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This paper serves two purposes: it provides a summarized scientific history of carbon sequestration in relation to the soil-plant system and gives a commentary on organic wastes and SOC sequestration. The concept of soil organic carbon (SOC) sequestration has its roots in: (i) the experimental work of Lundegårdh, particularly his in situ measurements of CO2 fluxes at the soil-plant interface (1924, 1927, 1930); (ii) the first estimates of SOC stocks at the global level made by Waksman [Waksman, S.A., 1938. Humus. Origin, Chemical Composition and Importance in Nature, second ed. revised. Williams and Wilkins, Baltimore, p. 526] and Rubey [Rubey, W.W., 1951. Geologic history of sea water. Bulletin of the Geological Society of America 62, 1111-1148]; (iii) the need for models dealing with soil organic matter (SOM) or SOC dynamics beginning with a conceptual SOM model by De Saussure (1780-1796) followed by the mathematical models of Jenny [Jenny, H., 1941. Factors of Soil Formation: a System of Quantitative Pedology. Dover Publications, New York, p. 288], Hénin and Dupuis [Hénin, S., Dupuis, M., 1945. Essai de bilan de la matière organique. Annales d'Agronomie 15, 17-29] and more recently the RothC [Jenkinson, D.S., Rayner, J.H., 1977. The turnover of soil organic matter in some of the Rothamsted classical experiments. Soil Science 123 (5), 298-305] and Century [Parton, W.J., Schimel, D.S., Cole, C.V., Ojima, D.S., 1987. Analysis of factors controlling soil organic matter levels in great plains grasslands. Soil Science Society of America Journal 51 (5), 1173-1179] models. The establishment of a soil C sequestration balance is not straightforward and depends greatly on the origin and the composition of organic matter that is to be returned to the system. Wastes, which are important sources of organic carbon for soils, are taken as an example. For these organic materials the following factors have to be considered: the presence or absence of fossil C, the potential of direct and indirect emissions of non-CO2 greenhouse gases (CH4 and N2O) following application and the agro-system which is being used as a comparative reference.

The GEFSOC soil carbon modelling system was built to provide interdisciplinary teams of scientists, natural resource managers and policy analysts (who have the appropriate computing skills) with the necessary tools to conduct regional-scale soil carbon (C) inventories. It allows users to assess the effects of land use change on soil organic C (SOC) stocks, soil fertility and the potential for soil C sequestration. The tool was developed in conjunction with case-studies of land use and management impacts on SOC in Brazil, Jordan, Kenya and India, which represent a diversity of land use and land management patterns and are countries where sustaining soil organic matter and fertility for food security is an on-going problem. The tool was designed to run using two common desktop computers, connected via a local area network. It utilizes open-source software that is freely available. All new software and user interfaces developed for the tool are available in an open source environment allowing users to examine system details, suggest improvements or write additional modules to interface with the system. The tool incorporates three widely used models for estimating soil C dynamics: (1) the Century ecosystem model; (2) the RothC soil C decomposition model; and (3) the Intergovernmental Panel on Climate Change (IPCC) method for assessing soil C at regional scales. The tool interacts with a Soil and Terrain Digital Database (SOTER) built for the specific country or region the user intends to model. A demonstration of the tool and results from an assessment of land use change in a sample region of North America are presented. [Copyright &y& Elsevier]

