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Soil Carbon: Science, Management and Policy for Multiple Benefits

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This book brings together the essential evidence and policy opportunities regarding the global importance of soil carbon for sustaining Earth's life support system for humanity. Covering the science and policy background for this important natural resource, it describes land management options that improve soil carbon status and therefore increase the benefits that humans derive from the environment. Written by renowned global experts, it is the principal output from a SCOPE rapid assessment process project. 
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31 Chapters

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1 The Global Challenge for Soil Carbon



The Global Challenge for Soil Carbon

Steven A. Banwart*, Helaina Black, Zucong Cai, Patrick T. Gicheru,

Hans Joosten, Reynaldo Luiz Victoria, Eleanor Milne,

Elke Noellemeyer and Unai Pascual


Soil carbon in the form of organic matter is a key component of the soil ecosystem structure. The soil carbon content is an important contributing factor in the many flows and transformations of matter, energy and biodiversity – the essential soil functions that provide ecosystem services and life-sustaining benefits from soil. These goods and services include food production, water storage and filtration, carbon storage, nutrient supply to plants, habitat and biodiversity. Soil functions provide natural capital as a means of production for the ongoing supply of the essential goods and services. Soil carbon content and soil functions are under threat worldwide due to resource demands and the increasing intensification of land use. Land degradation is characterized by soil carbon losses, loss of soil structure and associated loss of fertility, and the physical loss of bulk soil by erosion. Soil carbon accumulation is associated with plant productivity, wet conditions that ensure water supply to vegetation and lack of physical disturbance to the soil. Carbon accumulation is also associated with decreased organic matter decomposition in the soil, created by cool conditions that reduce the rate of microbial activity and wet conditions that create an O2 diffusion barrier from the atmosphere and reduced aerobic microbial respiration during organic matter decomposition. The environmental conditions for the accumulation of soil carbon also provide important clues to management approaches to reverse soil carbon losses and to increase soil carbon content under widely different environmental conditions around the world. Soil management strategies can be developed from the natural cycling of soil carbon, by reducing physical disturbances to soil, enhancing vegetation cover and productivity and through improved water management. These approaches are essential in order to prevent and reverse the loss of soil functions where land is degraded and to enhance soil functions where actively managed land is undergoing intensification of use. Improved soil carbon management provides an important opportunity in land management worldwide, to meet increasing resource demands and to create resilience in soil functions that arise from the intense pressures of land use and climate change.


2 Soil Carbon: a Critical Natural Resource – Wide-scale Goals, Urgent Actions



Soil Carbon: a Critical Natural

Resource – Wide-scale Goals,

Urgent Actions

Generose Nziguheba*, Rodrigo Vargas, Andre Bationo,

Helaina Black, Daniel Buschiazzo, Delphine de Brogniez,

Hans Joosten, Jerry Melillo, Dan Richter and Mette Termansen


Across the world, soil organic carbon (SOC) is decreasing due to changes in land use such as the conversion of natural systems to food or bioenergy production systems. The losses of SOC have impacted crop productivity and other ecosystem services adversely. One of the grand challenges for society is to manage soil carbon stocks to optimize the mix of five essential services – provisioning of food, water and energy; maintaining biodiversity; and regulating climate. Scientific research has helped develop an understanding of the general SOC dynamics and characteristics; the influence of soil management on SOC; and management practices that can restore SOC and reduce or stop carbon losses from terrestrial ecosystems.

As the uptake of these practices has been very limited, it is necessary to identify and overcome barriers to the adoption of practices that enhance SOC. Actions should focus on multiple ecosystem services to optimize efforts and the benefits of SOC. Given that depleting SOC degrades most soil services, we suggest that in the coming decades increases in SOC will concurrently benefit all five of the essential services.


