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A national soil carbon monitoring system for agricultural land in New Zealand

This study will improve New Zealand’s ability to report greenhouse gas emissions and removals under international climate change agreements and satisfy a growing desire by primary industry organisations and individual farmers to know how New Zealand’s soil carbon stocks might be changing.

Background

Under the United Nations Framework  Convention on Climate Change and the Paris Agreement, New Zealand is obliged to  report annually its national man made (anthropogenic) greenhouse gas emissions  and removals. Currently, national-scale  changes in soil carbon are estimated using a statistical model that predicts changes  in soil carbon for changes between broad land-use classes (e.g. pasture to cropping).  This is a well-established approach, but assumes that  soil carbon is at steady state if the land-use class doesn’t change (even if  management practices within the land use change), and that changes in a land-use  class result in changes in soil carbon over a period of twenty years to a new  steady state.

There is  currently limited data from direct measurements of soil carbon change through  time in New Zealand with which to test these assumptions. The evidence  available indicates that soil carbon is largely constant in flat and rolling  pastoral land, except for organic/peaty soils where soil carbon is declining.  There is some evidence that hill country pastoral soils gained carbon between  about 1980 and 2010, but it is not clear how widespread these  gains were, and whether they have continued since 2010. The information available is also largely based on  historical soil survey sampling sites which were not explicitly selected to be  representative of all agricultural land in New Zealand.

A new approach

The new study is  designed to directly measure changes in soil carbon through time using a statistically  designed framework to ensure unbiased, representative monitoring across all  agricultural land in New Zealand, grouped into the five broad land-use classes:

  1. Cropland
  2. Horticulture
  3. Dairy pasture
  4. Flat-rolling drystock pasture
  5. Hill-country drystock pasture

The approach is guided by best practice as documented  in recent international publications co-authored by New Zealand researchers  (FAO, 2019, Smith et al, 2019). Where practical, monitoring sites will be  randomly selected from nodes on the national 8 km grid system (to match with  previous soil carbon sampling in native forests) using a ‘balanced sampling’  approach. Average soil carbon stocks in New Zealand’s agricultural soils are  about 100 tonnes per hectare in the top 30 cm. The monitoring system is designed  to be able to detect a change of about 2 tonnes of carbon per hectare over 12  years, within each of the five broad land  use classes.

The overall plan is to monitor changes in soil carbon at  a total of 500 sites (about 100 within each land class) across New Zealand  (Figure 1). Baseline soil carbon measurements will be made at 110 nationally  representative sites each year for the next four years to take the total number  of new sites to 440. Sixty sites in hill country drystock pastures were sampled  in 2018 and make up the remainder of the 500 total sites. Sites will then be resampled  using a 4–5 year rolling scheme to determine any change  in soil carbon within each of the five broad land-use classes. An advantage of a  rolling baseline sampling and resampling approach is that field teams will be  sampling a manageable number of sites each year, providing continuity and  avoiding the need to train new teams for a big effort every few years. Obtaining  a nationally representative sample each year also reduces the likelihood that regional  or annual climatic extremes (e.g. drought) will influence results.

Soil profiles and sampling sites will be well described  by pedologists, with data and physical soil samples stored in the National Soil  Database Repository and National Soil Archive respectively. Archived soils will  be a valuable resource for multiple purposes in the future such as calibrating  new sensor technologies. Strict data management and privacy protocols will be  applied.

The monitoring system is largely designed to  provide statistically robust results for the broad land uses and management  regimes at the national scale. If finer resolution is desired (e.g. detection  of changes within specific regions, cropland types, or dairy farming  intensities) then more monitoring sites will be required. Soil carbon data  collected via separate industry or individual farmer initiatives could augment  the national monitoring effort and address additional questions, provided soil  sampling and processing are carried out using consistent methods and are  entered into the national database.

How might you be involved?

Figure 1 shows the approximate distribution  of the randomly selected monitoring sites across New Zealand. If a random  sampling site falls on land you own or manage, someone from the project team (or  an assisting primary industry organisation) will be in touch regarding  permission to collect soil samples.

This initiative will not only provide improved  data for national and international reporting on whether and how soil carbon  stocks in New Zealand are changing, but also indicate to the primary sector where  there could be scope to increase and/or retain soil carbon stocks, with  potential benefits for soil health and the climate.

Figure 1. Approximate distribution of the monitoring sites across New Zealand

Figure 1. Approximate distribution of the monitoring sites across New Zealand

Funding

Manaaki Whenua – Landcare Research has received funding from The New Zealand Agricultural Greenhouse Gas Research Centre (NZAGRC) and the Ministry for Primary Industries (MPI) to commence the first phase of a new nationwide baseline soil carbon measurement study.

References

FAO. 2019. Measuring and modelling soil carbon stocks and  stock changes in livestock production systems: Guidelines for assessment  (Version 1). Livestock Environmental Assessment and Performance (LEAP)  Partnership. Rome, FAO. 170 pp. Licence: CC BY-NC-SA 3.0 IGO.

Smith, P., Soussana, J.-F., Angers, D., Schipper, L.,  Chenu, C., Rasse, D.P., Batjes, N.H., van Egmond, F., McNeill, S., Kuhnert, M.,  Arias-Navarro, C., Olesen, J.E., Chirinda, N., Fornara, D., Wollenberg, E.,  Álvaro-Fuentes, J., Sanz-Cobena, A., Klumpp, K., How to measure, report and  verify soil carbon change to realize the potential of soil carbon sequestration  for atmospheric greenhouse gas removal. Glob Change Biol. 2019; 00: 1– 23. https://doi.org/10.1111/gcb.14815.

The  national soil carbon monitoring project has been developed as a collaboration  between Manaaki Whenua and the University of Waikato, and funded by the NZAGRC  and MPI. Funding for the initial statistical design phase came from the  Ministry for Primary Industries via the Global Research Alliance on Greenhouse  Gases.

 

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