Carbon sequestration in the soil

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The carbon bond in the ground ( English soil carbon sequestration , SCS for short) refers to methods of soil management , the organic carbon content increase of the soil, by the earth's atmosphere net CO 2 and other environmentally harmful are removed from carbon compounds. Because of this net carbon extraction, SCS is a negative emission technology .

Methods of sequestering carbon in the soil

How much carbon remains in the soil depends on the balance of carbon inputs into the soil (e.g. from crop residues such as stubble or roots , or manure such as manure ) and the carbon losses from the soil (mostly through respiration , aggravated by soil disturbances). In addition to carbon dioxide, methane (CH 4 ) also plays an important role here. Therefore, practices that either increase inputs, decrease losses, or both can promote SCS. SCS is thus a collection of management types that influence the balance between carbon inputs and outputs in favor of carbon formation in the soil.

This includes practices, in which it is assumed that they net out the bottom of carbon: use of cover crops and leaving crop residue on the field , recovery of organic residue on the field by fertilization (here, the fertilizers are farmyard manure , liquid manure , compost typical) , Planting deep-rooted plants , grass cover leys in crop rotations , agroforestry , diversified crop rotations, deep plowing and no-till methods .

For example , at the Chair of Forest Economics, Faculty of Economics, Technical University of Munich , the carbon sequestration in Cameroon in forest management was examined as part of a dissertation in 2016 .

Methods of calculation

The Federal Office for the Environment (FOEN) in Switzerland uses the digital soil map and two methods for categorizing land use , NOLU04 (with 46 categories) and an AREA-derived method (with 27 categories) to define its own soil category system: the so-called combination categories (CC). Two of these combination categories are arable land (CC number 21) with 390 kha in 2017 and grassland (CC number 31) with 922 kha in 2017.

Farmland and grassland represent the types of soil to which estimates for optimal SCS rates from the literature can be applied. A French study estimated the potential for SCS on agricultural land at 0.63 t C per hectare per year (note that here C means carbon and not CO 2 ). A linear extrapolation of this potential to the entire Swiss arable land results in a potential of 925 kt CO 2 per year. Swiss long-term tests for SCS on grassland show an SCS potential of 0.28 t C per hectare and year.

Current carbon sequestration using the example of Switzerland

The annual changes in carbon stocks in Swiss soils have been calculated using models by Agroscope, part of the EAER , since 2019 . These data are combined with other data in a report on Switzerland's greenhouse gas inventory, an annual publication by the FOEN. Various components of the changes in carbon stocks in Swiss soils are taken into account. A net increase in carbon stocks in soils is interpreted as the rate of SCS in Switzerland resulting from the few land management techniques currently in use (which are not optimized for SCS). According to the greenhouse gas inventory, the SCS rate in Switzerland was negligibly low from 2000 to 2020.

This results in a Swiss grassland potential of a total of 945 kt CO 2 per year. Together, arable land and grassland result in a potential of 1.87 million t CO 2 per year. This corresponds to around 4–5% of the total production- related CO 2 emissions in Switzerland in 2018. These estimates are between that of Smith et al. specified minimum of 0.03 t C per year and the maximum of 1 t C per hectare and year for arable land and grassland on a global average and are therefore considered plausible.

The combined potential for SCS in Switzerland - both from agriculture (0.7 million t CO 2 per year) and from the soil (1.9 million t CO 2 per year) - is based on estimates from 2019 to 2.6 million t CO 2 per year. This estimate is the result of a combination of data from the Federal Office for the Environment (FOEN) on arable land and estimates of the carbon sequestration potential of the soil per hectare (ha) from the literature for geographical locations with similar climatic and geological properties to those of Switzerland. This corresponds to around 6–7% of Switzerland's annual CO 2 emissions in 2018.

Contribution from deep plowing

Deep plowing

Another land management approach, deep plowing , could offer additional SCS potential. Studies from Germany and New Zealand show that the relocation of not easily degradable carbon to greater depths of the soil, where it is stored due to longer residence times, can deliver substantial sequestration gains. Beuttler et al. estimate that the application of this deep plowing technique on 5000 ha of soil could offer an annual sequestration potential of 15.4 million tons of CO 2 over 20 years. This corresponds to an annual potential of 770 kt CO 2 per year.

With deep plowing, the cumulative SCS total in Switzerland over two decades would be 15.4 million t CO 2 . In addition, SCS practices would need to be maintained to avoid carbon re-entering the atmosphere: there is a risk of carbon gains reversing if practices are not stabilized. It also means that once the soils are saturated, the costs associated with SCS practices will persist.