3 Soil Carbon Transition Curves: Reversal of Land Degradation through Management of Soil Organic Matter for Multiple Benefits



Soil Carbon Transition Curves:

Reversal of Land Degradation through

Management of Soil Organic Matter for

Multiple Benefits

Meine van Noordwijk*, Tessa Goverse, Cristiano Ballabio,

Steven A. Banwart, Tapas Bhattacharyya, Marty Goldhaber,

Nikolaos Nikolaidis, Elke Noellemeyer and Yongcun Zhao


Soils provide important ecosystem services at the local, landscape and global level. They provide the basis for crop, livestock and forestry production and help mitigate climate change by storing carbon. With expectations of a growing bioenergy supply to meet global energy demand added to the imperative to feed a global population of 9 billion people by mid-century and beyond, coupled with higher per person food demands than currently provided, the challenges to keep agricultural and rangeland soils healthy and productive are daunting. In this paper, we explore the existence of a common pattern in the use of soils under increasing demand for productivity – here described as a soil carbon transition curve: a rapid decline of soil carbon due to human clearing of natural vegetation for agricultural land use and management practices, followed by a


4 From Potential to Implementation: An Innovation Framework to Realize the Benefits of Soil Carbon



From Potential to Implementation:

An Innovation Framework to Realize the Benefits of Soil Carbon

Roger Funk*, Unai Pascual, Hans Joosten, Christopher Duffy,

Genxing Pan, Newton la Scala, Pia Gottschalk, Steven A. Banwart,

Niels Batjes, Zucong Cai, Johan Six and Elke Noellemeyer


This chapter addresses the mismatch between existing knowledge, techniques and management methods for improved soil carbon management and deficits in its implementation. The paper gives a short overview of the evolution of the concept of soil carbon, which illustrates the interactions ­between scientific, industrial, technical, societal and economic change. It then goes on to show that sufficient techniques are available for the large-scale implementation of soil organic carbon (SOC) sequestration.

A subsequent analysis of the bottlenecks that prevent implementation identifies where issues need to be addressed in order to enable robust, integrated and sustainable SOC management strategies.


In this chapter, we address the need for the wide-scale implementation of a strategy for improved soil organic carbon (SOC) management. Such a strategy can be denoted generally as innovation, but would also include new methods of governance. In this paper, we define innovation as the improved use of novel but readily available methods or technologies. In addition, it includes a change of the current behaviour by managing SOC in a different way. A broad range of SOC management methods and techniques has been proposed and tested. Many have already demonstrated their applicability and advantages, including implementation strategies


5 A Strategy for Taking Soil Carbonin to the Policy Arena



A Strategy for Taking Soil Carbon into the Policy Arena

Bas van Wesemael*, Michael Stocking, Francesca Bampa,

Martial Bernoux, ­Christian Feller, Patrick T. Gicheru,

Philippe Lemanceau, Eleanor Milne and Luca Montanarella


Soil organic carbon (SOC) has a relatively low profile in the policy arena. Here, we discuss the different steps of the policy-making process as well as the actors involved at the local, national and international scale. The first part analyses the policy-making process. The policy imperative consists of building up and maintaining SOC. The policy profile and discourse focuses on raising awareness. The policy rationale includes the economic and social benefits as well as the soil as capital. The policy support concerns the tools and programmes available. The second part of the chapter deals with the actors, from the advocates and institutions to the governance. For more detailed information, the reader is guided in each of these sections towards the chapters of the background document. Finally, recommendations are given at each of these levels for increasing the profile of SOC in the policy arena.


6 Soil Formation



Soil Formation

Marty Goldhaber* and Steven A. Banwart

Essentially, all life depends upon the soil . . . There can be no life without soil and no soil without life; they have evolved together.

USDA Yearbook of Agriculture (1938) by Charles E. Kellogg


Soil formation reflects the complex interaction of many factors, among the most important of which are (i) the nature of the soil parent material, (ii) regional climate, (iii) organisms, including humans,

(iv) topography and (v) time. These processes operate in Earth’s critical zone; the thin veneer of our planet where rock meets life. Understanding the operation of these soil-forming factors requires an interdisciplinary approach and is a necessary predicate to charactering soil processes and functions, mitigating soil degradation and adapting soil management to environmental change. In this chapter, we discuss how these soil-forming factors operate both singly and in concert in natural and human modified environments. We emphasize the role that soil organic matter plays in these processes to provide context for understanding the benefits that it bestows on humanity.


