Key Considerations in Sequestering Carbon as Soil Organic Carbon

Photosynthesis:

Photosynthesis is the main process that sequesters (stores) carbon in soils. Plants absorb carbon from the atmosphere as carbon dioxide and convert it into plant biomass, which becomes soil organic carbon (SOC). The rate of conversion of plant biomass to SOC limits the rate at which carbon is sequestered in soils.

To increase SOC by 0.1% in the top 10 cm of the soil requires a large amount of plant biomass, with sufficient nutrients such as N, P and S, to form new SOC. Roots play a major role in the transfer of plant biomass to SOC.

Management:

Most cropping and agricultural areas have greatly reduced, or even ceased, exploitive land management practices such as intensive conventional tillage, with early hot stubble burns and numerous tillage operations with disc ploughs. Modern cropping and agricultural practices are much less intensive and exploitive. The widespread adoption of these improved, more sustainable practices means there is minimal capacity for further changes in land management practices to sequester SOC. Sequestering carbon by changing cropping practices now requires even more attention to agronomy, the management of nutrients and the refinement of pasture and grazing practices: even then, gains may be minimal.

To effectively sequester carbon as SOC through changes in land management practices requires a knowledge of the initial levels of SOC at a given locality and the expected levels after the changes take effect.

Economics:

A potential limitation to the development of carbon sequestration projects is the expected return per hectare from trading in carbon. A recent trading price for carbon in Australia was $12 per tonne of CO₂ equivalent[1], which is equivalent to about 3.66 x 12 = $44/tonne of C sequestered (CER 2019[2]). Given that the maximum rate of carbon sequestration appears to be 0.5–1.0 Mg C/ha/yr, this provides an annual income based on carbon trading of $22 to $44/ha. This is a lower return than can be achieved by cropping and/or grazing, which is often in the range of $400 to $800/ha/yr. Therefore, carbon trading projects designed to sequester carbon as SOC need to be undertaken in conjunction with, or in addition to, existing cropping and grazing agricultural enterprises.

Measurement:

The measurement of SOC is a critical requirement in the management of soil carbon. However, it is important to know the purpose for which the measurements are being made.

If the measurements are being made to assess soil health or soil condition, the measurement of soil carbon percentage as grams of carbon per 100g of oven dry soil is sufficient (usually expressed as SOC%). The cost of this measurement is low.

However, if the object is to trade in carbon and receive payment for sequestering carbon it is necessary to measure SOC stocks (tonnes C/ha/depth). This is a costly measurement as it requires the measurement of soil bulk density and soil sampling strategies that meet a higher standard of statistical rigour.

If local soil conditions are known, it may be possible to use SOC% to obtain an estimate of the initial soil carbon conditions and stocks to assess the feasibility of a proposed carbon sequestration project. Local knowledge can help identify what levels of SOC are achievable based on soil type and climate.

Soil sampling depth is also a consideration: most (but not all) changes in soil organic carbon occur in the top 30 cm of the soil profile and it can become very costly to measure soil carbon to greater depths.

Potential:

The possibility of increasing SOC levels will be better where soils currently have low SOC due to overcropping and/or overgrazing but also have a high potential to produce biomass.

Areas of higher rainfall and moderate climate favour plant growth and potentially offer more opportunities to sequester carbon by changing land management as there are larger differences between soil carbon levels due to land management. In lower rainfall areas, the potential to sequester SOC is more limited, other than on degraded sites.
Land with better quality soils and higher land use capability provides improved chances of responding to climate potential by improving biomass and therefore increasing SOC.
Land which has been denuded and scalded due to land management history and erosion and is currently unproductive will respond to increased biomass if the moisture regime is improved. A specific example, confirmed by scientific studies, is water ponding on scalded land in semi-arid areas of NSW. Scalded soils have very low SOC levels and revegetation by ponding rainwater can significantly increase these levels. The land management system for water ponding and its specific operations have been clearly defined and well documented.

Land management scenarios which do not reflect these potential scenarios may have limited ability to sequester additional levels of SOC. In particular, where strong adoption of improved and appropriate land management practices consistent with sustainable agriculture has already occurred. In these cases, carbon sequestration has reached a threshold which is governed by natural limitations.