In some soils there is also the risk that an existing humus layer will be destroyed, which has a negative effect on soil fertility .

Saturation of carbon uptake in soils

The estimates have one major caveat: carbon stocks in soils tend to reach a saturation point. As soon as this saturation point is reached, further carbon inputs cease to translate into a higher carbon content of the soil. Beuttler et al. estimate that at the rates considered here, the soils will be saturated after about two decades. However, the estimation of the saturation time is uncertain. This is because the point in time at which saturation is reached mainly depends on the current C-storage compared to the potential of the maximum C-storage. For example, West and Post estimated that in 67 long-term experiments, the time to saturation for soils with crop rotation and no-tillage is around 15 years. Smith estimates that soil will become saturated with carbon after 10-100 years, depending on the soil, climate, and SCS characteristics. The IPCC uses a standard saturation time of 20 years.

Global SCS potential

The global SCS potential is orders of magnitude greater than the Swiss potential. The literature review by Fuss et al. Over twenty-three different studies give an estimate of the mean global SCS potential of 4.28 Gt CO 2 per year and a mean potential of 3.68 Gt CO 2 per year. This corresponds to around 9-11% of current global emissions. A more recent estimate by Lal shows a much higher potential of around 9 Gt CO 2 per year, which is around 23% of global emissions per year. Lenton estimates that a maximum annual potential of approx. 3.3 Gt CO 2 per year can be achieved for approx. 3.2 Gt CO 2 per year. 12.5 years can be achieved. It should be noted, however, that due to saturation effects and the possible re-release of carbon after the end of the SCS practice, the total cumulative potential of SCS is limited.

Cost of implementing SCS

The scientific literature gives several estimates of the cost of a tonne of soil-bound carbon dioxide, but these are highly dependent on geographic location and soil composition. In the review by Fuss et al. only three papers were found that provide estimates for the cost of SCS. According to the authors' estimates, around 20% of global SCS could be realized at negative costs of between -45 $ and 0 $ per t CO 2 -Eq. lie. Around 80% could cost between $ 0 and $ 10 per t CO 2 -eq. will be realized. The total cost of global implementation under these conditions would be $ 7.7 billion. These estimates suggest great potential for scalability. In Switzerland, the only cost estimate for SCS is from Beuttler et al. and amounts to 0–80 CHF per t CO 2 . These estimates ignore the opportunity cost of carbon: the cost of climate damage caused by not implementing SCS. These costs are considerable: Nordhaus estimates them at around $ 30 per tonne of CO 2 .

Technical challenges and risks

A key challenge for the implementation of SCS is the validation or independent verification of successful sequestration of carbon in the soil. This is seen as a prerequisite for the formation of a viable market. The development of methods for the cost-effective measurement of soil carbon is an active area of ​​research.

Avoiding the emission of other greenhouse gases (e.g. N 2 O) is a potentially undesirable side effect of SCS. Smith notes that many of the negative effects can be overcome with an appropriate portfolio of SCS techniques.

It remains uncertain where the saturation levels for carbon retention are in any particular soil type.

Another challenge is the reversibility of carbon sequestration in the soil. Soil carbon sequestration is susceptible to reversal if soil management techniques are adversely altered. It is expected that a re-release would occur within years. It also remains uncertain how the lack of durability of the SCS-derived soil carbon can be addressed by various methods, e.g. B. through approaches that increase the long-lived carbonaceous components in the plant roots.

Soil carbon enrichment requires the addition of plant nutrients, especially nitrogen, phosphorus and potassium. The addition of these nutrients without proper management techniques could exacerbate fertilizer leaching into watercourses.