7 Soil Carbon Dynamics and Nutrient Cycling



Soil Carbon Dynamics and

Nutrient Cycling

David Powlson*, Zucong Cai and Philippe Lemanceau


The quantity of organic carbon in soil and the quantity and type of organic inputs have profound impacts on the dynamics of nutrients. Soil organic matter itself represents a large reservoir of nutrients that are released gradually through the action of soil fauna and microorganisms: this is especially important for the supply of N, P and S to plants, whether agricultural crops or natural vegetation. Organic matter also modifies the behaviour and availability of nutrients through a range of mechanisms including increasing the cation exchange capacity of soil, thus leading to greater retention of positively charged nutrient ions such as Ca, Mg, K, Fe, Zn and many micronutrients. Carboxyl groups in organic matter, and in root exudates or microbial metabolites, form complexes with various metal ions, usually increasing their availability to plants. In some cases, the formation of stable complexes has a detoxifying effect, for example by making Al and Cu less available to plants or microorganisms. Organic matter influences soil physical conditions greatly, especially through the formation or stabilization of aggregates and pores; this indirectly influences the availability of water and dissolved nutrients to plant roots. Organic matter and organic inputs are the source of energy for heterotrophic soil organisms, variations in organic carbon content and composition, impacting biome size, diversity and activities. These complex interactions between organic carbon and the soil biome require additional research to be fully understood. The implications for nutrient dynamics differ between nutrient-rich situations such as agricultural topsoils and nutrient-poor environments such as subsoils or boreal forests. In agricultural soils, excessive inputs of organic matter in manures can lead to pollution problems associated with losses of N and P.


8 Soil Hydrology and Reactive Transport of Carbon and Nitrogen in a Multi-scale Landscape



Soil Hydrology and Reactive Transport of Carbon and Nitrogen in a Multi-scale


Christopher Duffy* and Nikolaos Nikolaidis


This chapter examines the role of soil in water filtration, its impact on carbon and nitrogen in

­biogeochemical transformations and its relation to the larger landscape, where soil functions are key to clean water, essential to human sustenance. Soil composition and chemical weathering are essential factors in soil structure and formation, which affect the hydrologic properties of soil and chemical transport significantly. The role of clays on aggregation, carbon (C) sequestration, pH, etc., carbon/­ nitrogen/phosphorus (C/N/P) cycles and plant growth (plant exudates) on reactive transport are examined. Examples of water filtration and solute transformation of a functioning soil (producing clean water) and of failure to transform (producing toxicity and contamination) are presented. Special focus is given on the parameterization of hydrologic and reactive transport models that cover a range of scales from soil profile, to hill slopes and the catchment. A variety of modelling strategies presently exist for biogeochemical modelling, and typically each focuses on a particular scale, with scale-­ appropriate processes, mechanisms and states. They range from bottom-up approaches, where plotscale studies use intensive monitoring and detailed local modelling of process-level biogeochemical cycles for C and N, to regional- and continental-scale approaches to simulating the C–N dynamics in atmospheric models that, by necessity, may neglect the details of the processes understanding obtained in the plot-scale research. Ecosystem approaches extend the plot-scale models for C–N and water to landscape scales maintaining systematic processes, but may not include detailed geospatial structure and coupled hydrodynamic processes of the larger catchment and river basin. A strategy for merging scales and concepts intrinsic to plot-, landscape- and catchment-scale carbon-based biogeochemical research is proposed. The approach will describe existing process models for each scale of research, including hydrologic impacts. We propose a strategy for an integrated hydrodynamic approach to numerical modelling that can resolve the geospatial characteristics of water, carbon and nitrogen cycles over entire catchments, including first- and second-order streams, which link plot and hill-slope studies within this framework. The approach integrates plot-to-catchment scales for mesoscale model application for scales that range from 100 m to 105 km2. The approach has important implications for ongoing soils research at Critical Zone Observatories, which are advanced field research facilities, to scale up their science understanding to larger domains and will improve the prospect of carbon–nitrogen management greatly.


