It’s important we know what’s happening to soil carbon stocks in New Zealand, because a small increase or decrease could have significant impacts on carbon footprints at farm, industry and national scales.
The National Soil Carbon Monitoring Project, a collaboration between Manaaki Whenua and the University of Waikato, and funded by the Ag Emissions Centre, is currently assessing whether soil carbon stocks under New Zealand’s agricultural land are increasing or decreasing, and whether land use influences any change that may be occurring. Data will also improve our ability to predict how SOC stocks are likely to change when land use changes and these will inform improvements to the national soil carbon inventory model.
In previous issues of Pūtaiao we have reported on progress in benchmarking SOC stocks across New Zealand. Samples were collected from 500 representative agricultural sites across the country, and analysed using consistent methods over a period of 6 years, which is an impressively constrained timeframe especially given interruptions to data collection caused by COVID and by extreme environmental events such as Cyclone Gabrielle during this time.
Now, we are pleased to report, the full results of the benchmark sampling are in! For the first time, spatially representative measurements of SOC stocks for agricultural land on mineral soils are available for New Zealand, separated into five broad land-use classes.
Mean slope-corrected SOC stocks for the 0–30 cm depth for all agricultural land on mineral soils in New Zealand are 101.3 tC/ ha. The 30–60 cm layer contributed on average a further 36.9 tC/ha. Stocks for the 0–30 cm layer were highest under dairy pasture (110.7 tC/ha), followed by hill country drystock pasture (104.3 tC/ ha), flat-rolling drystock pasture (98.7 tC/ ha), and perennial horticulture (84.8 tC/ ha), and were lowest under cropland (80.1 tC/ha). The differences between broad land use classes cannot be attributed to land use alone, since the location of different land uses is often related to soil type and climate. For example, there are proportionally more dairy farms on Allophanic Soils than for the other land-use classes and Allophanic Soils have naturally high background soil carbon stocks.
These data provide a crucial benchmark against which to compare future sampling, to determine whether SOC stocks are changing in agricultural land on mineral soils across the country.
Having reached this milestone, the intended next steps will be:
- Resampling all 500 sites by 2028 (4–6 years after the benchmark sampling) to determine whether SOC stocks are changing through time.
- Retaining the samples in the National Soil Archive to allow further analyses as new questions and technologies arise.
- Consider expanding the scope of the survey to include carbon stocks on drained Organic Soils used for agriculture in New Zealand.