Key Criteria for Projects to Sequester Carbon as SOC:

General Criteria

For a proposed project to successfully sequester carbon as SOC, clearly the initial levels of SOC must be less than those expected under the changed land management systems and practices. Those land management practices and operations that have provided strong evidence for sequestering carbon as SOC are therefore most important. The general requirements are:

The proposed land management systems must be clearly defined, including the land management operations that are to be implemented.
There must be strong evidence that the land systems and land management operations will sequester carbon as SOC for the specific climate and soil types.
It is essential to measure the initial levels of SOC to clarify the capacity of the proposed land management systems to sequester carbon as SOC.
A rigorous accounting system is required to be able to demonstrate changes in SOC where carbon trading and rewards for improving SOC are expected.
A rigorous field sampling methodology must be employed to assess trends in SOC over time. This must adequately account for field and seasonal variability, most particularly if carbon trading and rewards for improvements in soil carbon are expected.

Examples of Projects to Sequester Carbon as SOC:

Water Ponding

Water ponding on scalded land in the semi-arid areas of NSW is a documented example of a potentially successful project to sequester carbon as SOC (see references 1, 2, 3, 4). The land management system of water ponding and its operations are clearly defined. The degraded, scalded soils to which water ponding is applied have very low SOC levels and revegetation after water ponding can provide significant increases in SOC.

CAMBI Project

A pilot scheme was developed in Central West New South Wales to trial the use of a market-based instrument to encourage farmers to change farm management and to increase SOC levels (see references 5,6,7).

The changes in SOC stocks on the farms that were successfully contracted for the pilot were based on the initial measured levels of SOC and the predicted levels of SOC after 5 years. The 10 contracted farms were those that submitted the lowest bid per Mg CO2-e.

Four land uses were specified in the pilot:

Reduced tillage cropping;
Reduced tillage cropping with organic amendments (e.g. biosolids or compost);
Conversion from cropping land to permanent pasture;
Conversion from cropping land to permanent pasture with organic amendments.

At each site a minimum of 10 locations were sampled and soil was analysed for total carbon (LECO elemental analyser) and soil bulk density was measured. The SOC stocks (0–0.3m soil depth) were assessed before and after the pilot (2012-2017) and calculated on the initial equivalent soil mass, with 60% of sites showing a significant increase in SOC stocks. Pasture had a higher rate of SOC sequestration than reduced tillage cropping and sites with organic amendments had higher rates of SOC sequestration than without organic amendments. The results of the pilot demonstrated increases in SOC, using quantification methods consistent with the current Measurement Method of the Australian Government’s Emissions Reduction Fund Policy used to generate Australian Carbon Credit Units.

Further information: Questions about the sequestration of soil carbon – some comments about the Soil Carbon Forum 22nd April 2021

References

1. Cunningham, G.M. (1987). Reclamation of scalded land in western New South Wales: a review. Journal of Soil Conservation, New South Wales, 43 (2), 52-61.

2. Thompson, R. (2008). Waterponding: reclamation technique for scalded duplex soils in western New South Wales rangelands. Ecological Management and Restoration, 9(3), 170-181.

3. Read, Z.J., Murphy, B. and Greene, R.S.B. (2012). Soil carbon sequestration potential of revegetated scalded soils following waterponding. Australian and New Zealand Soils Conference, Hobart, 2-7 December 2012.

4. Ringrose-Voase, A.J., Rhodes, D.W. and Hall, G.F. (1989). Reclamation of a scalded, red duplex soil by waterponding. Australian Journal of Soil Research, 27, 779-795.

5. Lorimer-Ward, K., Badgery, W., Crean, J., Murphy, B., Rawson, A., Pearson, L., Simmons, A., Andersson, K., Warden, E., Packer, I., Trengove, D. and Kovac, M. (2013). Bridging the gap between science, economics and policy to develop and implement a pilot Market Based Instrument for soil carbon. In: Proceedings 22nd International Grassland Congress, 15–19 September 2013, Sydney, NSW. (Eds D. Michalik, G. Millar, W. Badgery and K. Broadfoot), 1811–1815. (New South Wales Department of Primary Industries, Orange).

6. Badgery, W., Murphy, B., Cowie, A., Orgill, S., Rawson, A., Simmons, A. and Crean, J. (2020). Soil carbon market-based instrument pilot – the sequestration of soil organic carbon for the purpose of obtaining carbon credits. Soil Research – https://doi.org/10.1071/SR19331

7. Pearson, L.J., Crean, J., Badgery, W., Murphy, B., Rawson, A., Capon, T. and Reeson, T. (2012). Soil carbon sequestration in mixed farming landscapes: insights from the Lachlan Soil Carbon Project. In: Proceedings of the 56th AARES Conference, 7–10 February 2012, Fremantle, WA.