9 Climate Change Mitigation



Climate Change Mitigation

Martial Bernoux* and Keith Paustian


Terrestrial ecosystems play a major role in regulating the concentrations of three greenhouse gases

(CO2, CH4 and N2O), of which CO2 is the most important in terms of the impact on the global radiative balance. Soils play a major role in the global carbon (C) cycle and CO2 dynamics; thus, management of soil carbon appears essential and more and more inevitable.

The capacity of natural and managed agroecosystems to remove carbon dioxide from the atmosphere in a manner that is not immediately re-emitted into the atmosphere is known as carbon sequestration: carbon dioxide is absorbed by vegetation through photosynthesis and stored as carbon in biomass and soils, and released through autotrophic and heterotrophic respiration. Forests, croplands and grasslands can store large amounts of carbon in soils for relatively long periods. Soils are the larger terrestrial pool of organic carbon. Moreover, soil carbon sequestration is beneficial for soil quality, both over the short term and long term, and can be achieved through land management practices adapted to the specific site characteristics. The ability of soils to sequester carbon depends on climate, soil type, vegetation cover and land management practices.


10 Soil Carbon and Agricultural Productivity: Perspectives from Sub-Saharan Africa



Soil Carbon and Agricultural

Productivity: Perspectives from

Sub-Saharan Africa

Andre Bationo*, Boaz S. Waswa and Job Kihara


Soil carbon plays a key role in maintaining crop productivity in the soils in sub-Saharan Africa (SSA).

This is more so considering that most smallholder farmers cannot afford the use of adequate amounts of inorganic fertilizers to restore the proportion of nutrients lost through crop harvests, soil erosion and leaching. Complicating the situation is the huge proportion of land under threat of degradation in the form of soil erosion and nutrient decline. There are numerous opportunities for improving soil carbon as a basis of ensuing sustainable agriculture. This paper discusses the role of soil carbon in agricultural production, with special focus on sub-Saharan Africa. First, the paper presents a discussion on the functions of soil carbon (biological, chemical and physical). This is followed by a look at the causes of carbon variation across agroecosystems. Management of soil carbon and productivity is evaluated in the context of resource availability, quality and soil organic matter pools. Drawing from the integrated soil fertility management practices in Africa, the paper discusses various strategies for organic carbon management and the implication of the same on crop productivity and soil properties.


11 Soil as a Support of Biodiversity and Functions



Soil as a Support of Biodiversity and Functions

Pierre-Alain Maron* and Philippe Lemanceau


The soil is a major reservoir of biological diversity on our planet. It also shelters numerous biological and ecological processes and therefore contributes to the production of a considerable number of ecosystem services. Among the ecological, social and economic services identified, the role of soil as a reservoir of diversity has now been well established, along with its role in nutrient cycling, supporting primary productivity, pollution removal and storing carbon.

Since the development of industrialization, urbanization and agriculture, soils have been subjected to numerous variations in environmental conditions, which have resulted in modifications of the diversity of the indigenous microbial communities. As a consequence, the functional significance of these modifications of biodiversity, in terms of the capacity of ecosystems to maintain the functions and services on which humanity depends, is now of pivotal importance. The concerns emanating from the scientific community have been reiterated in the Millennium Ecosystem Assessment (MEA, 2005) published by the policy makers. This strategic document underlines the need to consider biodiversity as an essential component of ecosystems, not only because of its involvement in providing services essential to the well-being of human societies but also because of its intrinsic value in terms of a natural patrimony that needs to be preserved. This objective cannot be raised without the improvement of our ability to predict the effects of environmental changes on soil biodiversity, ecosystem functioning and the associated services; this requires a better quantification of soil biodiversity at different temporal and spatial scales, and its translation into biological functioning. Major advances in molecular biology since the mid-1990s have allowed the development of techniques to investigate and resolve the diversity of soil microbial communities (Maron et al., 2007).