See also

Individual evidence

  1. ↑ a b c d e f g Sabine Fuss, William F Lamb, Max W Callaghan, Jérôme Hilaire, Felix Creutzig: Negative emissions — Part 2: Costs, potentials and side effects . In: Environmental Research Letters . tape 13 , no. 6 , May 21, 2018, p. 063002 , doi : 10.1088 / 1748-9326 / aabf9f .
  2. ↑ a b c d e Christoph Beuttler, Sonja G. Keel, Jens Leifeld, Martin Schmid, Nino Berta, Valentin Gutknecht, Nikolaus Wohlgemuth, Urs Brodmann, Zoe Stadler, Darja Tinibaev, Dominik Wlodarczak, Matthias Honegger, Cornelia Stettler: The Role of Atmospheric Carbon Dioxide Removal in Swiss Climate Policy . Ed .: Federal Office for the Environment FOEN. Bern August 2019.
  3. ↑ https://www.fwl.wzw.tum.de/forschung/dissertationen.html
  4. ↑ a b Federal Office for the Environment FOEN: Switzerland's Greenhouse Gas Inventory 1990–2017 . ( admin.ch ).
  5. ↑ Bénédicte Autret, Bruno Mary, Claire Chenu, May Balabane, Cyril Girardin: Alternative arable cropping systems: A key to increase soil organic carbon storage? Results from a 16 year field experiment . In: Agriculture, Ecosystems & Environment . tape 232 , September 16, 2016, ISSN  0167-8809 , p. 150-164 , doi : 10.1016 / j.agee.2016.07.008 .
  6. ↑ Sonja G. Keel, Thomas Anken, Lucie Büchi, Andreas Chervet, Andreas Fliessbach: Loss of soil organic carbon in Swiss long-term agricultural experiments over a wide range of management practices . In: Agriculture, Ecosystems & Environment . tape 286 , December 1, 2019, ISSN  0167-8809 , p. 106654 , doi : 10.1016 / j.agee.2019.106654 .
  7. ↑ National Soil Monitoring (NABO). Agroscope, accessed on May 16, 2020 .
  8. ↑ a b c d e Pete Smith: Soil carbon sequestration and biochar as negative emission technologies . In: Global Change Biology . tape 22 , no. 3 , 2016, ISSN  1365-2486 , p. 1315-1324 , doi : 10.1111 / gcb.13178 .
  9. ^ A b c Pete Smith, Daniel Martino, Zucong Cai, Daniel Gwary, Henry Janzen: Greenhouse gas mitigation in agriculture . In: Philosophical Transactions of the Royal Society B: Biological Sciences . tape 363 , no. 1492 , February 27, 2008, p. 789-813 , doi : 10.1098 / rstb.2007.2184 , PMID 17827109 .
  10. ↑ Viridiana Alcántara, Axel Don, Reinhard Well, Rolf Nieder: Deep plowing increases agricultural soil organic matter stocks . In: Global Change Biology . tape 22 , no. 8 , 2016, ISSN  1365-2486 , p. 2939-2956 , doi : 10.1111 / gcb.13289 .
  11. Jump up ↑ Marcus Schiedung, Craig S. Tregurtha, Michael H. Beare, Steve M. Thomas, Axel Don: Deep soil flipping increases carbon stocks of New Zealand grasslands . In: Global Change Biology . tape 25 , no. 7 , 2019, ISSN  1365-2486 , p. 2296-2309 , doi : 10.1111 / gcb.14588 .
  12. ↑ Annie Francé-Harrar : The last chance - for a future without need , new edition 2007, page 564
  13. ↑ Martin Wiesmeier, Rico Hübner, Peter Spörlein, Uwe Geuß, Edzard Hangen: Carbon sequestration potential of soils in southeast Germany derived from stable soil organic carbon saturation . In: Global Change Biology . tape 20 , no. 2 , 2014, ISSN  1365-2486 , p. 653-665 , doi : 10.1111 / gcb.12384 .
  14. ^ Tristram O. West, Wilfred M. Post: Soil Organic Carbon Sequestration Rates by Tillage and Crop Rotation . In: Soil Science Society of America Journal . tape 66 , no. 6 , 2002, ISSN  1435-0661 , p. 1930-1946 , doi : 10.2136 / sssaj2002.1930 .
  15. ↑ a b Timothy M. Lenton: The potential for land-based biological CO 2 removal to lower future atmospheric CO 2 concentration . In: Carbon Management . tape 1 , no. 1 , October 1, 2010, ISSN  1758-3004 , p. 145-160 , doi : 10.4155 / cmt.10.12 .
  16. ↑ a b Pete Smith: Agricultural greenhouse gas mitigation potential globally, in Europe and in the UK: what have we learned in the last 20 years? In: Global Change Biology . tape 18 , no. 1 , 2012, ISSN  1365-2486 , p. 35-43 , doi : 10.1111 / j.1365-2486.2011.02517.x .
  17. ^ William D. Nordhaus: Revisiting the social cost of carbon . In: Proceedings of the National Academy of Sciences . tape 114 , no. 7 , February 14, 2017, ISSN  0027-8424 , p. 1518–1523 , doi : 10.1073 / pnas.1609244114 ( pnas.org [accessed May 16, 2020]).
  18. ↑ Pete Smith, Jean-Francois Soussana, Denis Angers, Louis Schipper, Claire Chenu: How to measure, report and verify soil carbon change to realize the potential of soil carbon sequestration for atmospheric greenhouse gas removal . In: Global Change Biology . tape 26 , no. 1 , 2020, ISSN  1365-2486 , p. 219-241 , doi : 10.1111 / gcb.14815 .