A new book from Burleigh Dodds Science Publishing Limited:

Chapter Title: Mechanisms of soil organic carbon sequestration and implications for management*

Authors: Ingrid Kögel-Knabner, Technical University of Munich, Germany; Martin Wiesmeier, Technical University of Munich and Bavarian State Research Center for Agriculture, Germany and Stefanie Mayer, Technical University of Munich, Germany

*This chapter features in our book: ‘Understanding and fostering soil carbon sequestration’.

Introduction and definitions

A major potential for increasing carbon sequestration in mineral soils is in agricultural systems under cropland use (Amelung et al., 2020). Understanding organic carbon (OC) sequestration in (mineral) soils requires considering the pathways and the associated different types of organic matter (OM) input. As
pointed out in Box 1, OC sequestration refers to ‘the process of transferring CO2 from the atmosphere into the soil of a land unit, through plants, plant residues and other organic solids which are stored or retained in the unit as part of the soil organic matter (humus)’ (Chenu et al., 2019; Olson et al., 2014). If we accept this definition, all processes are relevant that lead to a storage or retention of OC in soils. A number of mechanisms have been described that lead to the retention of OC in soils. As the OC that enters the soil is in dynamic equilibrium, all the different OM pools that are retained in a soil must be considered. SOC sequestration implies raising soil organic carbon (SOC) levels, where they are currently undersaturated, and to maintain maximal OC levels in well-managed soil systems (Lehmann et al., 2020).

Olson et al. (2014) pointed out that it is essential to strictly differentiate between the application of any of OM to soils from sources external or outside a land unit (e.g. amendments like manure, compost, biochar) and OC sequestration sensu strictu. Sequestration of OC in soils as defined here (Box 1) requires that atmospheric CO2 is fixed through photosynthesis and stored in the soil. No atmospheric CO2 is converted and stored as a result of amendment transfer and it does not add to reducing atmospheric CO2 levels. Therefore, we will not consider the application of organic amendments in this chapter. As organic amendments may in specific cases influence SOC sequestration through their impact on plant growth and soil microbial functioning, their management is discussed in Chapter 9 of this book.

Organic matter input to soils

Organic C enters the soil mainly as:
• aboveground litter or crop residues,
• belowground litter or crop residues, and
• rhizodeposition.

Both above and belowground litter or crop residues are mainly composed of OC bound in large polymers (celluloses, hemicelluloses, lignin, cutin, suberin) in leaves, stems, twigs and other woody debris, or roots, with only a small contribution of low-molecular weight organic components (Kögel-Knabner, 2017). They are either deposited on the mineral soil surface, or in different soil depths as root litter. Incorporation of OC from aboveground litter occurs via bioturbation or leaching of soluble components. In contrast, rhizodeposition consists mainly of low molecular weight compounds released from roots into the surrounding soil at various depths.

Long-term OC storage in soils occurs primarily when OC derived from plant biomass is stabilized in soils as soil OM. Plant biomass makes up the majority of OC input also to agricultural soils. But we have to take into account that OM is also added to cultivated soils through fertilization and waste disposal (e.g. liquid manure, compost, sludge, animal excreta, biochar, biogas digestate), which contribute significant amounts (Jacobs et al., 2020). Soils are often also contaminated with organic constituents from the petroleum
and coal chemistry/industry, as well as from coal combustion, e.g. tar oil, coal dusts, black carbon, specifically in industrial-urban areas (Kiem and KögelKnabner, 2003; Schmidt and Noack, 2000), as well as plastics (Rillig et al., 2021). Geogenic C such as kerogen or black shale can also be inherited from the parent material (Fox et al., 2020 and references therein). This short listing demonstrates the large diversity of OM input to soils. Lehmann et al. (2020) suggest that the molecular diversity of the organic compounds rather than the material properties of individual compounds controls decomposition in soils. As pointed out above, the amendments help to increase the OC content and stocks of a soil, but may not help sequestering OC in soils. At the same time, it is important to return organic residue materials to soils, rather than burning them or using them otherwise, e.g. for energy production or production of chemicals.

Read the full chapter here