12 Water Supply and Quality



Water Supply and Quality

David Werner* and Peter Grathwohl


Water is filtered while passing through soil, and soil organic carbon plays an important role in water purification through the retention of organic and inorganic pollutants. But no filter lasts forever, and no matter how strongly pollutants adsorb to soil organic matter, they will not be retained indefinitely and will eventually break through organic carbon-rich soil horizons and may reach the groundwater.

Therefore, the organic pollutant biodegradation in soil by microorganisms is a most important complementary process to pollutant retention by sorption. Soil organic carbon also shapes soil microbial communities and activities as an important substrate and habitat, and forms the metabolic capabilities and activities leading to pollutant breakdown in soils. If released into soil pore water, dissolved or colloidal organic matter may cause problems for drinking water supply as a carrier of associated pollutants, by giving taste, odour or colour to water and through the formation of disinfection by-products.


13 Wind Erosion of Agricultural Soils and the Carbon Cycle



Wind Erosion of Agricultural Soils and the Carbon Cycle

Daniel E. Buschiazzo* and Roger Funk


Wind erosion is an important process of both progressive and regressive pedogenesis in arid and semi-arid environments around the world. In semi-arid regions, which are influenced by carbon-poor dust depositions from deserts, the properties as a sink area should be maintained to enable C enrichment by continued soil formation. On agricultural land, wind erosion is a soil-degrading process, resulting mainly from the very effective sorting processes. Coarse particles remain in the field, whereas the finest and most valuable parts of the soil get lost, like particles of the silt and clay fractions and soil organic matter. The latter is not regarded in most carbon balances, although this particulate loss can reach considerable amounts. The processes of wind erosion are subject to a great spatial and temporal variability, making its quantification difficult. In this chapter, we expose wind erosion in the context of its influence on soil organic carbon and prove considerable losses by first measurements.


14 Historical and Sociocultural Aspects of Soil Organic Matter and Soil Organic Carbon Benefits



Historical and Sociocultural Aspects of Soil Organic Matter and Soil Organic

Carbon Benefits

Christian Feller*, Claude Compagnone, Frédéric Goulet and Annie Sigwalt


In this chapter, soil organic matter (SOM) benefits will be considered from two different perspectives:

(i) the scientific perception of ‘SOM benefits’ between the 18th century and today; and (ii) how various contemporary religions and societies, including farmers of Western cultures, perceive soil and SOM benefits.

Perceptions of the benefits of SOM (or humus) varied greatly in Western culture according to changes in historical scientific theories. Different periods can be considered. In the first part of 19th century, the

‘theory of humus’ by Thaer, dealing with a large popularity of SOM management for soil humus, was considered as the main nutrient for plants. In 1840, the new ‘theory of the mineral nutrition of plants by

­Liebig demonstrated that humus was not the main source of nutrients for plants, with the consequence that there was no important need to manage organic fertilization: the popularity of humus was largely decreasing. With the emergence of environmental problems due to bad SOM management, the popularity of OM management is newly increasing. The best example is the concept that soil could be a large reservoir for atmospheric carbon sequestration, and this confers special attention to plant residue management.


15 The Economic Value of Soil Carbon



The Economic Value of Soil Carbon

Unai Pascual*, Mette Termansen and David J. Abson


Soil carbon has an economic value insofar as it is associated with an asset that provides benefits for humans. Demonstrating and measuring the economic value of soil carbon can provide valuable information for policy making. It makes explicit that soil carbon is not freely available. It signals the scarcity of the resource from a social point of view and also the extent to which investment in soil carbon should be prioritized relative to other investments. It also helps policy makers determine what type of economic instruments or incentives are necessary to align privately and socially optimal soil conservation decisions. In other words, the economic valuation of soil carbon provides information to help assess how efficiently a particular land management can reallocate the goods and services from soil to different and often competing uses. The chapter stresses the importance of the context of value formation by linking human preferences, knowledge and institutions to soil carbon. Then, by means of a conceptual framework, the chapter links the types of ecosystem services (supporting, regulating, provisioning and cultural services) derived from soil carbon to economic value components using the total economic value approach. Mapping the ecosystem service values of soil carbon needs to account for who appropriates the different values (private versus social values), whether the values are direct or indirect, so as to avoid double counting. Emphasis is given to the natural insurance value of soil carbon.