Soil organic carbon (SOC) plays a vital role in ecosystem function, determining soil fertility, water holding capacity and susceptibility to land degradation. In addition, SOC is related to atmospheric CO2 levels with soils having the potential for C release or sequestration, depending on land use, land management and climate. The United Nations Convention on Climate Change and its Kyoto Protocol, and other United Nations Conventions to Combat Desertification and on Biodiversity all recognize the importance of SOC and point to the need for quantification of SOC stocks and changes. An understanding of SOC stocks and changes at the national and regional scale is necessary to further our understanding of the global C cycle, to assess the responses of terrestrial ecosystems to climate change and to aid policy makers in making land use/management decisions. Several studies have considered SOC stocks at the plot scale, but these are site specific and of limited value in making inferences about larger areas. Some studies have used empirical methods to estimate SOC stocks and changes at the regional scale, but such studies are limited in their ability to project future changes, and most have been carried out using temperate data sets. The computational method outlined by the Intergovernmental Panel on Climate Change (IPCC) has been used to estimate SOC stock changes at the regional scale in several studies, including a recent study considering five contrasting eco regions. This ‘one step’ approach fails to account for the dynamic manner in which SOC changes are likely to occur following changes in land use and land management. A dynamic modelling approach allows estimates to be made in a manner that accounts for the underlying processes leading to SOC change. Ecosystem models, designed for site scale applications can be linked to spatial databases, giving spatially explicit results that allow geographic areas of change in SOC stocks to be identified. Some studies have used variations on this approach to estimate SOC stock changes at the sub-national and national scale for areas of the USA and Europe and at the watershed scale for areas of Mexico and Cuba. However, a need remained for a national and regional scale, spatially explicit system that is generically applicable and can be applied to as wide a range of soil types, climates and land uses as possible. The Global Environment Facility Soil Organic Carbon (GEFSOC) Modelling System was developed in response to this need. The GEFSOC system allows estimates of SOC stocks and changes to be made for diverse conditions, providing essential information for countries wishing to take part in an emerging C market, and bringing us closer to an understanding of the future role of soils in the global C cycle. [Copyright &y& Elsevier]

Soil aggregation can influence the storage of carbon in soil. The role of soil structure on soil respiration was investigated by destructive treatment (crushing, dry–wetting events). The soil was a Vertisol under market gardening, a 5-year-old pasture and a long-term pasture. When the soil structure was affected by crushing, soil respiration increase from 1 to 3.4 times. This increase, which represented the amount of protected soil organic carbon (SOC), was much higher in soil under pasture than under market gardening cultivation. There was a positive correlation between the amount of protected SOC and total SOC content. The rate of decay of the protected SOC seemed to be slightly higher than the rate of decay of the unprotected SOC. Because Vertisol is a clayey soil dominated by 2/1 clays, it showed a high capacity to protect mineralisable SOC from mineralisation in macro-aggregates compared with literature results. Even if the protected SOC in aggregates amounted to less than 10% of the total SOC, it represented more than 50% of the mineralisable SOC of a pasture soil. This mechanism seemed then to be an important factor to understand SOC storage in soil under pasture. [Copyright &y& Elsevier]

Prolonged cultivation may result in declines in total soil organic matter (SOM) and cause changes in the composition of its labile pools (carbohydrates) and in soil aggregation. The effects of long-term (19 years) tropical pasture (GP) and market-gardening (MG) on the distribution of carbon (C) and monosaccharides (MS) (individual carbohydrate monomers) across water-stable aggregates were investigated on a Vertisol (clayey, smectitic, Leptic Hapludert) of Martinique (French West Indies). Our objective was to distinguish whether the carbohydrate components identified, important stabilizing agents for aggregation, were altered after a change in land use. Surface (0–10 cm) soil samples were collected in GP and MG plots, and separated by wet-sieving into four aggregate classes (secondary organomineral separates) prior to analyses of C and monosaccharides. Concurrently, various parts of Digitaria decumbens, the vegetation dominating pasture, were analyzed for monosaccharides.The highest losses of C upon cultivation were observed in the larger aggregates 200–2000 μm where about 41% were in the form of particulate organic matter (POM) and the remaining mainly in the form of an organo-clay 0–5 μm fraction. Cultivation also affected the proportion of soil C present as carbohydrates in the aggregates. All the aggregates of the pasture soil were significantly richer in monosaccharides than those in the cultivated soil, the aggregates 200–2000 μm accounting for most of the soil carbohydrates. Whatever the separates considered in the pasture soil, glucose was the dominant sugar monomer, followed by xylose or mannose. The 0–5 and 5–20 μm aggregates containing more mannose than xylose—a sugar essentially of plant origin—displayed a microbial character. We concluded on a microbial storage of organic matter in aggregates <20 μm. In addition, it appeared that xylose which was highly depleted within macroaggregates after cultivation might be responsible for the management-induced changes in aggregate stability. This sugar, mainly associated with the 0–20 μm fraction, probably originated from plant exudates. [Copyright &y& Elsevier]