Soil carbon – measure to manage

There is more carbon in the top one metre of soil than there is in all above-ground vegetation and in the atmosphere. Could we manage our farms to increase soil organic carbon levels, potentially off-setting the emissions from other farm activities?

The Food and Agriculture Organization of the United Nations (FAO) estimates that historically, land-use conversion and soil cultivation have been responsible for about one-third of greenhouse gas emissions to the atmosphere. It also estimates that soils can sequester around 20Pg (billion tonnes) of carbon in 25 years, more than 10% of the anthropogenic emissions. The opportunity, then, is to improve management of agricultural land to mitigate climate change by reducing greenhouse gas emissions and storing more carbon in soils.

To take advantage of this opportunity, first we need an accurate understanding of current soil carbon stocks. The existing national Soil Carbon Monitoring system provides estimates of changes in soil carbon stocks with land use change at a national scale but is not designed for farm-scale estimations.

Trans-Tasman collaboration

We have used Landcare Research Core funding, coupled with Ministry for Primary Industries (MPI) Global Research Alliance funds, to collaborate with the University of Sydney and CSIRO, Australia, to develop innovative methods for mapping soil organic carbon stocks at the farm scale.

We are using the New Zealand Agricultural Gas Research Centre (NZAGRC) national soil carbon model to derive baseline values for a property. We then use finer scale environmental data to apportion the baseline value within the property boundary.

The finer scale environmental data that we are using includes elevation and electromagnetic data. Airborne LiDAR surveys provide very accurate elevation data, typically at one square metre resolution, and we obtain further attribute datalayers from the digital elevation map, which include slope, aspect, wetness and radiation incidence. These attributes relate to organic matter accumulation and decomposition and improve the performance of the spatial prediction model of soil carbon stocks.

A statistical analysis of the spatial layers is used to decide on soil sampling positions. Soils are scanned in the field at these positions using a sensing technique called Vis-NIR spectroscopy. This is a cost effective approach allowing more geo-referenced soil carbon values to be determined for the same cost as traditional lab analytical methods. The method scans soil cores at 1cm intervals, improving depth resolution, and detecting subtle but important changes that may occur centimetre by centimetre down the soil profile. This will help improve our understanding of soil processes at resolutions that are largely impractical using standard laboratory methods.

While it is being developed for soil organic carbon, the method can also be applied to mapping other soil attributes, such as texture and moisture characteristics.

We are liaising closely with MPI over our new techniques for farm-scale soil carbon analysis. A key reason that soil carbon is not currently part of the Emissions Trading Scheme (ETS) is that the uncertainty of estimating soil carbon stock changes is considered to be too large. This research aims to reduce the uncertainty of farm-scale soil carbon stock predictions.

Critical information for farmers

There is real value in a farmer having a detailed understanding of soil organic carbon stocks on their property and being able to monitor changes over time. It is the critical information required for change management decisions to sequester soil carbon, remove carbon dioxide from the atmosphere, and to maintain or enhance soil quality.

Accurate, internationally-respected techniques for measuring soil carbon will support farmers’ carbon accounting and carbon farming initiatives.

As well as opening the door to a much-needed greenhouse gas mitigation approach, the ability to audit changes in soil carbon levels will also help farmers’ understanding of the health of their soils.

The continued degradation of global soils due to depletion of organic carbon levels through intensive land use and cultivation is now generally acknowledged as a concern. It is being addressed by new ecological economics concepts such as ‘soil security’¹ and ‘soil natural capital’². These concepts aim to provide effective assessment of the soil to support global food security requirements, as well as sustainability of the soil resource for global population well-being.

References

¹ McBratney, A., Field, D.J., Koch, A 2014 The dimensions of soil security. Geoderma 213: 203–213
² Costanza, R, de Groot, R, Sutton, P, van der Ploeg, S, Anderson, S.J., Kubiszewski, I., Farber, S., Turner, K, 2014 Changes in the global value of ecosystem services. Global Environmental Change 26: 152–15