.”

The really interesting thing is that this was done some years ago and assumed the use of plantlets for establishment of the Miscanthus. But in reality for commercial plantings - say 10 ha and upwards - all the planting will now tend to be done with rhizomes. Miscanthus New Zealand Ltd (MNZ) already has a rhizome planting machine operating in New Zealand and it is safe to say that it is considerably more cost-effective than manual planting. So while the economics of growing Miscanthus is better than it was at the time of Dr Renquist’s research, MNZ is confident that the GHG emissions during production of the planting material and planting are also almost certainly lower than indicated in this research.

In addition to the very low GHG emissions from the plantlet production, establishment, growing and harvesting of Miscanthus, it has been recognised internationally that Miscanthus stands increase the amount of sequestered soil carbon. As long as the crop continues to be grown and harvested, the amount of carbon stored in the underground rhizome system together with the amount of carbon stored in the continually increasing soil organic matter is quite significant - perhaps in the order of 2 tonnes of carbon per hectare per year.

Dr Renquist spoke of soil carbon sequestration, saying: “This process is an additional factor making Miscanthus even better with respect to GWP, based on the large amount of soil organic carbon it can add to the soil from its massive rhizome system.”

He went on to say “… more C is sequestered this way than all the GWP impacts from growing the crop, giving Miscanthus a negative GWP footprint.” It is hard to imagine any other crop that can be grown on New Zealand rural land having such a clear and ongoing carbon negative footprint.

New Zealand primary producers get a lot of flak from well-intentioned critics who do not look at the big picture. They are forever being reminded that 47% of New Zealand’s greenhouse gas emissions come from agriculture. What those critics forget is that if the New Zealand farmers stopped producing their products – mostly food - and as a result dramatically cut NZ’s GHG emissions, somebody else would have to produce those products. They would almost certainly not do it as efficiently as the New Zealand farmers do. However, it also makes sense for those New Zealand farmers to establish Miscanthus stands on their properties, partly because it is good economics and environmentally friendly with low commercial risk and partly because it will help to reduce that 47% of NZ’s net GHG emissions.

In Dr Renquist’s research he demonstrated how effective Miscanthus can be when used as fuel. The key factor is the energy ratio - the amount of energy stored in the crop divided by the amount of energy used to produce it. Dr Renquist said “The energy ratio is a widely-used simple measure to compare fuels. … … the ratio between gross output and gross input [for Miscanthus] was extremely high (348.3/2.7 = 129:1).”

So this is yet another reason to grow Miscanthus. It is time to identify an area on your land and to ask MNZ for a quotation for its establishment in Miscanthus.

The next article will be on use of Miscanthus for boiler fuel. Keep an eye out for it.

ENDS

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