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).
FIGURE 1. The impact of plant photosynthesis and respiration on carbon in the soil vs. carbon dioxide in the atmosphere. Gains in soil carbon can be related to the restorative phase of the rotation and losses of soil carbon can be related to the depletive phase of the rotation (diagram from Agmatters website www.agmatters.nz).
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.
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 tillage leads to increased soil organic matter which results in greater soil carbon pools. Again, it’s not especially straight forward.
- Soil organic matter and soil carbon are closely related because organic matter contains, on average, 58% carbon. Soil organic matter also includes hydrogen, oxygen and small amounts of nitrogen, phosphorous, calcium and magnesium.
- Soil carbon stocks often fluctuate across the rotation time, depending on how much organic matter is entering the soil organic pool and how much is being removed via decomposition. Typically, rotations will includes restorative and depletive stages.
- Optimal amounts of soil organic matter and carbon vary depending on soil type (soil texture dictates how much carbon your soil can hold), climate and management. In general, the minimum target range for carbon in New Zealand’s cropping soils is 2–3%. Soils with less than 1% organic carbon content are considered functionally impaired.
Does reducing tillage increase soil carbon and improve soil function?
Different establishment methods affect how soil carbon is distributed, but reducing tillage, by itself, does not necessarily increase total soil carbon down the profile. This is because the mixing effect of inversion can result in greater soil carbon storage and protection at depth. However, if a ‘no-till system’ includes crops which return more organic matter than is being decomposed, soil carbon may increase. Even if there are no absolute differences in carbon stocks
between establishment methods, differences in how the carbon is distributed down the profile can be important. For example, having more soil organic matter at the surface can improve surface soil structure and water holding capacity, reducing the risk of erosion and runoff.
Chertsey Establishment Trial
In 2021, soil carbon stocks in the top 7.5 cm (t/ha) of the no-till establishment plots at the Chertsey Establishment Trial were significantly greater than those in the inversion plots, while the inversion plots had significantly greater carbon stocks at 15–30 cm (due to the mixing effect). In absolute terms, carbon stocks down the profile did not differ between the no-till and inversion treatments, but positioning of the soil organic matter down the profile did influence some soil functional attributes such as surface soil structure and water holding capacity. These functional attributes contribute to resilience when the system is under pressure. For example, moisture monitoring from the Chertsey Establishment Trial (December 2021 to January 2022) showed that the no-till plots (NT) had greater volumetric water content (VWC) than the tilled plots (T), particularly in the top 10 cm (Figure 2). This may explain why the yields from the autumn feed wheat in the dryland no-till plots (11.7 t/ha) were significantly greater than those from the dryland cultivated plots (11 t/ha).
FIGURE 2. Volumetric water content (VWC), before and after a rainfall event, at the Chertsey Establishment Trial (December 2021 to January 2022) to 10 cm depth. NT = no tillage. T = inversion tillage.
Reference
IPCC, 2018: Annex I: Glossary [Matthews, J.B.R. (ed.)].
In: Global Warming of 1.5°C. An *IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas
emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R.
*The IPCC (Intergovernmental Panel on Climate Change) is the United Nations body for assessing the science related to climate change.