ReviewThe knowns, known unknowns and unknowns of sequestration of soil organic carbon
Graphical abstract
Highlights
► Soil has great potential to become a managed sink for CO2 in agroecosystems. ► Increasing the SOC pool is also beneficial to secure soil fertility. ► This review discusses perspectives and practices to sequester more soil carbon. ►It also highlights methods and models used to estimate soil carbon pools. ► This knowledge and technology underpins decisions to protect the soil resource.
Introduction
Approximately 8.7 Gt (1 gigaton = 1 billion tonnes) of carbon (C) are emitted to the atmosphere each year on a global scale by anthropogenic sources (Denman et al., 2007, Lal, 2008a, Lal, 2008b). However, the atmospheric increase has been in the order of 3.8 Gt C yr−1 (rate of increase in the year 2005, Denman et al., 2007), highlighting the important regulatory capacity of biospheric C pools (Le Quéré et al., 2009). In this context, soil organic carbon (SOC) and its potential to become a ‘managed’ sink for atmospheric carbon dioxide (CO2) has been widely discussed in the scientific literature (e.g. Kirschbaum, 2000, Post and Kwon, 2000, Guo and Gifford, 2002, Lal, 2004a, Lal, 2004b, Lal, 2008a, Lal, 2008b, Post et al., 2004, Post et al., 2009, Smith, 2008, Chabbi and Rumpel, 2009, Luo et al., 2010b). Here, we use the term SOC to define C in soil derived from organic origins. The term soil organic matter (SOM) is also used frequently in the literature and is generally agreed to contain about 58% SOC (i.e. elemental C). SOM is a mixture of materials including particulate organics, humus and charcoal along with living microbial biomass and fine plant roots.
To reward ‘good’ management of the soil C pool leading to enhanced soil carbon sequestration (SCS), there are a number of overarching questions that need to be considered in relation to the potential of the soil–plant system to ‘sequester’ organic C, where sequestering soil carbon requires a stipulated duration timeframe (usually 100 years) in order to be considered a ‘permanent’ increase under managed agricultural systems. SCS implies an increase in soil C for a defined period against a baseline condition where the increased C is sourced from atmospheric CO2. This implication helps to frame the following questions:
What is the value in increasing the inputs to soil organic matter aside from its role in potential sequestration? For how long must this increase be maintained to be considered as SCS? Consequently, what is more important, long-term SCS or the functioning of the soil? Are these roles essentially inseparable?
How can the benefits of SCS be promoted among policy makers/farmers/landholders (i.e. the potential of SCS to mitigate climate change, the use of SCS as a platform for sustainable agriculture) and how can suitable answers to questions such as measurement, modelling, monitoring and permanence for SCS and/or management advice be followed through? Is there a need to improve models of SOM dynamics in order (i) to demonstrate better understanding of the functioning of the soil ecosystem and (ii) to better assist landholders/farmers with management decisions? Can both of these questions be addressed within the same model?
This article synthesizes current soil C research and highlights a number of key research areas.
Section snippets
Soil—A terrestrial pool of organic carbon
Globally, the quantity of C stored in the soil is second only to that in the ocean (38,400 Gt). While the terrestrial biotic C pool is ∼560 Gt of organic C (Fig. 1), the soil C pool is more than four times this figure. The organic C pool capacity of world soils has been variously estimated for principal biomes (refer to Table 1). For instance, approximately 2344 Gt of organic C is stored in the top three meters of soil, with 54% or 1500 Gt of organic C stored in the first meter of soil and about 615
SOC pools
As described above, SOM consists of a complex mixture of (partially) decomposed substances (i.e. organic molecules such as polysaccharides, lignin, aliphatic biopolymers, tannins, lipids, proteins and aminosugars) derived from plant litter as well as faunal and microbial biomass (Totsche et al., 2010). It also represents a variety of pools that are related to microbial function (Krull et al., 2003, Trumbore, 2009). These pools are divided according to biological stability (labile, stabile,
Model characteristics
There is a need to assess SOM dynamics and the sequestration potential of soil C at the landscape scale (Post et al., 2007) as well as simulate the response of soils to environmental pedoturbation (Smith et al., 1998). As well as measuring SOM, models of SOM dynamics are used to address the needs listed above (Post et al., 2007, Batlle-Aguilar et al., 2010). Ultimately, they should provide reliable predictions to the size of soil C stocks for different soil types, with differing management
Practical measures for enhancing soil carbon
SOC has received increasing attention due to its potential capacity to play an important role in mitigating (human) GHG emissions (Wander and Nissen, 2004). At a global scale, this is due to:
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The large size of the soil C pool compared to other biologically ‘active’ pools (Paustian et al., 2000);
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The estimated loss of organic C from the soil pool due to anthropogenic influences over the last century. For instance, globally around 42–78 Gt of C have been lost due to soil management practices and
Conclusions
The ‘future of SOC research’ requires collaboration and communication between the ‘science community’ (Fig. 6a) and the ‘practice sector’ (Fig. 6c), facilitated by individuals that are knowledge brokers (Fig. 6b) as defined by Bouma et al. (2011) with “hard knowledge and social intelligence”. In order to be applicable to the practice sector, new findings in SOC dynamics need to be addressed in a conceptual framework for communicating the need for change as being ‘compatible’, ‘observable’,
Acknowledgements
This review paper stems from the Soil Carbon Summit held in Sydney, Australia, in January 2011. The Summit was convened by the Soil Carbon Initiative, a project of the Dow Sustainability Program at the United States Studies Centre, in collaboration with the Faculty of Agriculture and Environment, both at The University of Sydney, Australia. The authors would like to acknowledge the financial support of this work by the Dow Sustainability Program and ALCOA.
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