ARABLE
Arable crops and sequestration? As farm businesses begin to calculate their annual total on-farm greenhouse gas emissions, FAR is receiving a number of questions around annual crops and sequestration. The most common ones are: Can arable crops be included in carbon sequestration? If not, why not? How come it’s different in other countries? WORDS BY DIRK WALLACE & ABIE HORROCKS, FOUNDATION FOR ARABLE RESEARCH
Can arable crops be included in carbon sequestration? No.
Why can’t arable crops be included in carbon sequestration? Carbon sequestration is defined as the process of storing carbon in a carbon pool (IPCC, 2018). To use a banking analogy, we can think of carbon pools like a bank balance and any increases or decreases in that balance as a carbon flux (Figure 1). Growing annual arable crops is a good example of a short term carbon flux. Crops do remove carbon dioxide from the atmosphere as they grow, however, much of the carbon removed is quickly returned to the atmosphere. To come back to the banking analogy, a short-term flux is similar to if I paid you $1,000 every odd week and you paid me $1,000 every even week, at the end of the year our bank balances would look the same as they did at the start.
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R E AL FARM E R
Sequestering carbon means storing it long term, to keep it out of the atmosphere, much like permanently increasing that bank balance. Thus, acceptable on-farm carbon sequestration typically involves taking carbon from the atmosphere and storing it in either the soil profile or in permanent vegetation. Not having it rise and fall (flux) over time. One option for sequestering carbon is to store it deep in the soil where decomposition rates are lower. Currently, soil sequestration is not part of the national greenhouse gas accounting framework. If this changes in the future, careful inventory methods will need to be developed to account for the fluctuation in soil carbon across the rotation.
How come it’s different in other countries? Internationally, some voluntary carbon markets are paying growers for soil carbon. These voluntary markets are often not aligned with national emissions reductions programmes or regulation.
Currently, the United States Department of Agriculture (USDA) are developing a method to account for carbon sequestration in permanent pasture, wetlands and forests, however, at the same time, voluntary carbon markets are operating which are paying for soil carbon. These voluntary markets frequently produce payments for farmers by using a short term (10 years vs 100 years) view of carbon sequestration and base credit assumptions off satellite imagery and models rather than measured changes in soil carbon stocks. As far as we are aware, there are no voluntary carbon markets established in New Zealand, but if they do appear it will be important to understand what time frames they use and any associated liabilities.
Soil carbon, soil organic matter and tillage We’re also getting a number of questions around the links between soil organic matter, soil carbon and tillage. Most of these are focused on the assumption that reduced
