Global Change Biology (2015), doi: 10.1111/gcb.12959
RESEARCH REVIEW
Soil organic carbon across scales S H A R O N M . O ’ R O U R K E 1 , 2 , D E N I S A . A N G E R S 3 , N I C H O L A S M . H O L D E N 1 and A L E X B . MCBRATNEY2 1 UCD School of Biosystems Engineering, University College Dublin, Belfield, Dublin 4, Ireland, 2Faculty of Agriculture and Environment, The University of Sydney, Biomedical Building C81, 1 Central Avenue, Australian Technology Park, Eveleigh, Sydney, NSW 2015, Australia, 3Soils and Crops Research and Development Centre, Agriculture and Agri-Food Canada, 2560 Hochelaga Blvd, Quebec, G1V 2J3 QC, Canada
Abstract Mechanistic understanding of scale effects is important for interpreting the processes that control the global carbon cycle. Greater attention should be given to scale in soil organic carbon (SOC) science so that we can devise better policy to protect/enhance existing SOC stocks and ensure sustainable use of soils. Global issues such as climate change require consideration of SOC stock changes at the global and biosphere scale, but human interaction occurs at the landscape scale, with consequences at the pedon, aggregate and particle scales. This review evaluates our understanding of SOC across all these scales in the context of the processes involved in SOC cycling at each scale and with emphasis on stabilizing SOC. Current synergy between science and policy is explored at each scale to determine how well each is represented in the management of SOC. An outline of how SOC might be integrated into a framework of soil security is examined. We conclude that SOC processes at the biosphere to biome scales are not well understood. Instead, SOC has come to be viewed as a large-scale pool subjects to carbon flux. Better understanding exists for SOC processes operating at the scales of the pedon, aggregate and particle. At the landscape scale, the influence of largeand small-scale processes has the greatest interaction and is exposed to the greatest modification through agricultural management. Policy implemented at regional or national scale tends to focus at the landscape scale without due consideration of the larger scale factors controlling SOC or the impacts of policy for SOC at the smaller SOC scales. What is required is a framework that can be integrated across a continuum of scales to optimize SOC management. Keywords: aggregate, biome, landscape, management, profile, scale, soil organic carbon, soil particle, soil policy, soil security Received 23 February 2015 and accepted 12 April 2015
Introduction Much recent research on SOC has been driven primarily by the belief that soils can be used to offset increased concentrations of atmospheric CO2. Much effort has focused on scale extremes, to develop a mechanistic understanding of SOC at the particle and aggregate scale (Schmidt et al., 2011) and to estimate global and national SOC stock (Schrumpf et al., 2011). There is some concern that soil may contribute to elevated CO2 and global climate change (Davidson & Janssens, 2006; Smith et al., 2008; Blagodatsky & Smith, 2012). The response of SOC to changing global temperatures is being studied at small scales (see Von L€ utzow & K€ ogelKnabner, 2009) and could in theory be extrapolated from predicted net primary production using climate and land-cover data (Del Grosso et al., 2008) to larger scales. We now probably know enough about SOC to manage soils for SOC sequestration potential and stock proCorrespondence: Sharon M. O’Rourke, tel. +61 28 6271013, fax +61 2 8627 1099, e-mail: [email protected]
© 2015 John Wiley & Sons Ltd
tection. Whether this is operationally feasible at the farm level is unclear. Many complex challenges related to scale remain because SOC is a complex material with variable interactions of chemical composition with the mineral fraction and decomposition kinetics (Baldock & Nelson, 2000). Many climate, soil, crop and management factors affect SOC so quantitative data are not always transferable and applicable between sites, particularly in the case of land-use and land-cover change. Global issues such as climate change require consideration of SOC stock changes at biosphere or biome scales, but human interaction occurs at the landscape scale, with consequences at the pedon, aggregate and particle scales. An integration of our understanding of SOC at all scales is required to formulate solutions that are relevant and consistent from both scientific and policy perspectives because weak understanding of scale effects has limited the translation of SOC science into policy. Growing demands on soils to provide food, water and energy security, protect biodiversity and abate climate change has led to the concept of ‘soil security’, a
3561
3562 S . M . O ’ R O U R K E et al. framework integrating biophysical, social, economic and political sciences (McBratney et al., 2014). SOC might be a universal indicator of soil security (Koch et al., 2013), where SOC lost from soil reduces soil security. Our objective is to review the processes controlling stabilization and distribution of SOC across scales, to examine the current synergy between science and policy and outline how SOC can be integrated into the framework of soil security relevant at each scale.