16 Measuring and Monitoring Soil Carbon



Measuring and Monitoring

Soil Carbon

Niels H. Batjes* and Bas van Wesemael


Soils are the largest terrestrial reservoir of organic carbon, yet great uncertainty remains in estimates of soil organic carbon (SOC) at global, continental, regional and local scales. Compared with biomass carbon, changes in SOC associated with changes in land use and management, or climate change, must be monitored over longer periods. The changes are small relative to the very large stocks present in the soil, as is their inherent variability. This requires sensitive measurement techniques and due consideration for the minimum detectable difference (MDD). Relationships between environmental and management factors and SOC dynamics can be established using experimental field trials, chronosequence studies and monitoring networks. Soil monitoring networks (SMNs), for example, can provide information on direct changes of SOC stocks through repeated measurements at a given site, as well as data to parameterize and test biophysical models at plot scale. Further, they can provide a set of point observations that represent the (mapped) variation in climate/soil/land use and management at national scale, allowing for upscaling. SMNs must be designed to detect changes in soil properties over relevant spatial and temporal scales, with adequate precision and statistical power. Most SMNs, however, are in the planning or early stages of implementation; few networks are located in developing countries, where most deforestation and land-use change is occurring. Within these monitoring networks, sites may be organized according to different sampling schemes, for example regular grid, stratified approach or randomized; different statistical methods should be associated with each of these sampling designs. Overall, there is a need for globally consistent protocols and tools to measure, monitor and model SOC and greenhouse gas emission changes to allow funding agencies and other organizations to assess uniformly the possible effects of the impacts of land-use interventions, and the associated uncertainties, across the range of world climate, soils and land uses.


17 Modelling Soil Carbon



Modelling Soil Carbon

Eleanor Milne* and Jo Smith


Models that describe the dynamics of soil organic carbon (SOC) can be useful tools when estimating the impacts of land cover, land management and climate change on ecosystems. The development of

SOC models started with single-compartment models that assumed a constant decomposition rate. As understanding of SOC dynamics improved, these were replaced by models with different compartments with varying decomposition rate constants. Models that deal with the decomposition of SOC as a continuum have been developed, but they require complex mathematics and are therefore less popular. Compartmentalized soil carbon models are at the core of complex models such as CENTURY and

DNDC, which describe nutrient turnover in the entire ecosystem both above and below ground. The majority of such models have been developed using data from temperate ecosystems as studies on SOC stock change in temperate areas outnumber those from tropical areas. Application to tropical and subtropical areas therefore requires substantial parameterization and testing, and the availability of appropriate data sets remains a challenge.


18 Valuation Approaches for Soil Carbon



Valuation Approaches for Soil Carbon

David J. Abson*, Unai Pascual and Mette Termansen


Valuation of soil carbon can be understood as the process for assigning ‘weights’ to soil carbon when these are inadequately represented in decision making processes. There are different types of weights or ‘values’ that can be assigned to soil carbon. One approach is to assign monetary weights to such resources using economic valuation models. The total set of such monetized weights is referred to as total economic value (TEV). The different components of the value of soil carbon differ both conceptually and with respect to how they can be measured or manifested. There are various methods for quantifying soil carbon values that differ with respect to the types of values they are suitable or able to assess. This chapter reviews the various valuation approaches that can be applied to estimate different components of the TEV of soil carbon. In this respect, it discusses how soil carbon values can be estimated through both stated and reveal preferences methods, and places particular emphasis on the production function approach. In addition other approaches are presented, including the preventive or mitigation expenditure (marginal abatement costs) approach and the social cost of carbon approach. Lastly, the chapter addresses the question of how economic values can be included in economic decision making processes.


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