NZ’s soil scientists are exploring how our soil carbon stocks are changing across a range of agricultural land uses and physical environments, and under different farm-management practices.
Their long-term goal is to confirm actions that NZ farmers can take to maintain or increase their soil carbon stocks. Soil carbon is vital for farm productivity and an important factor in climate change.
The research relies on statistically and scientifically robust data, which needs to be spatially representative because soil carbon can vary significantly across relatively short distances and vertically down through the soil profile.
It also needs to cover timescales long enough to show any meaningful change. That’s particularly important in NZ, because current knowledge suggests our soil carbon stocks under flat-rolling pastoral land are generally quite stable.
In this article, I’ll look at what’s being done at different scales to fill the evidence gaps.
At the national scale, researchers have begun the first phase of a 12-year study funded by the New Zealand Agricultural Greenhouse Gas Research Centre.
It will improve our estimates of soil carbon stocks under the broad agricultural land uses of cropland, perennial horticulture, dairy, flat-rolling drystock and hill-country drystock. More importantly, it will determine whether changes are occurring over time within any of those land uses.
The study will sample about 500 carefully selected sites around the country, to a depth of 60cm.
It will use very precise sampling, measurement and analysis protocols to ensure the data generated are comparable between sites, sampling rounds and other measuring and monitoring programmes.
Data from the study will also improve our estimates of how soil carbon stocks change if land use changes. It will therefore increase the accuracy of national reporting on soil carbon stock changes required under the international climate change agreements NZ has signed.
But what about measuring and monitoring at the individual farm scale?
Many farmers want to know how much carbon their own soils contain and how those stocks are changing.
It’s possible to measure at the farm scale, but the approach isn’t easy and can be expensive if robust data are desired.
So, before beginning, it’s important to be very clear about why you want to measure.
For example, you might simply want a one-off measurement from a single location (such as a point in a paddock) at a single point in time, to get an indication of how much carbon your soils contain.
Or, you might want comprehensive quantification at representative points across your entire farm, starting with an accurate baseline measurement and then repeating measurements with time to determine whether soil carbon stocks have changed.
While costs will vary significantly depending on the number of soil samples needed, the basics of soil sampling and processing are the same regardless of your objective.
They include careful sampling of a known volume of soil (usually with a soil corer, to a minimum depth of 30cm), then sending the sample(s) to a laboratory for processing (which includes weighing, drying and sieving), analysis of organic carbon and calculation of carbon stocks.
A study to accurately quantify carbon stocks across your whole farm and monitor changes through time needs careful design, by a specialist, to ensure enough samples are collected and they represent the range of soil types, slopes and management activities on your farm.
To monitor changes, a minimum of two measurements are required: a baseline and a subsequent sampling, typically three to five years later. However, measuring more frequently (say, every one or two years) will increase confidence that any changes are ongoing.
The number of samples required (and associated costs) will vary depending on the size and topography of your farm, the spatial variability of soil carbon stocks and stock changes, and the study design used.
For example, at least 50 sampling sites would be required to detect a change in soil carbon of about two tonnes per hectare (about a 2% change) on a typical dairy farm, should such a change occur between two sampling times.
More sampling sites would be required on more variable hill country farms.
The cost to design a statistically robust study for a typical dairy farm and undertake benchmark soil sampling, processing and analysis would probably be between $10,000 and $15,000.
Subsequent measurements and analysis to determine whether carbon stocks have changed from the benchmark sampling would cost between $5,000 and $10,000 each.
It’s important to note that statistically robust benchmarking and monitoring over time at the whole farm scale will not, alone, conclusively determine the impact that different management practices have on soil carbon. That’s the subject of other research, described by Louis Schipper in last week’s article.
Farmers interested in finding out more about their options to measure soil carbon are welcome to contact me at firstname.lastname@example.org
You can also view the full report the research team prepared for the Ministry for Primary Industries (MPI) on this subject on the Ag Matters website: www.agmatters.nz/assets/On-farm-soil-carbon-benchmarking-and-monitoring-approach_final-report_June2019-v2.pdf
The acquisition of data at these different scales is enabling scientists to gradually build a clearer picture of how New Zealand’s soil carbon measures up, and where farmers need to focus their attention to maintain or increase their own stocks.
For updates on their work, and for more on soil carbon and other aspects of primary production and climate change, visit www.agmatters.nz
Soil carbon stock changes and NZ’s GHG Inventory
The Ministry for Primary Industries advises that because of the current uncertainty about how New Zealand’s soil carbon stocks are changing within any land use and how changes are influenced by specific management practices, including losses through soil erosion, the New Zealand National Greenhouse Gas (GHG) Inventory does not account for changes in soil carbon stocks within a land use.
Reporting is limited to accounting for soil carbon stock changes when land use changes – for example, from pasture to forest. See the Ministry for the Environment website for more.
Who am I? Dr Paul Mudge is a soil scientist at Manaaki Whenua – Landcare Research in Hamilton, where he conducts and coordinates research on interactions between carbon, nutrient and water cycling in agroecosystems. He is leading the multi-million-dollar long-term national soil carbon monitoring programme funded and coordinated by the New Zealand Agricultural Greenhouse Gas Research Centre (NZAGRC).