Particle scale Primary mineral particles (sand;
RESEARCH REVIEW
Soil organic carbon across scales S H A R O N M . O ’ R O U R K E 1 , 2 , D E N I S A . A N G E R S 3 , N I C H O L A S M . H O L D E N 1 and A L E X B . MCBRATNEY2 1 UCD School of Biosystems Engineering, University College Dublin, Belfield, Dublin 4, Ireland, 2Faculty of Agriculture and Environment, The University of Sydney, Biomedical Building C81, 1 Central Avenue, Australian Technology Park, Eveleigh, Sydney, NSW 2015, Australia, 3Soils and Crops Research and Development Centre, Agriculture and Agri-Food Canada, 2560 Hochelaga Blvd, Quebec, G1V 2J3 QC, Canada
Abstract Mechanistic understanding of scale effects is important for interpreting the processes that control the global carbon cycle. Greater attention should be given to scale in soil organic carbon (SOC) science so that we can devise better policy to protect/enhance existing SOC stocks and ensure sustainable use of soils. Global issues such as climate change require consideration of SOC stock changes at the global and biosphere scale, but human interaction occurs at the landscape scale, with consequences at the pedon, aggregate and particle scales. This review evaluates our understanding of SOC across all these scales in the context of the processes involved in SOC cycling at each scale and with emphasis on stabilizing SOC. Current synergy between science and policy is explored at each scale to determine how well each is represented in the management of SOC. An outline of how SOC might be integrated into a framework of soil security is examined. We conclude that SOC processes at the biosphere to biome scales are not well understood. Instead, SOC has come to be viewed as a large-scale pool subjects to carbon flux. Better understanding exists for SOC processes operating at the scales of the pedon, aggregate and particle. At the landscape scale, the influence of largeand small-scale processes has the greatest interaction and is exposed to the greatest modification through agricultural management. Policy implemented at regional or national scale tends to focus at the landscape scale without due consideration of the larger scale factors controlling SOC or the impacts of policy for SOC at the smaller SOC scales. What is required is a framework that can be integrated across a continuum of scales to optimize SOC management. Keywords: aggregate, biome, landscape, management, profile, scale, soil organic carbon, soil particle, soil policy, soil security Received 23 February 2015 and accepted 12 April 2015
Introduction Much recent research on SOC has been driven primarily by the belief that soils can be used to offset increased concentrations of atmospheric CO2. Much effort has focused on scale extremes, to develop a mechanistic understanding of SOC at the particle and aggregate scale (Schmidt et al., 2011) and to estimate global and national SOC stock (Schrumpf et al., 2011). There is some concern that soil may contribute to elevated CO2 and global climate change (Davidson & Janssens, 2006; Smith et al., 2008; Blagodatsky & Smith, 2012). The response of SOC to changing global temperatures is being studied at small scales (see Von L€ utzow & K€ ogelKnabner, 2009) and could in theory be extrapolated from predicted net primary production using climate and land-cover data (Del Grosso et al., 2008) to larger scales. We now probably know enough about SOC to manage soils for SOC sequestration potential and stock proCorrespondence: Sharon M. O’Rourke, tel. +61 28 6271013, fax +61 2 8627 1099, e-mail: [email protected]
© 2015 John Wiley & Sons Ltd
tection. Whether this is operationally feasible at the farm level is unclear. Many complex challenges related to scale remain because SOC is a complex material with variable interactions of chemical composition with the mineral fraction and decomposition kinetics (Baldock & Nelson, 2000). Many climate, soil, crop and management factors affect SOC so quantitative data are not always transferable and applicable between sites, particularly in the case of land-use and land-cover change. Global issues such as climate change require consideration of SOC stock changes at biosphere or biome scales, but human interaction occurs at the landscape scale, with consequences at the pedon, aggregate and particle scales. An integration of our understanding of SOC at all scales is required to formulate solutions that are relevant and consistent from both scientific and policy perspectives because weak understanding of scale effects has limited the translation of SOC science into policy. Growing demands on soils to provide food, water and energy security, protect biodiversity and abate climate change has led to the concept of ‘soil security’, a
3561
3562 S . M . O ’ R O U R K E et al. framework integrating biophysical, social, economic and political sciences (McBratney et al., 2014). SOC might be a universal indicator of soil security (Koch et al., 2013), where SOC lost from soil reduces soil security. Our objective is to review the processes controlling stabilization and distribution of SOC across scales, to examine the current synergy between science and policy and outline how SOC can be integrated into the framework of soil security relevant at each scale.
Particle scale Primary mineral particles (sand;
